Railway Tunnels

Railway Tunnels

Indian Railways Institute of Civil Engineering,Pune 411001

November 2018

FOREWORD

Indian Railway Institute of Civil Engineering (IRICEN)regularly publishes text books and monographs on varioussubjects dealt by and relevant to the Civil Engineers ofIndian Railways.

In last few years, lot of tunneling works are being executedby Indian Railways and many of them in Himalayan geology,one of the toughest geology in the world to construct ormaintain any underground structures. Considering this,IRICEN has started special course on “Tunneling” from year2018 onwards for the Indian Railway officials dealing withtunnels. During these courses, most of the trainee officershad lot of doubts and queries, with a desire to have answersto all these doubts and queries in one common documentor publication. To address this, Shri R. K. Shekhawat, SeniorProfessor/IRICEN, has authored this book. The contentsof the books are based on various Text Books/ResearchPapers/ Publications and lectures notes of eminent teacherson the subject, as listed in the Bibliography and Referencesat the end of book. This book covers all technical aspectsabout Planning of tunnels, Construction of tunnels andInspection & Maintenance of tunnels. It is expected thatthis publication will be useful for all the engineers dealingwith tunnels in general, and engineers of Indian Railway inparticular.

Suggestions for improvement, including need for addition/deletion of any specific topics, may be forwarded to IRICEN.These suggestions will be considered in future revision ofthis book.

Ajay GoyalNovember, 2018 Director

IRICEN, [email protected]

mailto:[email protected]

PREFACE

Before early 1990s, when construction of Konkan Railwaycommenced, most of the tunnels constructed in IndianRailways network were in the areas where the rocks werestrong “igneous” type, not having much of geologicaldiscontinuities or other problems (like joints, folds, faults,shear zones, water charged zones, high tectonic stressesetc.). This is reflected in the fact that these tunnels aregenerally unlined and even after so many years there isnot much of structural problems in these tunnels, exceptsome instances of water seepage (due to no drainagesystem being provided in these tunnels) or problem of stonepieces falling at isolated places.

In Konkan Railway network, out of total 92 tunnels of totallength 84.8 km, only 6 - 7 tunnels (Pernem Tunnel, OldGoa Tunnel, Verna Tunnel, Honnaver Tunnels and part ofByndoor Tunnel) were located in soft or weak grounds. Allother tunnels were located in Basalt, a competent igneousrock. But these few tunnels in soft ground (of total lengthless than 5 km and maximum length of any tunnel being1.5 km only) presented peculiar problems and contributedto delay in completion of project in targeted time.

With the Indian Railway network getting expanded to allparts of the country, tunnels are being/have to be excavatedin Jammu & Kashmir, Himachal Pradesh, Uttarakhand andthe Northeastern States, mostly in the Himalayas. TheHimalayan Geology, being one of the youngest geology ofthe world, contains many geological discontinuities andsurprises in the form of joints, faults, folds, shear zones,water charged areas, high tectonic stresses, vulnerableslopes etc. This presents a great challenge to tunnelengineers in excavation and maintenance of tunnels in theseareas. In fact, some of the railway projects in these areashave been delayed due to problem related with tunnels.Therefore, understanding of the all the issues relevant toconstruction and maintenance of tunnels has becomefurther important for engineers of Indian Railways.

There are many books and research papers on the subjectof tunnels, but they are mostly written by foreign authors,and they may not be sufficient to tackle the difficulties inHimalayan region. Dr. Bhawani Singh, and some otherauthors with him, have written about tunneling in weakrocks (relevant to Himalayan region). Thus, forunderstanding the subject of tunneling in entirety, one hasto refer to a number of Text Books, Research Papers andother Publications. This may not be possible and practicablefor every field engineer. While organizing training courseson Tunnels at RDSO and teaching the subject of Tunnels atIRICEN, it was observed that the trainee officers alwaysexpressed need for a common document/ publication,containing technical material covering all activities relatedto railway tunnels. This book is an effort in that direction.It contains all the relevant aspect of tunneling i.e. Planning,Construction, Inspection and Maintenance of RailwayTunnels.

The contents of the books are based on various Text Books/Research Papers/Publications (listed in the Bibliography)and learning/lectures notes on the subject, which the authorhad good fortune of learning from stalwarts on the subjectespecially Dr. Bhawani Singh (Retired Professor, IIT/Roorkee) and Dr. K. G. Sharma (Emeritus Professor, IIT/Delhi). The author expresses his indebtedness to them.The technical material collated for publishing Indian RailwayTunnel Manual, during posting of the author as Director/GE/RDSO, has also been referred at some place, which isacknowledged.

The support and help rendered by Shri. Ghansham Bansal(Chief Vigilance Officer/Delhi Metro), Shri B. Ravi Kumar(Senior Instructor) and Shri Sabyasachi Roy (SeniorInstructor), in proof reading of the book and offeringvaluable suggestions, is also appreciated.

It is felt that this book will be useful to engineers of IndianRailways. However, there is always scope for improvementin any publication. Therefore, suggestions for improvement

are welcome from all the readers and the same may pleasebe forwarded for incorporation in the future editions.

November, 2018 R. K. ShekhawatSenior Professor

IRICEN, [email protected]

mailto:[email protected]

Chapter-1Rock Material and Rock Mass 1

1. Rocks 12. Origin of Rocks 13. Rock Discontinuities 4

3.1 Rock Joints 43.2 Faults 53.3 Folds 63.4 Bedding Planes 6

4. Rock Material and Rock Mass 75. Scale Effect 86. Groundwater 97. Inhomogeneity and Anisotropy 108. In-situ Stresses 10

8.1 Vertical Stresses and Overburden 108.2 Horizontal Stresses

(Tectonic Stresses) 119. Special Rocks 11

9.1 Weathered Rocks 119.2 Soft Rocks and Hard Soils 129.3 Swelling Rocks 129.4 Crushed Rock 12

Chapter-2Engineering Properties of Rocks 15

1. Determination of Rock Properties 151.1 Direct Methods 151.2 Indirect Methods 16

2. Test Methods 162.1 Laboratory Tests 162.2 In-situ Tests 17

3. Lab Test Samples 174. Strength and Deformation Tests 18

4.1 Uniaxial Compressive Strength Test 184.2 Tri-axial Compression Test 194.3 Young’s Modulus and Poisson’s Ratio 21

TABLE OF CONTENTS

Description Page

4.4 Brazilian Tensile Strength Test 214.5 Shear Strength 234.6 Point Load Index Test 25

5. Physical and Engineering Properties Tests 275.1 Density, Porosity and Water Content 275.2 Hardness 285.3 Abrasivity 285.4 Permeability 295.5 Wave Velocity 30

6. Failure Criteria of Rock Material 316.1 Mohr-Coulomb Criterion 316.2 Hoek-Brown Criterion 33

Chapter-3Rock Mass Classification Systems 37

1. Rock Quality Designation (RQD) 392. Terzaghi’s Rock Mass Classification 413. Rock Mass Rating (RMR) 434. Rock Mass Quality (Q) 445. Geological Strength Index (GSI) 476. Tunnelman’s Ground Classification for Soils 497. Examples of estimating RMR, Q and GSI 52

7.1 Example – 1 527.2 Example – 2 537.3 Example – 3 557.4 Comparison of rock Class/Quality 56

8. Correlation between RMR, Q & GSI 569. Squeezing behaviour of rock mass 5710. Empirical relations for rock mass strengths 58

10.1 Compressive Strength of Rock Mass 5910.2 Tensile Strength of Rock Mass 59

Appendix-3.1: Assessment of RMR value 60Appendix-3.2: Assessment of Q values 63

Chapter-4Geotechnical Investigations 69

1. Phases of Geotechnical InvestigationProgram 691.1 Preliminary Geotechnical

investigations for Feasibility Studies 69

(A) Collection, organization & studyof available data 69

(B) Preliminary Survey 70(C) Conducting investigations to

compare alternative alignmentsand for arriving at a conceptualpreliminary design 71

1.2 Preconstruction Planning andEngineering Phase 74(A) Topographical Survey 74(B) Subsurface Investigations 75(C) Detailed Geological Mapping 82(D) Groundwater investigation 83(E) Structure & Utility

pre-construction survey 851.3 Geo-technical Investigations during

Construction phase 85Tunnel Seismic Prediction 87

2. Assessing exploration requirements 893. Geotechnical investigation Program 91

Chapter-5Tunnel Surveying 93

1. Type of Surveys 931.1 Preliminary Surveys 93

1.1.1 Equipment and Techniques 941.2 Utility Surveys 95

1.2.1 Equipment and Techniques 951.3 Preliminary Survey Network 96

1.3.1 Survey Control 972. Surveying steps in alignment control 983. Recommendations for framing contract 99

Chapter-6Choice of Tunnel System, Alignment andShape/Size of Cross-section 101

1. Choice of Tunnel System 1011.1 Escape Distances 1011.2 Lateral Exits/Access 1021.3 Parallel Service & Safety Tunnel 103

2. Choice of Alignment 1043. Shape and Dimensions of the

Cross-section 1053.1 Serviceability requirements 1053.2 Geological/geotechnical conditions 1063.3 Construction aspects 106

4. Shape of Railway Tunnels 1074.1 D Shaped 1074.2 Horseshoe Shaped 1084.3 Elliptical/Circular Shape 109

Chapter-7Tunnel Design and Tunnel Supports 111

1. Concept of Stabilization of a Cavity 1112. Properties of Supports 114

2.1 Stiffness 1142.2 Bond 96 1152.3 Time of installation 115

3. Type of Tunnel Supports 1153.1 Steel ribs/Steel sets 1153.2 Lattice Girders 1173.3 Rock Bolts 117

3.3.1 Reinforcement by Rock Bolts 1243.3.2 Rock bolting pattern 1293.3.3 Floor Bolting 130

3.4 Concrete Lining 1303.4.1 Cast in-situ Concrete Lining 1313.4.2 Precast Segmental Lining 134

3.5 Shotcrete 1373.5.1 Initial Shotcrete Lining 1403.5.2 Final Shotcrete Lining 142

4. Design of tunnel lining 1445. Need for Design Consultant 1446. Initial and Final Support System 144

Chapter-8Empirical Methods of Tunnel SupportDesign 145

1. Terzaghi’s Rock Load factors 1452. Modified Terzaghi’s Theory 147

3. Support based on Rock QualityDesignation 149

4. Supports based on RMR Classification 1515. Standup time v/s Unsupported span 1536. Supports based on Q system 1547. Elements of commonly used Excavation

and Support Classes (ESC) in Rock andSoft ground 158

8. Example of estimating supportrequirement 158

9. Limitations of Empirical Approach 161Appendix-8.1 Elements of commonly usedESC in Rock 162Appendix-8.2 Elements of commonlyused ESC in Soft Ground 165

Chapter–9Analytical and Numerical Methods ofTunnel Support Design 169

1. Analytical Methods 1691.1 Examine Software 1691.2 RocSupport Software 169

2. Numerical Methods 170

Chapter–10Tunnel Excavation Methods 173

1. Selection of Tunnel Excavation Method 1732. Excavation Methods for Rock Tunnels 1733. Drill and Blast Method 173

3.1 Drilling 175(i) Wedge Cut or “V” Cut Drilling 176(ii) Cone/ Pyramid/ Diamond Cut 176(iii) Burn (Parallel) Cut Holes 176

3.2 Loading 1773.2.1 Choice of Explosives 1773.2.2 Requirement of Explosives 178

3.3 Blasting 1793.3.1 Choice of Initiation System &

Selection of Delay Sequence 179

3.3.2 Controlled Blasting 1793.3.3 Air Column Method 1803.3.4 Efficiency of Blasting 180

3.4 Ventilation (De-fuming) 1803.5 Mucking (removing rubble) 1813.6 Scaling (Dislodging) 1813.7 Supporting (Securing) 1813.8 Geological Mapping 182

4. Full Face v/s Partial Face Excavation 1824.1 Full-Face Excavation 1824.2 Partial Face Excavation 182

5. Excavation by Road header 1856. Mechanized Tunnelling 1877. Tunnelling Shields 1888. Tunnel Boring Machine 1899. Stages of TBM Construction 19110. Types of TBM 192

10.1Hard Rock TBMs 192(I) Gripper TBM 194(II) Shielded TBM 195

10.2Soft Ground TBMs 196(I) Mechanical Support TBM 197(II) Compressed Air TBM 197(III) Slurry Shield TBM 197(IV) Earth Pressure Balance TBM 199

10.3Special purpose TBMs 20011. Back-up systems for TBM 20012. Selection of TBM 20013. Excavation Methods for Soft Ground Tunnels 201

13.1 Multiple Drift Method 20113.2 Excavation by Tunnel Shields 201

14. Stipulation about machinery in contract 201

Chapter–11Tunnelling Methods/Philosophies 203

1. Conventional Tunnelling 2032. Ground (Rock) Support Interaction 204

2.1 Ground Reaction Curve (GRC)and Support Reaction Curve (SRC) 205

3. Observation Method of Tunnelling 210

4. New Austrian Tunnelling Method (NATM) 2124.1 Definition of NATM 2124.2 Principles of NATM 2134.3 Construction Sequence 2144.4 Limitations of NATM 2144.5 Pre-requisites for NATM 2154.6 Advantages of NATM 215

5. Norwegian Method of Tunnelling (NMT) 2156. ADECO-RS Method of Tunnelling 216

6.1 Important Components in Tunnelling 2166.2 Deformation Response of the

Medium 2176.3 Some Terms used in this

Methodology 2196.4 Experimental and Theoretical

Research 2206.5 Type on Interventions 221

6.5.1 Protective Interventions 2216.5.2 Reinforcement Interventions 222

6.6 Behaviour Categories of Core Face 223(A) “Category-A” – Stable Core

Face 223(B) “Category-B” – Stable Core

Face in Short Term 224(C) “Category-C” – Unstable Core

Face 2246.7 Stages of ADECO-RS Approach 225

(A) Design Stage 225(B) Construction Stage 226

6.8 Tunnelling with ADECO-RS Approach 2276.9 Difference between NATM and

ADECO-RS 2276.10Case History with ADECO-RS Method 228

Chapter–12Instrumentation and Monitoring ofTunnels 231

1. Need for Geotechnical Instrumentation 2312. Purpose of Instrumentation and

Monitoring 232

2.1 Design and Design Verification 2322.2 Construction Control 2322.3 Safety/Stability 2322.4 Regulatory/Environmental

requirements 2332.5 Performance Monitoring 2332.6 Contractual Documentation 233

3. Items to consider in Instrumentation andMonitoring Program 2343.1 Project Conditions 2343.2 Mechanisms that control behaviour 2343.3 Purpose of Instrumentation 2343.4 Parameters to be Monitored 2343.5 Expected values of parameters

recorded 2353.6 Instrument selection 2353.7 Location for Installation 2353.8 Threshold Values 2353.9 Remedial action and Implementation 2363.10Factors affecting measurements 2363.11Ensuring Data Correctness 236

4. Instruments used 2364.1 Surface Settlement 236

(A) Borehole Extensometer 237(B) Multiple Point Borehole

Extensometer 2374.2 Sub-surface Horizontal Movement 237

(A) Borehole Extensometer 238(B) Inclinometer 238

4.3 Diameter or Width Change 240(A) Tape Extensometer 240(B) 3-D Optical Targets 242

4.4 Tilt 2434.5 Load or Stress in structural Supports 243

(A) Load Cell 243(B) Pressure Cell 243(C) Measuring Anchors 245(D) Shotcrete Strain Meter 245

4.6 Pore Water Pressure (Piezometer) 2464.7 Groundwater Level (Water level

sounder) 246

4.8 Vibrations (Engineering Seismographs) 2465. Monitoring Sections at Construction Stage 248

5.1 Standard Monitoring Section 2485.2 Principal Monitoring sections 249

(A) Measurements 249(B) Number of Instruments 249(C) No. of Principal Monitoring

Sections 2495.3 Surface Monitoring 250

6. Stages in Geotechnical Monitoring 2507. Graphical Presentation of Monitoring Data 251

7.1 Time Displacement Diagram 2517.2 Vector Diagram 2517.3 Lines of Influence 2527.4 Trend Lines 253

8. Control Limits in Observational Approach 2548.1 Type of Control Limits 255

(A) Alert Level 255(B) Alarm Level 255(C) Action Level 255

8.2 Threshold Values for Control Limits 256(A) Control Limits-Displacement

Velocity 256(B) Control Limits-Differential

Settlement 257(C) Control Limits-Trend Lines 257(D) Control Limits-Shotcrete Strain 258

9. Ignoring Instrumented Data 25810. Contract Document 260

Chapter–13Drainage and Water Proofing in Tunnels 261

1. Stages of providing Drainage 2611.1 Pre-drainage 261

(A) Diverting water channels 261(B) Ground Water Pumping 262(C) Grouting 262(D) Ground Freezing 262

1.2 Drainage during Construction 263(A) Construction of Cutoff Walls 263

(B) Use of Plastic Gutter,Channels Pipes 264

(C) Drainage or Dimpled Mats 264(D) Dewatering Pumps 265(E) Drainage Boreholes before

Tunnel Advance 266(F) Drainage Boreholes after

Tunnel Advance 2661.3 Permanent Drainage System 268

2. Extent of Waterproofing 2682.1 Watertight/Waterproof Tunnel 2682.2 Partially Drained Tunnel 2692.3 Fully Drained Tunnel 270

3. Requirements of Watertightness in Tunnels 2714. Waterproofing Systems 273

4.1 Rigid Systems 2734.2 Flexible Systems 273

(A) Bitumen Waterproofing Layer 273(B) Use of Geo-synthetics 274(C) Sprayed Waterproofing Layer 275

Chapter–14Tunnelling in Weak/Soft Grounds 277

1. Conventional Methods 2771.1 Belgian Method 2771.2 German Method 2781.3 English Method 2791.4 Austrian Method 279

2. Ground Improvement 2793. Drainage 2814. Grouting 281

4.1 Location for Grouting 2824.2 Groutability of Ground 2824.3 Types of Grouting 2834.4 Grouting Material 284

5. Ground Freezing 2855.1 Refrigeration Process 2855.2 Ground Freezing Techniques 286

6. Face Consolidation 2867. Advance Supporting or Pre-supporting 287

7.1 Mechanical Pre-cutting 2877.2 Spiles 2887.3 Umbrella Arch 289

8. DRESS Method 2909. Tunneling in Swelling Grounds 29410. Tunnelling in Squeezing Grounds 29711. Comparison between Squeezing &

Swelling 30112. Tunnelling in Seismic Areas 301

Chapter–15Problems and Hazards in Tunnelling 305

1. Rock Burst 3052. Chimney Formation or Daylighting 3073. Face Collapse 3084. Water Ingress 3095. Portal Collapse 3126. Toxic Gases and Geothermal Gradient 3127. Timely Decision 3138. Failure Analysis 314

Chapter–16Safety Aspects in Tunnel Construction 315

1. Applicable Regulations 3152. Risk Assessment 3153. Project Safety Plan (PSP) 3154. General Safety Measures 316

4.1 Basic Philosophy 3164.2 Personal Protective Equipment 3164.3 Access Control System 3174.4 Signage 3174.5 Safety Management and Training 3194.6 Medical Facilities 3204.7 Ventilation 3214.8 Noise Protection 3224.9 Lighting 3234.10Communication system 323

(A) Warning Signs and NoticeBoards 323

(B) Telephone System 324(C) CCTV System 324

4.11Fire Protection 3244.12Housekeeping 3274.13Working with Machinery &

Equipment 3294.14Insects, Leeches, Vermins and

Snakes 3294.15Emergency Management System 329

5. Safety Requirement for Various Activities 3305.1 Drilling and Blasting 330

(A) Drilling Operations 330(B) Blasting Operations 330(C) Inspection after Blasting 332(D) Misfires 332(E) Scaling and Mucking 333(F) Explosives Disposal 333(G) Explosives Accountal 334

5.2 Installation of Supports 3345.3 Structural Steel Erection 3345.4 Scaffolds 3345.5 Working Platforms 3355.6 Concreting 335

(A) Mixing Plant 335(B) Pumped Concrete 335(C) Grouting and Shotcreting 335

5.7 Welding and Cutting 336(A) General 336(B) Oxy-acetylene Cutting and

Welding 336(C) Gas Cylinders 336(D) Hoses and Torches 336(E) Electric Arc Welding and

Cutting 3375.8 Paints 337

Chapter–17Safety Aspects in Operation in Tunnels 339

1. General 3392. Important Safety Aspects 340

2.1 Tunnel Ventilation 3402.2 Effect of Movement of Train inside

Tunnels 340(A) Air Quality Deterioration 340(B) Thermal Environment Hazards 341(C) Pressure Transient Hazards 341

2.3 Permissible Values of Pollutants 3412.4 Types of Ventilation Systems 342

(A) Natural Ventilation 342(B) Artificial Ventilation 342

2.5 Tunnel Illumination System 3432.6 Design of Tunnel Lighting Systems 3442.7 Normal Tunnel Lighting 3442.8 Emergency Tunnel Lighting 3442.9 Power Supply System 3452.10Cables 3452.11 Leaky Feeder 3462.12Sensors inside Main and Escape

Tunnels 3462.13Safety Walkway near tunnel portal 346

3. Infrastructure Measures 3463.1 Speed Monitoring/Signaling System 3463.2 Train Radio 3473.3 Train Detection 3473.4 Train Control Equipment 3483.5 Arrangement of Switches 3483.6 Access Control 3483.7 Double-bore Single-track Tunnels 3493.8 Fire, Smoke and Gas detection

in tunnels 3503.9 Fire Extinguishing System 3503.10Ventilation System 3503.11Escape Routes 3523.12Emergency Tunnel Lighting 3523.13Emergency Telephone/

Communication Means 3533.14Escape Distances 3543.15Vertical Exits/Access 3543.16Lateral Exits/Access 3553.17Cross Passages 355

3.18Parallel Service and Safety Tunnel 3563.19Access to Tunnel Entrance and Exits 3563.20Rescue Areas at Tunnel Entrance or

Exit 3573.21Water Supply (as access, in tunnel) 3583.22Electrical Supply for Rescue Services 3593.23Radio Installation for Rescue Services 3593.24Control Systems 3603.25Tunnel Rescue Train 3603.26Road/Rail Vehicle for Rescue 361

4. Prevention of Fire on Rolling Stock 3614.1 Fire Load and Prevent Fire Spreading 3614.2 Onboard Fire detection 3624.3 Onboard Fire Extinguishing Equipment 3624.4 First-aid Equipment Onboard 3634.5 Escape equipment & design of coaches 363

5. Operations Measures 3635.1 Regulations for carriage of

Dangerous Goods 3635.2 Emergency Information for

Passengers 3645.3 Emergency and Rescue Plans 3645.4 Information on carriage of

Dangerous Goods 3645.5 Provision of Rescue Equipment 365

6. Additional Measures for Very Long Tunnels 3656.1 Segmentation of Overhead Lines 365

7. Safety Measures for Existing Tunnel 3668. Mock Drills 366

Chapter–18Tunnel Portals 367

1. Need for Portal 3672. Design of Portal 3673. Construction of Portal 368

Chapter–19Shafts 373

1. Need for Shafts 3732. Location of Shafts 373

3. Shape of the Shaft 3744. Design of Shaft 3745. Collar 3756. Area around Shaft 3767. Shaft excavation in Soft Ground 3768. Support Systems used for Shaft

Excavation 3778.1 Timber Sheet Piling 3778.2 Steel Sheet Piling 3788.3 Soldier Piles and Lagging 3808.4 Liner Plates 3818.5 Horizontal Ribs and Vertical Lagging 3838.6 Slurry Walls 3848.7 NATM System 386

9. Shaft excavation in Soft and Wet Ground 3869.1 Lowering of Groundwater 3869.2 Open Pumping 3879.3 Wellpoint System 3879.4 Deep Wells 3879.5 Freezing 388

10. Shaft excavation in Rock 38810.1Raises 38910.2Temporary Supports 389

11. Other Mechanized Methods 39012. Lining of Shafts 390

Chapter–20Inspection and Maintenance of Tunnels 393

1. Pre-requisites for Tunnel Inspection 3931.1 Qualifications of Inspecting Officials 3931.2 Equipment/Tools for Inspection 3931.3 Advance Preparation 395

(A) Study of Tunnel Records 395(B) Marking of Reference System 395(C) Tunnel Inspection Record 395(D) Inspection Methodology 396(E) Ensuring Safety 396

2. Common Structural Defects 3962.1 Concrete Structures 398

(1) Scaling 398

(2) Cracking 398(3) Spalling 398(4) Pop-outs 399(5) Efflorescence 399(6) Staining 399(7) Hollow Area 400(8) Honeycomb 400(9) Leakage 400

2.2 Steel Structures 400(1) Corrosion 400(2) Cracks 401(3) Buckles and kinks 401(4) Leakage 401(5) Protection System 401

2.3 Masonry Structures 402(1) Masonry Units 402(2) Mortar 402(3) Shape 402(4) Leakage 402

2.4 Connection Bolts 4032.5 Shotcrete/SFRS 4032.6 Rockbolts 403

3. Major Items to be inspected 4043.1 Portal (including cuttings at

approaches) 4043.2 Section of tunnel in relation to

moving dimensions 4043.3 Tunnel Roof, Walls and Invert 4053.4 Tunnel Refuges and Walkway 4053.5 Drainage 4053.6 Track 4053.7 Ventilation, Lighting, Telephone

Communication, Firefightingpreparedness and Electrical/Mechanical Systems 405

4. Inspection Documentation 4064.1 Recording of Defects 4064.2 Condition Rating System 4074.3 Repair Priority Classifications 408

4.4 Specialized Testing Reports 4094.5 Tunnel Inspection Report 409

5. Role/Duties of Inspecting Officials 4105.1 Inspection by SSE/P.Way or

SSE/Works 4105.2 Inspection by ADEN 4115.3 Inspection by Divisional/Sr.

Divisional Engineer 4115.4 Special Inspection 412

6. Maintenance and Repair of Tunnels 4136.1 Portal and Approach Cuttings 413

(A) Surface Drainage 413(B) Catch Water Drain 413(C) Removal of loose boulders etc. 413(D) Structural Repairs 414(E) Access Road 414(F) Weep Holes 414

6.2 Inside Tunnel 4146.3 Systematic Approach for Repairs 415Appendix 20.01: Tunnel Inspection

Register 416Appendix 20.02: Tunnel Inspection

Register 419Appendix 20.03: Supplementary Tunnel

Information Register 422

Chapter–21Metro Tunnels 425

1. Metro Tunnels Excavation 4252. Pre-cast Lining 4273. Building Condition Survey and

Vibration Limit 4284. Impact on the Structures 4285. Subsidence 4296. Portal and Cut Slopes 430

Appendix 21.1: Building DamageClassification 432

Chapter–22Miscellaneous 435

1. Ballastless Track in Tunnels 4351.1 Ballasted Track Structure 4351.2 Ballastless Track Structure 4371.3 Ballastless Track Systems 438

2. Cut and Cover Tunnelling 4382.1 Bottom-Up Construction 4402.2 Top-Down Construction 443

3. Micro-tunnelling 4463.1 Guidance System 4483.2 Control Container 4493.3 Remote Hydraulic Power pack 4493.4 Main Drive Power Container 4503.5 Procedure of Micro-tunnelling 4503.6 Utility in Railways 450

4. Provision of OHE 451

INDEX 452

Bibliography and References 456

1

ROCK MATERIAL AND ROCK MASS

Some basic concepts about rocks and rock mass arebeing presented in this chapter, which will be helpful inappreciating the other issues related with tunnelling.

1. Rocks: A mineral is solid inorganic material of theearth having known chemical properties, crystalstructure (Fig. 1.01), hardness and cleavages pattern.A rock is solid aggregate of one or more minerals thathave been cohesively brought together by a rock formingprocess (Fig. 1.02). Various properties of mineralscombined with their arrangement in the rock inter-aliainfluence the properties of rocks.

Fig. 1.01: Quartz Fig 1.02: QuartzCrystals Crystals in Granite

2. Origin of Rocks: Rocks are as dynamic as rest ofthe nature, but they change very slowly. The oldestrocks were formed with formation of planet earth, about4.6 billon years ago. But they were built, eroded andrebuilt in the process called “rock cycle” (Fig. 1.03).

Igneous rocks are formed by volcanic activity. As themagma cools under earth's surface in magma chambers,intrusive igneous rocks are formed (common exampleis Granite). As the lava gets out to the surface, it cools,and extrusive igneous rocks are formed. Commonexamples of igneous rocks are Granite, Diorite, Gabbro,Rhyolite, Andesite and Basalt (Fig. 1.04 & Fig. 1.05).

CHAPTER-1


Chapter-1

2

Fig. 1.03: Rock Cycle

Granite Diorite GabbroFig 1.04

Rhyolite Andesite BasaltFig 1.05

These rocks erode mainly through wind and rain andthe eroded material ends up on the ground or finds itsway to waterways. In the bottom of oceans, lakes andrivers, this material accumulates in the form of layers,

3


known as lithification, with older layers overlain byyounger layers. The older layers buried deep turn intoSedimentary rocks under the pressure from youngerlayers. Some sedimentary rocks end up on the surface,by uplifting, and/or erosion. Common examples ofsedimentary rocks are Shale, Sandstone, Rock salt andConglomerate (Fig. 1.06 & Fig. 1.07).

Fig 1.06: Shale Sand Stone

Fig 1.07: Rock Salt Conglomerate

Pre-existing rocks metamorphose into Metamorphicrocks, by pressures and temperatures, wherein it isstretched and compressed but not melted. Somemetamorphic rock ends up on the surface by upliftingforces and erosion. Others can get buried even deeper,where the temperatures are so hot that the rock startsmelting - it turns into magma melt. Different forcesmay bring it closer to the surface again, where thetemperatures may get so low that it "freezes" (this isstill at a temperature of thousands of degree Celsius)and turns into an intrusive igneous rock. Or, it may endup exploding in a volcano and freeze on the surface ofthe earth to an extrusive igneous rock. Commonexamples of metamorphic rocks are Slate, Phyllite,Schist, Gneiss, Marble and Quartzite (Fig. 1.08 & Fig.1.09).

Chapter-1

4

Slate Phyllite SchistFig 1.08

Gneiss Marble QuartziteFig 1.09

And the cycle continues by erosion, burial, uplift and soon. All the rocks at any time on the Earth, are in somestage of that cycle.

3. Rock Discontinuities: The rocks are not found inthe form of monolithic continua but they consist of intactrock material and number of discontinuities, also knownas geological structures, which are planes/sources ofweaknesses. These discontinuities are introduced eitherduring the rock formation stage or later on due to variousforces acting on it over a prolonged period of time. Thesediscontinuities play a significant role at all stages intunneling i.e. deciding alignment of tunnels, design oftunnel supports, and construction & maintenance oftunnels. Some of the common discontinuities are asfollowing.

3.1 Rock Joints: Joints are most common rockdiscontinuity. A joint is a break in the continuity ofthe rock material without any movement along thejoint surface (Fig. 1.10). A series of parallel jointsis called as “Joint Set” and two or more joint sets

5


intersecting each other produce a “Joint System”.Joints are normally in parallel sets and theirspacing can vary from a few to few ten cent-imeters.

Fig. 1.10: Rock Joints

3.2 Faults: Fault is a discontinuity in rock, acrosswhich there has been significant displacement as aresult of rock mass movement (Fig. 1.11 & Fig.1.12). A fault plane is the plane that representsthe fracture surface of a fault. A fault trace orfault line is the place where the fault can be seenor mapped on the surface. Since most of the timefaults do not consist of a single clean fracture,term fault zone is also used to refer to zone offault plane. The fault zone may contain crushedparent rock material or other material with orwithout water entrapped in it.

Fig. 1.11: Components of a Fault

Chapter-1

6

Fig. 1.12: Faults

3.3 Folds: Fold is a bended and planar rockstrata, as a result of tectonic forces or movements.They are often associated with high degree offracture and relatively weak and soft rock. Ananticline is a fold that is convex upwards and asyncline is a fold that is concave upwards (Fig.1.13). Any underground excavation located at thebottom zone of a syncline fold is expected to havehorizontal stresses of compressive in nature andany underground excavation located at the topzone of an anticline fold is expected to have lesshorizontal compressive stresses or tensile stresses.A symmetrical fold is one in which the axialplane is vertical and asymmetrical fold is one inwhich axial plane is inclined.

Anticline Fold Syncline FoldFig. 1.13

In most of the cases, the folds may be a combinationof many types of folds as shown in Fig. 1.14.

3.4 Bedding Planes: Bedding plane is an interfacebetween sedimentary rock layers (Fig. 1.15). Somebedding planes could also become potentialweathered zones and ground water pockets.

7


Fig. 1.14: A Fault in Katra -Banihal Section of USBRL Project

Fig. 1.15: Bedding Planes

4. Rock Material and Rock Mass: For civil engineeringworks e.g. Foundations, Slopes and Tunnels, the scaleof projects may vary from a few tens to a few hundredmetres. In this extent, the rock encountered, oftentermed as rock mass, will contain the “rock material”in the form of intact rock blocks of various sizes and“rock discontinuities” (liken Joints, Faults, Folds, Beddingplanes etc.). The “rock material” will be strong, stiffand brittle material, very strong in compression butweak in tension; with even weak rock material havingcompressive strength of order of 40-50 MPa which ismore than strength of high strength concrete (M40 orM50 Grade Concrete). But behaviour of “rock mass” iscontrolled by the discontinuities and due to this, therock mass strength may be 1/2 to 1/10 of the rockmaterial strength. The discontinuities reduce rock massstrength, reduce rock mass quality and increasedeformation under load. Therefore, “Rock Mass = RockMaterial + Rock Discontinuities”.

Chapter-1

8

5. Scale Effect: In case of underground excavationlike tunnel, the behaviour of rock mass, to a large extent,also gets governed by the spacing/extent ofdiscontinuities vis-à-vis the dimension of excavation.In case, the rock mass is having very few discontinuitiesor the excavation dimension is less than the spacing ofthe discontinuities, then rock mass will behave like“Massive Rock”, without much of influence due todiscontinuities (Fig. 1.16).

Fig. 1.16: Scale Effect - Massive Rock

When the rock mass is having moderate number ofdiscontinuities or the excavation dimension is biggerthan the spacing of the discontinuities, then rock masswill behave like “Jointed or Blocky Rock”, with someinfluence due to discontinuities (Fig. 1.17).

Fig. 1.17: Scale Effect – Jointed or Blocky Rock

In case rock mass is having large number ofdiscontinuities or the excavation dimension issignificantly higher than the spacing of thediscontinuities, then rock mass will behave like “Heavily

9


Jointed Rock”, with significant influence due todiscontinuities (Fig. 1.18).

Fig. 1.18: Scale Effect – Heavily Jointed Rock

6. Groundwater: Groundwater is a very importantfactor for consideration in any underground excavationbecause:

(i) Water pressure contributes to the stresses onthe tunnel supports.

(ii) Presence of water alters the properties ofrock mass.

(iii) Presence of water increases the complexity inconstruction and maintenance of tunnels.

Most of the igneous and metamorphic rocks are verydense with interlocked texture and, therefore, haveextremely low permeability and porosity. But someclastic sedimentary rocks, typically sandstones, can beporous and permeable. Weathered rocks can also beporous and permeable.

Fig. 1.19: Water inflow in Tunnel T-3 of Udhampur –Katra Section (measured peak Discharge of 600–

1200 lit./Sec)

Chapter-1

10

Therefore, the “Rock Mass Behaviour” is influenced by“Rock Material”, “Discontinuities” and “Ground Water”.(Fig. 1.20)

Fig. 1.20: Behaviour of Rock Mass

7. Inhomogeneity and Anisotropy:Inhomogeneity represents property varying withlocations. Rocks exhibit great inhomogeneity, due to:

(i) Different minerals in a rock,(ii) Different bonding between minerals, and(iii) Existence of discontinuities.

Anisotropy is defined as properties being different indifferent direction. It occurs in both rock materials androck mass.Inhomogeneity and Anisotropy present challenge indesign and construction of tunnels.

8. In-situ Stresses: In-situ stresses are anothercomplexity in design and construction of tunnels. In-situ stresses can be of two types;

8.1 Vertical Stress and Overburden: Verticalstress in rock is the overburden stress, generatedby weight of the overlying material. The averagespecific gravity of rocks is about 2.7. The verticalstress at depth can be estimated as:

ZZσ (in MPa)0.027 z, Where: z is overburden (in m)

11


8.2 Horizontal Stress (Tectonic Stresses):Horizontal stresses in rock are primarily tectonicstresses. Tectonic stresses have huge variations inmagnitude, and can be exceptionally large. Fromlarge scale experimental observations (Fig. 1.23),it was observed that ratio of average horizontal

stresses ( XXσ + YYσ )/2 to vertical stress is between

0.5 to 3.0, mostly bounded between (100/z + 0.3)and (1500/z + 0.5).

From Fig. 1.22, it can be seen that at common depthsfor Civil Engineering applications (<1000 m), thevariation of horizontal stress is quite wide, whichbecomes narrow at depths of 2000m or more.

9. Special Rocks: There are many special types ofrocks which present a difficulty in design of tunnelsupports, construction of tunnel and inspection &maintenance of tunnels.

9.1 Weathered Rocks: All rocks disintegrateslowly as a result of Mechanical and Chemicalweathering (Fig. 1.21). Some weathered rocksshow structure and texture as normal rock, butdue to weathering, rock material strength issignificantly reduced.

Fig. 1.21: Weathered Rocks

Chapter-1

12

9.2 Soft Rocks and Hard Soils: Sedim-entaryrocks are formed by sediments (soils) through longprocess of compaction and cementation. Manytimes, this process is stopped before the sedimentsare completely solidified. The materials then couldbe highly consolidated but not fully solidified.Typically, these materials have low strength andhigh deformability, and when placed in water, theyoften can be dissolved, but in dry condition, theybehave as weak rock.

9.3 Swelling Rocks: Some rocks have the cha-racteristics of swelling, when exposed to water.Rock and soil containing considerable amount ofmontmorillonite minerals will exhibit swelling andshrinkage characteristics. Such rocks can causeexcessive pressures on the tunnel supports andcan lead to their collapse, in worst case, even aftercompletion of tunnel construction.

9.4 Crushed Rock: Characteristics of highly fract-ured and crushed rocks (Fig. 1.22) are quitedifferent from the massive rock mass. When suchmaterials are encountered, they need to beaddressed separately.

Fig. 1.22: Crushed Rock

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15

ENGINEERING PROPERTIES OF ROCKS

Due to various complexities, it is difficult to describeany single value for engineering properties of the rockas well as rock mass (Fig. 2.01). It has to be seen andunderstood in the overall context of the its’ usage, dulyconsidering effect of discontinuities, scale effect, in-situ stresses etc.

Fig: 2.01: Complexities in Rock Mass Properties

1. Determination of Rock Properties: Theengineering properties of intact rock material or rockmass can be determined by two methods:

1.1 Direct Methods: Direct methods consist ofLaboratory tests and Field (in-situ) tests. In-situtests are normally time consuming and conductingthem on the scale required for undergroundexcavation sometimes becomes very costly. Thedata obtained by direct tests, especially for rockmass, is many times not consistent.

CHAPTER-2

ENGINEERING PROPERTIES OFROCKS

16

Chapter-2

1.2 Indirect Methods: These methods are basedon empirical or theoretical correlations. Currentpractices for determination of rock mass propertiesrely heavily on indirect methods. Indirect methodscan also be used for verifying the results obtainedfrom direct methods.

2. Test Methods: Though the tests to be carried outon Rock Material or Rock mass will get dictated by theintended use of the test results, type of rock mass andother relevant factors, International Society of RockMechanics (ISRM) has suggested some categories oftests (after Brown, 1981) which are as following:

2.1 Laboratory Tests(A) For Characterization(i) Porosity, Density and Water content(ii) Absorption(iii) Hardness - Schmidt Rebound Hammer and

Shore Scleroscope(iv) Resistance to Abrasion(v) Point Load Strength Index(vi) Uniaxial Compressive Strength and

Deformability(vii) Swelling and Slake Durability(viii) Sound Velocity(ix) Petrographic description

(B) For Engineering Design(i) Tri-axial Strength and Deformability Test(ii) Direct Shear Test(iii) Tensile Strength Test(iv) Permeability(v) Time Dependent and Plastic Properties

17


2.2 In-situ Tests

(A) For Characterization(i) Discontinuity Orientation, Spacing and

Persistence, Roughness, Wall Strength,Aperture Filling, Seepage, Number of JointSets and Block Size

(ii) Drill Core Recovery and RQD(iii) Geophysical Borehole Logging(iv) In-situ Sound Velocity(B) For Engineering Design(i) Plate and Borehole Deformability Tests(ii) In-situ Uniaxial and Tri-axial Strength and

Deformability Test(iii) Shear Strength - Direct and Torsional Shear(iv) Field Permeability Measurement(v) In-situ Stress Determination

3. Lab Test Samples: For carrying out various tests,test samples (mostly cylindrical in shape) are prepared,as per the size specified in the relevant code orspecification (Fig. 2.02 & Fig. 2.03). The commonly usedL/D (Length/Diameter) ratio for lab test specimen is asunder:

Table-2.01: L/D Ratio for Lab Test Samples

Description of Test L/D RatioUniaxial or Tri-axial Compression 2 - 3Brazilian Test 0.5Point Load Index 1.0Punch Test 0.2 – 0.25Bending Test 3 - 7Single Shear 2Double Shear 3Oblique Shear 1Permeability 2

18

Chapter-2

Fig. 2.02: Specimen for different Tests

Ends (top and bottom) should be flat to 0.05mm. Endsshould be perpendicular to the axis of specimen within0.001 radian (3.5 minutes). Sides should be smoothand free of abrupt irregularities, straight to within0.3mm over full length of specimen.

Fig. 2.03: Strain Gauging of Test specimen

4. Strength and Deformation Tests: Some of themost commonly performed strength and deformationstests are covered in brief.

4.1 Uniaxial Compressive Strength (UCS) Test:UCS is the ultimate stress a cylindrical rockspecimen can take under axial load (Fig. 2.04). Itis important mechanical property of rock material,used in design, analysis and modelling.

Diameter of specimen must be greater than 10times the diameter of largest grain size. Preferabledia. is 45mm. But in no case the diameter of coresample should be less than 35mm. Specimen iskept in a loading frame and axial load iscontinuously increased till the specimen fails

19


(crumbles). At least 5 specimens are required togive a representative value. Along with axial load,the axial and lateral deformations (strains) arealso measured. (Ref. 42, IS:9143-1979).

Fig. 2.04: UCS Test for Rock

Fig. 2.05: Stress-Strain curve for UCS Test

4.2 Tri-axial Compression Test: True tri-axialcompression means three different principalstresses. But it is often simplified by conductingaxisymmetric tri-axial test wherein two lateralstresses equal to minor principal stress σ 3 areapplied by confining pressure around the sample intri-axial cell and the major principal stress isapplied as deviator stress (σ 1–σ 3) required to takesample up to failure for the given confiningpressure (Ref. 43, IS:13047-1991).

For various confining pressures/stresses, which isminor principal stress (σ 3), values of deviator

20

Chapter-2

stress (σ 1–σ 3) required to take sample up tofailure are measured, which in-turn gives themajor principal stress (σ 1). By this approach, a setof σ 1 and σ 3 values are obtained. By continuousmeasurements of deviator stress and axial strain,the values of Axial Stress and Axial Strain can beworked out/plotted for any given value of confiningstress (σ 3) (Fig. 2.06).

Fig. 2.06: Stress-Strain curve for Tri-axial Tests

Fig. 2.07: Sample of Tri-axial Test Results

21


The behaviour of rock in tri-axial compressionchanges with increasing confining pressure, withthe stress-strain behaviour in elastic regionremaining same as in uniaxial compression (Fig.2.06 & 2.07).

4.3 Young’s Modulus and Poisson’s Ratio:Young's Modulus and Poisson’s Ratio can beexperimentally determined from the Stress-Straincurve. They seem to be unaffected by change ofconfining pressure. High strength rocks also tendto have high Young's Modulus, depending on rocktype and other factors. For most rocks, thePoisson’s ratio is between 0.15 and 0.4. Typicalvalues/range of values for Young’s Modulus forvarious types of rocks are given in Fig. 2.08.

Fig. 2.08: Typical values of Young’s Modulus

4.4 Brazilian Tensile Strength Test: Rock mat-erial generally has a low tensile strength, due tothe pre-existing micro-cracks in the rock material.

22

Chapter-2

Fig. 2.09: Brazilian Test

Rock material tensile strength can be obtainedfrom several types of indirect tests and the mostcommon tensile test is “Brazilian test” (Fig. 2.09).When using the Brazilian test to determine theindirect tensile strength of rock (Ref. 44,IS:10082-1981), it is usually assumed that failureis the result of the uniform tensile stress normal tothe splitting diameter and that the tensilestrength, T0, is given by the formula:

T0 = 2*P / π*D*L

Where:

“P” is the applied load, “D” is the core diameterand “L” the core thickness (length of specimen)

23


Table 2.02: Typical Values of UCS and TensileStrength

Rock UC Strength Tensile(MPa) Strength

(MPa)Granite 100 -300 7 – 25Dolerite 100 -350 7 – 30Gabbro 150 -250 7 - 30Basalt 100 – 350 10 -30Sandstone 20 - 170 4 - 25Shale 5 - 100 2 - 10Dolomite 20 - 120 6 – 15Limestone 30 - 250 6 – 25Gneiss 100 - 250 7 - 20Slate 50 - 180 7 – 20Marble 50 - 200 7 – 20Quartzite 150 - 300 5 - 20

4.5 Shear Strength: Rocks resist shear stress bytwo internal mechanisms, Cohesion (c) andInternal Friction (Ø). Like soil, shear strength ofrock material can be determined by direct sheartest and by tri-axial compression test.

In Direct Shear Test, specimen is placed in ashear box which has two stacked rings to hold thesample; the contact between the two rings is atapproximately the mid-height of the sample (Fig.2.10). A normal stress is applied vertically to thespecimen, and the upper ring is pulled laterallyuntil the sample fails. The normal stress appliedand shear stress at failure is recorded. Severalspecimens are tested at varying normal stresses,to plot stress–strain curve, with peak shear stresson the y-axis and the normal stress on the x-axis(Fig. 2.11). The y-intercept of the curve which fitsthe test results is the cohesion (c), and the slope

24

Chapter-2

of the line or curve is the friction angle (Ø) (Ref.45, IS:12634-1989).

Fig. 2.10: Direct Shear Test Apparatus

Fig. 2.11: Plot of Shear Stress v/s Normal Stress

Fig. 2.12: Mohr Circles based on Tri-axial Tests

25


In Tri-axial Shear Test, from a series of tri-axialtests, peak stresses (σ 1) are obtained at variousconfining stresses (σ 3). Using each set of σ 1 andσ 3 values, numbers of Mohr Circles are drawn(Fig. 2.12). The slope of best fit tangent on thesecircles is friction angle (Ø) and y-intercept of thistangent is the cohesion (c).

4.6 Point Load Index Test: Point load test is asimple index test for rock material. It is anattractive alternative to UCS as it can providesimilar data at a lower cost, as block or irregularlump specimen can also be tested in this test. Theapparatus for this test consists of a rigid frame,two point load platens, a hydraulically activatedram with pressure gauge and a device formeasuring the distance between the loading points(Fig. 2.13 & Fig. 2.14).

This test gives the standard Point Load Index Is(50)for a 50mm diameter sample. Minimum of 10 testspecimens are required to find out the averagevalue of Point Load Index (Ref. 46, ASTM D5731-2016).

Is(50) = (P*1000)/(D1.5 * 50 ) in MPaa

Where,

“P” is Breaking Load in KN and “D” is distancebetween platens in mm.

Point Load Index can be correlated to Strengthsby various empirical formulas, as given below,and it can be used as an independent strengthindex also.

Compressive Strength: σ c 22 * Is(50)

(The multiplication factor of 22 can vary from 10 to 30)Tensile Strength: σ t 1.25 Is(50)

26

Chapter-2

Fig. 2.13: Test Setup for Point Load Testing

Fig. 2.14: Point Load Testing Machine

27


5. Physical and Engineering Properties Tests: Someof the most commonly performed tests for Physical andEngineering properties of rocks are described in brief.

5.1 Density, Porosity and Water Content:

Fig. 2.15: Rock Mass Matrix

Density=Bulk Mass/Bulk Volume

Porosity=Non-solid Volume/Bulk Volume

Water Content=Volume of Water/Bulk Volume

Dry density of rock material is generally between2.5-2.8 g/cm3. High density generally means lowporosity.

Porosity is generally low for crystalline rocks, e.g.Granite (<5%) and can be high for clasticSedimentary rocks, e.g. Sandstone (up to 50%).Porosity effects permeability.

Water content depends on saturation. Wet rocktends to have slightly lower strength.

28

Chapter-2

5.2 Hardness: Hardness of rock/rock mass is usedfor predicting cutting/boring rates for tunnel boringmachines and determination of rock quality forconstruction purposes etc. The rebound hammertest provides a means for rapid determination ofhardness.

Fig. 2.16: Rebound Hammer Test

Using Rebound Hammer Values, the hardness isarrived at using the standard charts supplied withthe equipment.

5.3 Abrasivity: Abrasivity of the rocks is used todesign the cutting tools of Tunnel Boring Machineor Road Headers or to predict wear of excavatorbuckets. It is determined most commonly in termsof Cerchar Abrasivity Index (CAI) (Ref. 47, ASTMD7625-2010).

Fig. 2.17: CERCHAR Abrasivity Test setup

29


The test is performed on a small freshly brokenrock sample, less than 25mm in size, which is heldin position by sample holder. The sample isscratched by a hardened sharp heat-treated alloysteel needle of a defined geometry, over a lengthof 10mm in 1 second, under a static load of 70kN(Fig. 2.17). The CAI is then calculated as 1/10thof di (average measured worn flat diameter oftesting needle, in tenth of mm).

Table 2.03: Typical values of CAI

Granite 4.5 – 5.3Diorite 4.2 – 5.0

Andesite 2.7 – 3.8Basalt 2.0 – 3.5Sandstone 2.8 – 4.2Shale 0.6 – 1.8Limestone 1.0 – 2.5Gneiss 3.5 – 5.3Slate 2.3 – 4.2Quartzite 4.3 – 5.9

5.4 Permeability: It is a measure of ability of thematerial to transmit fluids. It is of importance onlyfor porous rock masses, as it can be used topredict the amount and pressure of water likelycome through rock material, which has to betackled during construction phase and for design ofwaterproofing arrangements. However, in rockmass the flow is concentrated in fractures.

Coefficient of Permeability (k) is measured byDarcy’s law, using following formula (Fig. 2.18)(Ref. 48, IS:4348-1973):

Q = A * k * (h1- h2) / L

30

Chapter-2

Fig. 2.18: Darcy’s Law Setup

Where:

Q = Flow ratek = Coefficient of PermeabilityA = Cross section areah1 & h2 = Hydraulic headL = Length of specimen

5.5 Wave Velocity: Two types of waves arenormally used in velocity measurements i.e.Longitudinal (P) wave and Shear (S) wave. P waveis the fastest travelling wave and is mostcommonly used in wave velocity measurements.

The waves generated by a pulse generator aretransmitted into the sample of known length by atransmitter at one end of sample and received atother end of sample by a receiver. By using thetime of travel, the transmission velocity of thewave in the sample is calculated.

A well compacted rock mass will generally havehigh velocity as the grains are in good contact andwave travels faster through solid grains.

P wave velocity (Vp) of Gneiss and Quartzite is5000-7000 m/s and of Shale, Sandstone andConglomerate is 3000-5000 m/s.

31


S wave velocity (Vs) of Gneiss and Quartzite 3000-4000 m/s and of Shale, Sandstone andConglomerate is 2000-3000 m/s.

Fig. 2.19: Wave velocity measurement

Wave velocities are used to estimate the modulusof rock material, by using following equations:

Elastic Modulus Es=ρ Vp2(GPa),(g/cm3), (km/s)

Shear Modulus Gs=ρ Vs2(GPa),(g/cm3),(km/s)

Poisson’s Ratio sν =[1–2(Vs/Vp)2]/[2{1–(Vs/Vp)

2}]

Where:ρ = Density of material

sν = S Wave velocity in the material

sν = P Wave velocity in the material

The modulus values arrived at by using the aboveequations is slightly higher than the modulusdetermined from static tests.

6. Failure Criteria of Rock Material: There are twofailure criteria normally used for rock material.

6.1 Mohr-Coulomb Criterion is a mathematicalmodel describing the response of brittle materialsto shear stress as well as normal stress (Fig.

32

Chapter-2

2.20). The failure envelope, in a plot betweenNormal Stress (σ n) and Shear Stress ( ) isrepresented by a circle. Any point within the circleis stable and any point outside the circle isunstable, with the points on the periphery of thecircle being on the verge of instability.

Fig. 2.20: Mohr-Coulomb Failure Criterion

σ n = ½ (σ 1 + σ 3) + ½ (σ 1 - σ 3) cos2 = ½ (σ 1 - σ 3) sin2 = ¼ + ½ Ø

Where: σ n = Normal Stress on the failure plane = Shear Stress on the failure planeσ 1 = Major Principal Stressσ 3 = Minor Principal Stress

θ = Inclination of the failure plane

Shear strength is made up of two parts, a constantCohesion (c) and a normal stress dependent Angleof Internal Friction (Ø):

ζ = c + σ n tanØ

Actual tensile strength of rock material is lowerthan the criterion. A tensile cut-off is usually

33


applied at a selected value of uniaxial tensilestress, σ t’, at about 1/10 of σ c (Fig. 2.21).

Fig. 2.21: Compressive & Tensile Strength

The compressive strength is given by:σ c = (2c cosØ) / (1 – sin Ø)

The tensile strength is given by:σ t = (2c cosØ) / (1 + sin Ø)

6.2 Hoek-Brown Criterion: Mohr-Coulomb crit-erion is suitable for the low range of confiningstress only and at high confining stress, itoverestimates the strength. It also overestimatestensile strength. Since in most cases, rockengineering deals with shallow depth problems andlow confining stress, so Mohr-coulomb criterion iswidely used, due to its simplicity and popularity.

To cater for a wide range of compressive stressconditions, number of empirical strength criteriahave been introduced for practical use and one ofthe most widely used criteria is the Hoek-Brown(H-B) criterion for isotropic rock materials and rockmasses. The classic Hoek–Brown failure criterion,empirical in its formulation and based onnumerous experimental data, has been widelyused to predict the failure of rocks.

34

Chapter-2

Fig. 2.22: Generalized H-B Criterion

The generalized Hoek-Brown criterion is non-linear(parabolic) in form (Fig. 2.22) and is given by thefollowing equation:

σ 1 = σ 3 + (mb σ 3 σ ci + s σ ci2)a

Where,

σ 1 = Major Principal Stress

σ 3 = Minor principal Stress

σ ci = Compressive strength of rock material

mb, s & a = Constants depending on type/conditionof rock mass

Hock-Brown criterion for rock material is a specialform of the generalised equation when s =1,a = 0.5, mb= mi, thereby giving the followingequation:

σ 1 = σ 3 + (mi σ 3 σ ci + σ ci2)0.5

Values of constants mb, s & a are calculated usingequations given by Hoek and Brown with GSI(Geological Strength Index) and Disturbance Factor(D). Hoek and Brown have given detailedmethodology for estimating GSI (discussed indetail in Chapter-3) and D.

35


Using suitable software also (e.g. RocLab), valuesof these constants can be easily estimated withresults of tri-axial shear test (number of σ 1 andσ 3 values) as the input.

Typical values of mb and s, depending upon thequality of rock mass are given in Table 2.04(values of RMR and Q are discussed in Chapter-3).

As shown in Fig. 2.23, exact type of failure envelopedepends on the spacing of the discontinuities in therock mas vis-à-vis the size of the openings orexcavation (i.e. size affect).

Fig. 2.23: Types of H-B Failure Envelopes

36

Chapter-2

Tab

le 2

.04

: T

yp

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es

of

mb a

nd

s

37

ROCK MASS CLASSIFICATION SYSTEMS

Rock mass classification systems form the back boneof the empirical design approach and are widelyemployed in rock engineering. The rock massclassifications have been quite popular and being usedin feasibility designs. It has been experienced repeatedlythat when used correctly, a rock mass classification canbe a powerful tool in designs. In fact, on many projects,the classification approach serves as the only practicalbasis for the design of complex underground structures.The Gjovik Underground Ice Hockey Stadium of 60mwidth in Norway was also designed by the classificationapproach. (Ref. 4: Bhawani Singh & R. K. Goel).

Rock mass classifications systems have improvised withtime, to take into account various developments ingeological investigations as well as undergroundexcavations support systems (like rock bolts, shotcrete,steel fiber reinforced shotcrete etc.) and they have beenwidely used due to following reasons:

(i) It provides a common acceptable platform togeologist, designers, contractors and engineers, inunderstanding and expressing quality of rock for theintended purpose.

(ii) Rather than expressing the rock quality in subjectiveterms, it expresses rock quality in terms of a numericalvalue, thereby enabling better understanding of rockquality without any element of subjectivity.

In all rock mass classification systems, minimum ratingis assigned to poorest rock mass and maximum ratingis assigned to excellent rock mass. It must be notedthat no single classification system may cater to alltypes of rock masses from poorest to excellent.Therefore, it requires experience to select proper

CHAPTER-3

ROCK MASS CLASSIFICATIONSYSTEMS

38

Chapter-3

classification system for the given type of rock mass. Asound engineering judgment emerges out of workingin the field for a long time and no formula or ratingsystem can be a substitute for this.

Precaution should be taken that joint parameters arenot accounted twice, once for analysis and once forclassification. Any isolated variation or aberration inrock parameters should not be accounted, onceallowance has been given for such uncertainties. It isbetter to give a range of rating for each parameter. Inlinear classification systems like RMR and GSI, averageof rock mass rating is taken for design of supports,whereas in non-linear classification systems like Q thegeometrical mean of maximum and minimum valuesare considered for the design.

Empirical, numerical/analytical and observationalapproaches are various tools for engineering design oftunnel supports. The empirical approach, based on rockmass classification systems, is very popular because ofsimplicity and ability to manage uncertainties. However,in present day, a mix of all approaches is employed and“Design as you go” approach is adopted. It should benoted that Rock mass classification based designprocedures have inherent limitations and their use doesnot (and cannot) replace more elaborate designprocedures. The approach generally adopted is asunder:(i) In feasibility studies, empirical correlations maybe used to estimate rock parameters.(ii) At design stage, in-situ tests should beconducted for the major projects to determineactual rock parameters as well in-situ stresses.(iii) At the initial construction stage,instrumentation should be carried out on thesupported excavated surface and within the rockmass, for getting field data about actualdisplacements/stresses etc. and comparing themwith predicted values. Instrumentation is very

39


critical for a safe and steady tunneling rate. Theinstrumentation data should be utilized incomputer modelling for back analysis of the modeland its’ improvisation for further tunneling.(iv) At construction stage, forward analysis of rockstructures should be carried out using the backanalyzed model and parameters of rock mass.Repeated cycles of back analysis and forwardanalysis (BAFA) may eliminate many inherentuncertainties in geological mapping and knowledgeof engineering behaviour of rock masses.(v) In case of non-homogenous and complexgeological environment, slightly conservativevalues of rock parameters may be used for thepurpose of designing site specific remedialmeasures.(vi) Be prepared for the worst and hope for thebest. (Ref. 4: Bhawani Singh & R. K. Goel).

Some of the important Rock Mass Classification Systemsare discussed as under:

1. Rock Quality Designation (RQD): The RQD wasinitially proposed by Deere (1963) as an index ofassessing rock quality quantitatively and it has sincethen been the topic of various assessments for civilengineering projects. It has become a fundamentalparameter or property of rock mass, mainly due to itssimple definition.

RQD (%) = xi / L x 100

Where:

xi = Lengths of individual pieces of core 10 cm(obtained using NX-Size Core 54.7mm Dia.,with double tube core barrel using a diamondbit).

L = Total length of the drill run

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Chapter-3

Fig. 3.01: Rock fragments obtained in drill run

RQD represents degree of fracturing of rock mass. Itpartially reflects the in-situ rock mass quality. It is adirectionally dependant parameter and is used as acomponent in most of the “Rock Mass ClassificationSystems”.

Calculation of RQD is explained by an example, asunder:

41


Table 3.01: Rockmass Quality based on RQD

Description RQDA. Very Poor < 25B. poor 25 - 50C. Fair 50 - 75D. Good 75 - 90E. Excellent 90 - 100

2. Terzaghi’s Rock Mass Classification: This was oneof the earliest attempts (in 1946) to classify rock massfor engineering purpose. Terzaghi classified rock massesinto 9 categories, based on structural discontinuities,as detailed in Table 3.02.

Table 3.02: Terzaghi Rock Mass Classification

Rock Rock DescriptionClass Condition

I Hard and intact The rock is un-weathered. Itcontains neither joints nor haircracks. If fractured, it breaksacross intact rock. Afterexcavation, the rock may havesome popping and spallingfailures from roof. At highstresses, spontaneous andviolent spalling of rock slabsmay occur from the side or theroof. The unconfinedcompressive strength is equalto or more than 100 MPa.

II Hard stratified The rock is hard and layered.and schistose The layers are usually widely

separated. The rock may ormay not have planes ofweakness. In such rocks,spalling is quite common.

42

Chapter-3

III Massive, A jointed rock, the joints aremoderately widely spaced. The joints mayjointed or may not

be cemented. It may alsocontain hair cracks but thehuge blocks between the jointsare intimately interlocked sothat vertical walls do notrequire lateral support.Spalling may occur.

IV Moderately Joints are less spaced. Blocksblocky and are about 1m in size. The rockseamy may or may not be hard. The

joints may or may not behealed but the interlocking isso intimate that no sidepressure is exerted orexpected.

V Very blocky Closely spaced joints. Blockand seamy size is less than 1m. It consists

of almost chemically intactrock fragments which areentirely separated from eachother and imperfectlyinterlocked. Some sidepressure of low magnitude isexpected. Vertical walls mayrequire supports.

VI Completely Comprises chemically intactcrushed but rock having the character of achemically crusher-run aggregate. Thereintact is no interlocking.

Considerable side pressure isexpected on tunnel supports.The block size could be fewcentimeters to 30cm.

43


VII Squeezing rock Squeezing is a mechanical– Moderate process in which the rockdepth advances into the tunnel

opening without perceptibleincrease in volume. Moderatedepth is a relative term andcould be from 150 to 1000m.

VIII Squeezing rock The depth may be more than– Great depth 150m. The maximum recom-

mended tunnel depth is1000m.

IX Swelling rock Swelling is associated withvolume change and is due tochemical change of the rock,usually in presence of moistureor water. Some shales absorbmoisture from air and swell.Rocks containing swellingminerals such asmontmori l lonite, il l i te,kaolinite and others can swelland exert heavy pressure onrock supports.

3. Rock Mass Rating (RMR): The geo-mechanicsclassification or Rock mass rating (RMR) was developedby Bieniawski in 1973 in South Africa (Ref. 49,Bieniawski Z. T.) with significant evolutions later on,last being in 1984 in USA. To apply this system, thesite should be divided into a number of geologicalstructural units having their boundaries usuallycoinciding with a major structural feature such as afault or a change in rock type. Each unit should then beclassified separately. In some cases, within the samerock type, division of the rock mass into a number ofsmall structural units may be required due to significantchanges in discontinuity spacing or characteristics. Eachtype of rock mass shall be represented by a separate

44

Chapter-3

geological structural unit. Following six parameters areconsidered for each of the geological structural unit:

(i) Strength of Intact Rock Material(ii) Rock Quality Designation (RQD)(iii) Spacing of discontinuities(iv) Condition of discontinuities(v) Groundwater conditions(vi) Orientation of discontinuities

For each geological structural unit, ratings are assignedfor first five parameters, as detailed in Appendix 3.1.After summing up the ratings for these five parameters,a correction is applied for the sixth parameter (i.e.orientation of discontinuity vis-à-vis direction ofexcavation) as detailed in Appendix 3.1. The resultantrating, so obtained, is RMR.

While assigning the ratings, the typical rather than theworst conditions are evaluated, since this classificationbeing based on case histories, has a built-in safety factor.

This classification is more applicable to hard rocksituations and not found to be very reliable in very poorrock mass.

Based on RMR value, the rock mass quality is describedas given in Table-3 of Appendix 3.1.

4. Rock Mass Quality (Q): Barton, Lien and Lunde(1974) of the Norwegian Geotechnical Institute (NGI),on the basis of about 200 case histories of tunnels andcaverns, proposed a Tunnelling Quality Index (Q) (Ref.16, Barton N., Lien R. and Lunde J.). The numericalvalue of Q varies on a logarithmic scale from 0.001 to amaximum of 1000, but its value typically varies from0.01 to 100. The Rock Mass Quality (Q) is defined as:

SRFJ

JJ

JRQD w

a

r

n

Q

45


Where:

RQD = Rock Quality Designation 10

= 115 – 3.3 JV 100Jn = Joint Set NumberJr = Joint Roughness Number for critically

oriented joint setJa = Joint Alteration Number for critically

orientedjoint set

Jw = Joint Water Reduction FactorSRF = Stress Reduction Factor to consider

in-situstressesJv = Joint Volume Count

Numerical ratings are assigned to above six parameters,based on rock conditions. The ratings do not includejoint orientation to make the classification more generaland orientation of joints/discontinuities is consideredby taking lowest value of second quotient for computingQ for Most unfavourable joint.

The parameter Jn, representing the number of joint sets,is often affected by foliations, schistocity, slaty cleavagesor beddings etc. If strongly developed, these paralleldiscontinuities should be counted as a complete jointset. If there are few joints visible or only occasionalbreaks in rock core due to these features, then oneshould count them as “a random joint set” whileevaluating Jn.

The parameters Jr and Ja, represent roughness anddegree of alteration of joint walls or filling materials.These parameters should be obtained for the weakestcritical joint set or clay-filled discontinuity in a givenzone. If the joint set or the discontinuity with theminimum value of (Jr/Ja) is favorably oriented forstability, then a second less favorably oriented joint setor discontinuity may be of greater significance, and itsvalue (Jr / Ja) should be used when evaluating Q.

46

Chapter-3

The parameter Jw is a measure of water pressure, whichhas an adverse effect on the shear strength of joints.This is due to reduction in the effective normal stressacross joints. Water in addition may cause softeningand possible wash-out in the case of clay-filled joints.The value of Jw should correspond to the future groundwater condition where seepage erosion or leaching ofchemical can alter permeability of rock masssignificantly.

The parameter SRF is a measure of (i) looseningpressure in the case of an excavation through shearzones and clay bearing rock masses, (ii) rock stressσ c/σ 1 in a competent rock mass, where σ c is uniaxialcompressive strength of rock material and σ 1 is majorprincipal stress before excavation, and (iii) squeezingor swelling pressures in incompetent rock masses. SRFcan also be regarded as a total stress parameter. Someof the ratings for SRF have been altered by Grimstadand Barton (1993) (Ref. 50).

Tables for ratings for various parameters, includingalterations by Grimstad and Barton (1993), are givenin Appendix 3.2.

Table 3.03: Rock Mass Classification v/s Q

Group Q Classification

1 1000-400 Exceptionally good400-100 Extremely good100-40 Very good40-10 Good

2 10-4 Fair4-1 Poor

1-0.1 Very poor3 0.1-0.01 Extremely poor

0.01-0.001 Exceptionally poor

Many empirical correlations have been developedbetween RMR and Q, as given in the Table 3.04 below:

47


Table 3.04: Empirical correlations

Correlation Reference

RMR = 9.0 ln Q + 44 Bieniaswki (1976),Jethwa et al. (1982)

RMR= 5.9 ln Q + 43 Rutledge & Preston (1978)

RMR = 5.4 ln Q + 55 Moreno (1980)

RMR = 4.6 ln Q + 56 Cameron-Clarke and(drill core) Budavari (1981)

RMR= 5.0 ln Q + 61(In-situ results)

RMR = 10.5 ln Q + 42 Abad et al. (1984)

RMR = 8.7 ln Q + 38 Kaiser et al. (1986)

RMR = 9.1 ln Q + 45 Trunk & Homisch (1990)RMR= 7.0 ln Q +41 El-Naqa (1994)

(Bore cores)

RMR = 7.0 ln Q + 44(Scan lines)

RMR = 15 ln Q + 50 Barton (1995)

RMR and Q system or their variants are the most widelyused rock mass classification systems. Both incorporategeological, geometric and design/engineeringparameters to obtain a “value” of rock mass quality.But, they are empirical and require subjectiveassessment.

5. Geological Strength Index (GSI): Hoek and brown(1980) proposed a method for estimating the strengthof jointed rock masses, based upon assessment ofinterlocking of rock blocks and condition of the surfacesbetween these blocks. This method was modified overthe years and it was eventually extended recently forheterogeneous rock masses (Marinos and Hoek, 2000)in the form of evaluating a parameter, GeologicalStrength Index (GSI) (Ref. 51, Marinos, Marinos andHoek).

48

Chapter-3

The basic input required are uniaxial compressivestrength (σ ci) and a material constant (mi) that is relatedto the frictional properties of the rock. Ideally, thesebasic properties should be determined by laboratorytesting as described by Hoek and Brown (1997) but, inmany cases, the information is required beforelaboratory tests have been completed. To meet thisneed, tables that can be used to estimate values forthese parameters are reproduced in Table 3.05.

Table 3.05: Estimation of GSI

For better quality rock masses (GSI>25), the value ofGSI can be estimated directly from the 1976 version ofBieniawski’s Rock Mass Rating, with the Groundwaterrating set to 10 (dry) and the Adjustment for JointOrientation set to 0 (very favourable) (Bieniawski 1976),as under:

4

1iiR10GSI

For very poor quality rock masses, the value of RMR isvery difficult to estimate and the balance between the

49


ratings no longer gives a reliable basis for estimatingrock mass strength. Consequently, Bieniawski’s RMRclassification should not be used for estimating the GSIvalues for poor quality rock masses.

If 1989 version of Bieniawski’s RMR classification is used, thenGSI=(RMR89 – 5) where RMR89 has the Groundwater rating setto 15 and the Adjustment for Joint Orientation set to zero.

GSI can also be estimated using Q system, as under:

44JJ

JRQDlog9GSI

a

r

n

GSI can be used to describe rock quality as given inTable 3.06

Table 3.06: GSI and Rock Mass Quality

GSI Value 76-95 56-75 41-55 21-40 <20

Rock Mass VeryGood Fair Poor

VeryQuality good poor

6. Tunnelman’s Ground Classification for Soils:Anticipated ground behavior in soft ground tunnels wasfirst defined by Terzaghi (1950) by means of theTunnelman’s Ground Classification, a classificationsystem of the reaction of soil to tunneling operation.Heuer (1974) modified the Tunnelman’s roundClassification, as shown in Table 3.07 below:

Heading can beadvanced withoutinitial support, andfinal lining can beconstructed beforeground starts tomove.

Loess above watertable; hard clay,marl, cementedsand and gravelwhen not highlyoverstressed.

Table 3.07: Tunnelman’s Ground Classification

Classification Behavior Typical Soil Types

Firm

50

Chapter-3


Chunks or flakes ofmaterial begin todrop out of the archor walls sometimesafter the ground hasbeen exposed, dueto loosening or over-stress and “brittle”fracture (groundseparates or breaksalong distinctsurfaces, opposed tosqueezing ground).

Residual soils orsand with smallamounts of bindermay be fast ravelingbelow the watertable, slow ravelingabove. Stiff fissuredclays may be slowor fast ravelingdepending upondegree ofoverstress.

Raveling Slowraveling

Fastraveling

In fast ravelingground, the processstarts within a fewminutes, otherwisethe ground is slowraveling.

Ground squeezes orextrudes plasticallyinto tunnel, withoutvisible fracturing orloss of continuity,and withoutperceptible increasein water content.Ductile, plastic yieldand flow due tooverstress.

Ground with lowfrictional strength.Rate of squeezedepends on degreeof overstress.Occurs at shallow tomedium depth inclay of very soft tom e d i u mconsistency. Stiff tohard clay underhigh cover maymove incombination ofraveling at excav-ation surface andsqueezing at depthbehind surface.

Squeezing

Granular materialswithout cohesion areunstable at a slopegreater than their

Clean, dry granularmaterials. Apparentcohesion in moistsand, or weak

Running Cohesiverunning

51



Flowing A mixture of soil andwater flows into thetunnel like a viscousfluid. The materialcan enter the tunnelfrom the invert aswell as from theface, crown, andwalls, and can flowfor great distances,com-pletely fillingthe tunnel in somecases.

Below the watertable in silt, sand orgravel withoutenough claycontent to givesignificant cohesionand plasticity. Mayalso occur in highlysensitive clay whensuch material isdisturbed.

angle of repose(approx. 30°-35°).When exposed atsteeper slopes, theyrun like granulatedsugar or dune sanduntil the slopeflattens to the angleof repose.

cementation in anygranular soil, mayallow the material tostand for a briefperiod of ravelingbefore it breaksdown and runs.Such behavior iscohesive-running.

Swelling Ground absorbswater, increases involume and expandsslowly into thetunnel.

Highly pre-consolidated claywith plasticity indexin excess of about30, generally conta-ining significantper-centages ofmont-morillonite.

Anticipated ground behavior has been further expandedby various researchers/authors for various soilconditions (clays to silty sands, cohesive soils, siltysands, sands, gravels) above/below the water table.

52

Chapter-3

7. Examples of estimating RMR, Q and GSI7.1 Example - 1(A) Input values• Granite rock mass,• Contains 3 joint sets,• Average RQD is 88%,• Average joint spacing is 0.24m,• Joint surfaces are generally stepped and

rough, tightly closed and un-weathered withoccasional stains observed,

• The excavation surface is wet but notdripping,

• Average rock material uniaxial compressivestrength is 160 MPa, and

• The tunnel is excavated 150m below theground where no abnormal high in situ stressis expected.

(B) Calculation of RMR (Ref. Appendix-3.1)Sl. No. Parameter Rating

(i) Rock Material Strength = 160 MPa 12

(ii) RQD (%) = 88% 17

(iii) Joint Spacing (m) = 0.24 m 10

(iv) Condition of Joints: Very Rough, 30un-weathered and no separation

(v) Ground Water: Wet 7

Total = RMR = 76

Rock Mass Class - II : Good

(C) Calculation of Q (Ref. Appendix-3.2)Sl. No. Parameter Value of

(i) RQD (%) = 88 -

(ii) Joint Set Number – 3 sets Jn = 9

(iii) Joint Roughness Number –

Roughly stepped (undulating) Jr = 3

53


(iv) Joint Alteration Number – Unaltered,

Some Stains Ja = 1

(v) Joint Water Factor – Wet only

(Dry excavation or Minor flow) Jw = 1

(vi) Stress Reduction Factor:

σ c /σ 1 = 160/(150x0.027) = 39.5 SRF = 1

Q = (88/9) * (3/1) * (1/1) = 29

Rock Mass Class : Good (Ref. Table-3.03)

(D) Calculation of GSIRock Mass Structure: BlockyJoint Surface Condition: Very goodGSI = 75 + 5 (Ref. Table-3.05)Rock Mass Quality : Good (Ref. Table-3.06)

7.2 Example - 2(A) Input values• A sandstone rock mass,• Fractured by 2 joint sets plus random

fractures,• Average RQD is 70%,• Average joint spacing is 0.11m,• Joint surfaces are slightly rough, highly

weathered with stains but no clay found onsurface,

• Joints are generally in contact withapertures generally less than 1mm,

• Average rock material uniaxial compressivestrength is 85 MPa,

• Tunnel is to be excavated at 80m belowground level, and

• The groundwater table is 10m below theground surface.

54

Chapter-3



(ii) RQD (%) = 70% 13


(iv) Condition of Joints: Slightly Rough,

Highly weathered and Separation <1mm 20

(v) Ground Water: Water Pressure/Stress = 0.32 4

Total = RMR = 49

Rock Mass Class - III : Fair

(C) Calculation of Q (Ref. Appendix-3.2) Sl. No. Parameter Value of

(i) RQD (%) = 70 -

(ii) Joint Set Number – 2 sets plus random Jn = 6

(iii) Joint Roughness Number – Slightly

rough (Rough Planar) Jr = 1.5

(iv) Joint Alteration Number – HighlyWeathered, Only Stain (altered non-softened mineral coating) Ja = 2

(v) Joint Water Factor – 70m Water Head= 70 kg/m2 = 700 kPa Jw = 0.5

(vi) Stress Reduction Factor:

c / 1 = 85/(80x0.027) = 39.3 SRF = 1

Q = (70/6) * (1.5/2) * (0.5/1) = 4.4Rock Mass Class : Fair (Ref. Table-3.03)

(D) Calculation of GSIRock Mass Structure: BlockyJoint Surface Condition: Poor (Highly Weathered)GSI = 45 + 5 (Ref. Table-3.05)Rock Mass Quality : Fair (Ref. Table-3.06)

55


7.3 Example - 3(A) Input values• A highly fractured siltstone rock mass,• Has 2 joint sets and many random fractures• Average RQD is 41%,• Average joint spacing 5cm,• Joints appears continuous,• Joint surfaces are slickensided & undulating,

and are highly weathered,• Joints are separated by about 3-5mm,• Filled with clay,• Average rock material uniaxial compressive

strength is 65MPa,• Inflow per 10m tunnel length is approximately

50 litre/minute, with considerable outwash ofjoint fillings, and

• Tunnel is at 220m below ground.



(ii) RQD (%) = 41% 8


(iv) Condition of Joints: Continuous,

Slickensided, Separation 1-5 mm 10

(v) Ground Water: Inflow = 50 lit/min 4

Total = RMR = 34

Rock Mass Class - IV : Poor

(C) Calculation of Q (Ref. Appendix-3.2)Sl. No. Parameter Value of

(i) RQD (%) = 41 -

(ii) Joint Set Number – 2 sets plus random Jn = 6

56

Chapter-3

(iii) Joint Roughness Number – Slightlyrough (Rough Planar) Jr = 1.5

(iv) Joint Alteration Number – Slickensided,Undulating Ja = 2

(v) Joint Water Factor: Large inflow withconsiderable outwash Jw = 0.33

(vi) Stress Reduction Factor:σ c/σ 1 = 65/(220x0.027) = 11 SRF = 1

Q = (41/6) * (1.5/4) * (0.33/1) = 0.85

Rock Mass Class : Very Poor (Quite near to Poor)

(Ref. Table-3.03)

(D) Calculation of GSIRock Mass Structure: BlockyJoint Surface Condition: Very PoorGSI = 35 + 5 (Ref. Table-3.05)Rock Mass Quality : Poor (Ref. Table-3.06)

7.4 Comparison of Rock Class/Quality, asestimated from different classifications systems, issummarized in Table 3.08; which shows that therock class/quality obtained by using threedifferent/independent classification systems isalmost same.

Table 3.08: Summary of Rock Class/Quality

8. Correlation between RMR, Q and GSI:Researchers have developed many empirical correlationbetween these values, as given under and as shown inFig. 3.02:

57


RMR = 9 ln Q + (44+18)RMR = 13.5 log Q + 43

GSI = RMR – 5 (for GSI > 25)

9. Squeezing behaviour of Rock Mass: Squeezingis time dependent large deformation, which occursaround an opening, and is essentially associated withcreep caused by stress exceeding shear strength. Thedegree of squeezing is classified as under:

(i) Mild squeezing: Closure 1-3% of opening D(ii) Moderate squeezing: Closure 3-5% of D(iii) High squeezing: Closure > 5% of D

Fig. 3.02: Correlation between RMR & Q

Rate of squeezing is time and stress dependent. Usuallythe rate is high at initial stage, say, several cm/day,and it reduces with time. Squeezing may continue for along period.

Squeezing may occur at shallow depths in weak andpoor rock masses. Poor rock masses with moderatestrength at great depth may also suffer from squeezing.

58

Chapter-3

Based on the analysis of several case studies in variousparts of the world, Barton et al. (1974) and Singh et al.(1992) have established that it is possible to predictthe squeezing condition or otherwise, in anyunderground excavation, based on the value of “Q” and“Height of Overburden”. The dividing line between thesqueezing and non-squeezing line is given by theequation, as shown in Fig. 3.03 also:

H = 350 Q1/3

Where:

H = Overburden (in m)Q = Rock Mass Quality

Fig. 3.03: Boundary line for Squeezing

10. Empirical relations for rock mass strengths: Itis difficult to determine the strength (UCS, Strengthparameters) of jointed rock masses in the laboratoryas the samples need be undisturbed and sufficientlylarge to be representative of the discontinuity conditions.

59


To estimate strength of rock masses, empiricalcorrelations developed by various researchers areusually used, some of which are as under:

10.1 Compressive Strength of Rock Mass

σ cm / σ c = e [(RMR-100)/24]

σ cm / σ c = e [(RMR-100)/18]

σ cm = 5 γ (Q σ c / 100)1/3

Where:

σ cm = Compressive Strength of rock mass

σ c = Compressive Strength of rockmaterial

γ = Unit weight of rock mass (g/cc)

10.2 Tensile Strength of Rock MassBy Singh & Goel:

σ tm = 0.029 γ fc Q0.3

Where:

fc = σ c/100 for Q>10 and σ c > 100 Mpa

Otherwise fc = 1

γ = Unit weight of rock mass in g/cm3.

By Hoek & Brown:

σ tm = 0.5 σ c [mb - 4sm2b ]

Where: mb & s are material constants.

60

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61


62

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63


64

Chapter-3

65


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Chapter-3

67


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blank

69

GEOTECHNICAL INVESTIGATIONS

Successful planning, design, construction andmaintenance of a tunnel requires various geotechnicalinvestigations to obtain a broad spectrum of pertinenttopographic, geologic, geo-hydrological and structuralinformation and data.

Exploration programs for tunneling must be plannedby construction engineers in close cooperation withengineering geologists, geotechnical engineers &designers.

1. Phases of Geotechnical Investigation Program:The geotechnical investigation is generally carried outin following phases to obtain the information necessaryat each stage of the project:

1.1 Preliminary Geotechnical investigationsfor Feasibility Studies: In this phase, theemphasis is on defining the regional geology andthe basic issues of design and construction.Investigations at this stage are largely confined to:

(A)Collection, organization & study of availabledata: Available information should be collected andreviewed to identify existing conditions and featuresthat may impact the design and construction of theproposed tunnel, and can guide in planning the scopeand details of the subsurface investigation programto address these issues. Some possible sources ofavailable information are:• Topographical maps of Survey of India• Geological maps and reports of GSI• Geological maps and reports from agencies other

than GSI• Geotechnical investigation reports from other

agencies, working in that area

CHAPTER-4


70

Chapter-4

• Case histories of other underground works inthe region

• Details of land ownership (Government, Private,Forest), access routes, hydrological informationand environmental sensitivity from State Govt./Local bodies

• Satellite images and aerial photographs• Seismic Records• Records of landslides (caused by earthquakes or

other reasons), documented by the other agencies:can be useful to avoid locating tunnel portals andshafts at these potentially unstable areas.

(B) Preliminary Survey: The information studiesshould be followed by a preliminary survey. Initialon-site studies should start with a carefulreconnaissance over the tunnel alignment, payingparticular attention to the potential portal and shaftlocations. Features identified on maps and aerialphotographs should be verified. Rock outcrops, oftenexposed in highway and railway cuts, provide asource of information about rock mass fracturingand bedding and the location of rock typeboundaries, faults, and other geologic features.Features identified during the site reconnaissanceshould be photographed and documented.

The reconnaissance should cover the immediateproject vicinity, as well as a larger regional area sothat regional geologic, hydrologic and seismicinfluences can be accounted for. A preliminaryhorizontal and vertical control survey may berequired to obtain general site data for routeselection and for design. This survey should beexpanded from existing records, alignment posts andbenchmarks that are based on the same horizontaland vertical datum that will be used for final designof the structures. Additional alignment posts andbenchmarks can be established, as needed, tosupport field investigations and mapping.

71


(C) Conducting investigations to comparealternative alignments and for arriving at aconceptual preliminary design: Carrying outfollowing investigations at this stage would help incomparing alternative alignments and for arrivingat a conceptual preliminary design:(i) Preliminary Geological field mapping with

particular attention to features that couldsignify difficulties like slides (particularly inportal areas), major faults, thrusts etc. Themapping should identify major componentsof the stratigraphy and the geologic structure,which form the framework for zonation of thealignment and for the planning of theexplorations.

(ii) Selected exploratory borings in criticallocations

(iii) Geophysical explorations: Geophysicalmethods of exploration are often useful at theearlier stages of a project because they arerelatively inexpensive and can cover relativelylarge volumes of geologic material in a shorttime. The most commonly used techniquesare Seismic Refraction Survey and ElectricResistivity Survey.

(a) Seismic Refraction Survey

In seismic refraction survey, pulses of lowfrequency seismic energy are emitted by aseismic source such as a hammer-plate orweight-drop (Fig. 4.01). The type of sourceis dependent on local ground conditions andrequired depth of penetration. Explosivesare best for deeper applications but areconstrained by environmental regulations.

72

Chapter-4

Fig. 4.01: Seismic Refraction Survey

The seismic waves propagate downwardthrough the ground until they arereflected or refracted off subsurfacelayers. Refracted waves are detected byarrays of geophones spaced at regularintervals of 1-10m, depending on thedesired depth of penetration.

Geophones output data is time traceswhich are compiled and processed bythe seismograph. Interpretationtechniques are applied to the first arrivaltimes to calculate the seismic velocitiesof the layers and the depths ofindividual refracting interfaces. Theinterfaces are correlated with realphysical boundaries in the ground, suchas the soil-bedrock interface and otherlithological boundaries, to produce amodel of the subsurface groundstructure. The final interpretation ispresented in a format that is easilyunderstood by engineers (Fig. 4.02).

73


Fig. 4.02: Seismic Refraction Survey Output

(b) Electrical Resistivity Survey:Resistivity geophysical surveys measurevariations in the electrical resistivity ofthe ground, by applying small electriccurrents across arrays of groundelectrodes. The survey data is processedto produce graphic depth sections of thethickness and resistivity of subsurfaceelectrical layers (Fig. 4.03). Theresistivity sections are correlated withground interfaces such as soil and filllayers or soil-bedrock interfaces, toprovide engineers with detailedinformation on subsurface groundconditions.

Fig. 4.03: Electric Resistivity Survey Results

74

Chapter-4

(iv) Aerial Photography: To supplement existingdata.

(v) Hydrological Survey: To define the ground-water regime, aquifers, sources of water etc. Asa part of the hydro geological survey, all existingwater wells in the area should be located, theirhistory & condition assessed and groundwaterlevels taken. Mapping of permanent orephemeral streams and other water bodies andthe flows and levels in these bodies at varioustimes of the year is usually required.Additional hydro-geological work to be carriedout at a later stage includes measurements ofgroundwater levels or pressures in boreholes,permeability testing using packers in boreholesand sometimes pumping tests.

1.2 Preconstruction Planning and EngineeringPhase: More detailed geotechnical investigationshould be carried out in this phase to refine thetunnel alignment and profile once the generalcorridor is selected, and to provide the detailedinformation needed for design of tunnel & selectionof appropriate tunneling methodology.

As the final design progresses, additional geo-technical investigations might be required for fullercoverage of the final alignment and for selectedshaft and portal locations. Investigations carriedout during this phase are:

(A) Topographical Surveys: Detailed topographicmaps, plans and profiles should be developed toestablish primary control for final design andconstruction based on a high order horizontal andvertical control field survey.

Accurate topographic mapping is also required tosupport surface geology mapping and the layout ofexploratory borings. The principal survey techniquesinclude:• Conventional Survey

75


• Global Positioning System (GPS)• Electronic Distance Measuring (EDM) with

Total Station• Remote Sensing• Laser Scanning

(B) Subsurface Investigations: Subsurfaceinvestigation is the most important type ofinvestigations to obtain ground conditions, as it isthe principal means for:• Defining the subsurface profile (i.e. stratigraphy,

structure, and principal soil/rock types)• Determining soil/rock material properties and

mass characteristics• Identifying geological anomalies, fault zones and

other hazards (squeezing soils, etc.)• Defining hydro geological condit ions

(groundwater levels, aquifers, hydrostaticpressures, etc.)

• Identifying potential construction risksSubsurface investigations typically consist of:

(i) Borings to identify the subsurface str-atigraphy,and to obtain disturbed and undisturbed samplesfor visual classification and laboratory testing.The number, location, depth, sample types andsampling intervals for each test boring must beselected to match specific project requirements,topographic setting and anticipated geologicalconditions. When deciding boring locations,knowledge of the geology of the area is necessaryto determine fault locations.The layout of borings should take intoconsideration various factors like the structureof the material, stratigraphy, strike and dip ofthe rock, consistency of strata etc. For example,in steeply dipping rock, vertical borings from thesurface may be of little use. In such cases, it isimportant to angle the holes to cross the strata.

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Chapter-4

Table 4.01 presents general guidelines fromAASHTO (1988) for determining the spacing ofboreholes for tunnel projects:

Table 4.01: Guidelines for Borehole Spacing

Ground Conditions Typical Borehole Spacing(feet)

Cut-Cover Tunnels 100 to 300

Rock Tunneling

Adverse Conditions 50 to 200

Favourable Conditions 500 to 1000

Soft Ground Tunneling



Mixed Face Tunneling



The above guideline can be used as a startingpoint for determining the number and locationsof borings. However, for a long tunnel through amountainous area, it may not be economicallyfeasible or the time sufficient to perform boringsaccordingly. Therefore, engineering judgmentneed to be applied by experienced geotechnicalprofessionals to adapt the investigation program.For a tunnel having length of 1 km or more,borings may be initially spaced at about 100–300m (depending on anticipated geologicalcondition) along the tunnel alignment. If theborings or surface features indicate differencesin the material, then additional borings shouldbe done between the borings. This should bedone until the alignment between the boringscan be characterized with a high degree ofcertainty. In addition to the alignment, portallocations and shaft locations should bethoroughly investigated.

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In general, borings should extend to at least 1.5times tunnel diameter below the proposed tunnelinvert. However, if there is uncertainty regardingthe final profile of the tunnel, the borings shouldextend at least two or three times the tunneldiameter below the preliminary tunnel invertlevel.In some cases, especially for a long high covertunnel through a mountainous area, it may notbe economically feasible or the time sufficientto perform borings in accordance with aboveprovisions. In such cases recourse may have tobe taken for few borings in combination withadvance probe holes or pi lot tunnel.Notwithstanding above provisions, engineeringjudgment need to be applied in complexgeological and/or topographical conditions inconsultation with experienced geotechnicalprofessionals to plan and adapt suitableinvestigation program.Horizontal boreholes along tunnel alignmentsprovide a continuous record of ground conditionsand information which is directly relevant to thetunnel alignment. Although the horizontal drillingand coring cost per m may be much higher thanthe conventional vertical/inclined borings, ahorizontal borings can be more economical,especially for investigating a deep mountainousalignment, since one horizontal boring canreplace many deep vertical conventionalboreholes.A deep horizontal boring will need some distanceof inclined drilling through the overburden andupper materials to reach to the depth of thetunnel alignment. Typically the inclined sectionis stabilized using drilling fluid and casing andno samples are obtained.All borings should be properly sealed at thecompletion of the field exploration, if not

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intended to be used as monitoring wells.(ii) In situ testing to obtain useful engineering

and index properties by testing the material in-place to avoid the disturbance caused bysampling, transportation and handling ofsamples. In-situ tests can also aid in definingstratigraphy.The parameters to be tested would depend onthe nature of underground strata viz. Rock orSoil, and they should be finalized in advance inconsultation with design engineers. Appropriatetest methods should then be used to obtain thoseparameters with an acceptable degree of validity& reliability.

(iii) Geophysical investigations to quickly andeconomically obtain subsurface information over alarge area to help define stratigraphy and to identifyappropriate locations for performing borings.Geophysical method to be used should befinalized in advance in consultation with designengineer. Typical techniques of geophysical testsfor various geological conditions are tabulatedbelow:

Table 4.02: Geophysical Investigations

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The data from geophysical exploration mustalways be correlated with information from directmethods of exploration that allow visualexamination of the subsurface materials, directmeasurement of groundwater levels and testingof physical samples of soil and rock.Direct methods of exploration provide valuableinformation that can assist not only in theinterpretation of the geophysical data, but alsofor extrapolating the inferred ground conditionsto areas not investigated by borings. Conversely,the geophysical data can help determineappropriate locations for borings and test pitsto further investigate any anomalies that arefound.(iv) Laboratory testing provides a wide varietyof engineering properties and index propertiesfrom representative soil samples and rock coreretrieved from the borings.

Soil Testing: Testing is done on selectedrepresentative samples (disturbed andundisturbed) in accordance with relevantstandards. Table 4.03 shows common soillaboratory testing for tunnel design purposes.

Table 4.03: Common Lab Tests for Soil

Parameters to be tested should be finalized inadvance in consultation with designers.However, as a general guideline tests for Particle

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size distribution, Atterberg limits, moisturecontent, unit weight, unconfined compressivestrength and Tri-axial compressive strengthmust be carried out on the selectedrepresentative samples. In case undisturbedsamples are difficult to obtain, strengthproperties can alternatively be obtained fromempirical correlations using N-values fromstandard penetration tests. Consolidation testshould be carried out in case of clayey soils

Rock Testing: Table 4.04 summarizescommon laboratory testing for rocks, for tunneldesign purposes.

Table 4.04: Common Lab Tests for Rock

Parameters to be tested should be finalized inadvance in consultation with designers.However, as a general guideline tests forDensity, Porosity, Moisture Content, Point LoadIndex, Uniaxial compressive strength, Tri-axialcompressive strength, Tensile strength(Brazilian), Young’s modulus, Poisson’s ratio &Coefficient of permeability must be carried outon the selected representative samples. Whenthe rock contains clay minerals, then swellingindex should be determined. Hardness &Abrasivity tests would be additionally requiredin case mechanized tunneling is planned to beadopted.

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It is desirable to preserve the rock coresretrieved from the field properly until theconstruction is completed and disputes/claimsare settled. Photographs of rock cores (in coreboxes) should be kept for review by designers.

In addition to typical geotechnical, geological,and geo-hydrological data, subsurfaceinvestigation for a tunnel project must considerthe unique needs for different tunnelingmethods, i.e. cut-and-cover, drill and blast,NATM. Table 4.05 shows special considerationsfor various tunneling methods.

Table 4.05

Various geological conditions demand specialconsiderations for subsurface investigations assummarized in the Table 4.06.

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Table 4.06

(C) Detailed Geological Mapping: Detailedgeologic mapping should be carried out whichincludes mapping & plotting of joints, faults, andbedding planes. The geologist must then project thegeological conditions to the elevation of the proposedtunnel so that tunneling conditions can be assessed.Geologic mapping should characterize and documentthe condition of rock mass such as:• Discontinuity type• Discontinuity orientation• Discontinuity infilling• Discontinuity spacing

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• Discontinuity persistence• Weathering

In addition, following surface features should alsobe observed and documented during the geologicmapping program:• Slides, new or old, particularly in proposed portal

and shaft areas• Faults• Rock weathering• Sinkholes and karstic terrain• Groundwater springs

Fig. 4.04: Tunnel alignment zones

Based on detailed geological mapping, it should bepossible to divide the tunnel alignment into zonesof consistent rock mass condition (Fig. 4.04). Criteriafor zonation would be site specific, but factorsinvolving intact rock, rock mass and excavationsystem characteristics should be considered. Eachzone should be characterized in terms of averageexpected condition as well as extreme conditionslikely to be encountered.

(D) Groundwater investigation: Since gro-undwater is a critical factor for tunnels, specialattention must be given to defining the groundwater

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regime, aquifers, and sources of water, any perchedor artesian conditions, depth to groundwater, andthe permeability of the various materials that maybe encountered during tunneling.

Related considerations include the potential impactof groundwater lowering on settlement of overlyingand nearby structures, utilities and other facilities;other influences of dewatering on existing structures;pumping volumes during construction; the potentialimpact on water supply aquifers; and seepage intothe completed tunnel etc.

Groundwater investigations typically include mostor all of the following elements:• Observation of groundwater levels in boreholes• Assessment of soil moisture changes in the

boreholes• Installation of groundwater observation wells and

piezometers• Borehole permeability tests• Pumping tests

During subsurface investigation i.e. drilling andcoring, it is particularly important to note anddocument any groundwater related observationsmade during drilling or during interruptions to thework when the borehole has been left undisturbed.Even seemingly minor observations may have animportant influence on tunnel design and groundbehavior during construction.

Observation wells and piezometers should bemonitored periodically over a prolonged period oftime to provide information on seasonal variationsin groundwater levels. Monitoring duringconstruction provides important information on theinfluence of tunneling on groundwater levels,forming an essential component of constructioncontrol and any protection program for existingstructures and facilities.

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(E) Structure & Utility pre-construction survey:Structures located within the zone of potentialinfluence may experience disturbance due to soilmovement caused by tunnel excavation andconstruction in close proximity (e.g. cut-and-coverexcavation, shallow soft ground tunneling, etc.). Ifthe anticipated movement can induce potentialdamage to a structure, some protection measureswill be required, and a detailed preconstructionsurvey of the structure should be performed.Preconstruction survey should ascertain all pertinentfacts of pre-existing conditions, and identify featuresand locations for further monitoring.

The requirement for utility survey varies withtunneling methods and site conditions. Cut-and-cover tunnel and shallow soft ground tunnelconstructions, particularly in urban areas,extensively impacts overlying and adjacent utilities.Water, sewerage, storm water, electrical, telephone,fiber optic and other utility mains and distributionsystems may require excavation, re-routing,strengthening, reconstruction and/or temporarysupport, and may also require monitoring duringconstruction.

1.3 Geo-technical Investigations during Cons-truction phase: It sometimes becomes necessaryto perform additional subsurface investigations andground characterization during construction. Suchconstruction phase investigations serve a numberof important functions like:

• Verify initial ground support selection and fordesign/re-design.

• Documenting existing ground conditions forreference, in case of contractual claims.

• Assessing ground and groundwater conditionsahead of the advancing face, to reduce risksand improve the efficiency of tunnelingoperations. This enables forewarning of

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adverse tunneling conditions like potentialhigh water inflow, very poor ground etc.

• Verification of conditions assumed for finaltunnel lining design, including choice ofunlined tunnel.

• Mapping for the record, to aid in futureoperations, inspections, and maintenancework.

A typical construction phase investigation programwould include some or all of the following elements:

• Subsurface investigation (borings andgeophysical) from the ground surface.

• Additional groundwater observation wells and/or piezometers.

• Additional laboratory testing of soil and rocksamples.

• Geologic mapping of the exposed tunnel face:with due safety precautions.

• Geotechnical instrumentation.

• Probing in advance of the tunnel headingfrom the face of the tunnel: It typicallyconsists of drilling horizontally from thetunnel heading by percussion drilling orrotary drilling methods.

• Pilot Tunnels are small size tunnels (typicallyat least 2mx2m in size) that are occasionallyused for large size tunnels in complexgeological conditions.

Pilot tunnel may also be located adjacent to theproposed tunnel, using the pilot tunnel foremergency exit, tunnel drainage, tunnel ventilation,or other purposes for the completed project.

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Tunnel Seismic Prediction: Himalayas is the oneof the youngest geologies on earth having rockmass with fault zones, sheared zones, fracturedrock mass and being permeable and waterbearing. Even with the best of geotechnicalexploration, all these features cannot be capturedin advance. Therefore, geological prediction duringconstruction becomes inevitable in such cases.Tunnel Seismic Prediction (TSP) is one of suchmodern techniques, which has been used in manytunnels world over including tunnels in Himalayasregion. TSP can also explore water bearingformations in 3D image. It takes less time ascompared to probe drilling method. Once thegeological risk is identified and mapped properly,risk management becomes easy, cheaper andpredictable to large extent.

This is adopted as predictive method duringexcavation process, for both drill & blast and TBMtechniques, and no access face is required toperform the measurements. The TSP system is anunderground seismic reflection package comprisingmeasurement instrumentation and its owninterpretation software. By employing the principleof echo sounding, it serves to predict changes inrock physical properties ahead of and aroundspatially very restricted underground excavationssuch as tunnel tubes. TSP-3D is one such patentedtechnique developed by Amberg Technologies AGof Switzerland. In this method, acoustic signals areproduced by a series of 24 shots of usually 50 to100 grams of detonation cords aligned along onetunnel wall side and having additional shot linealong opposite tunnel wall side in case of morecomplex geology (Fig. 4.05). The 3 componentreceiver picks up the seismic signals which werebeing reflected from any kind of discontinuity inrock mass ahead (Fig. 4.06). The capability ofsystem to record full wave field of compressionaland shear wave in conjunction with analysis

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software enables determination of rock mechanicalproperties such as Poisson’s Ratio and Young’sModulus within the prediction area. The final 2Dand 3D result produced by the system softwarepresents boundary planes crossing the tunnel axiscoordinates ahead (Fig. 4.07).

Fig. 4.05: TSP Survey

Fig. 4.06: Principle of TSP

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Fig. 4.07: TSP Output

2. Assessing exploration requirements of aTunneling Project:

(1) Because of the complexities of geology andthe variety of functional demands, no two tunnelsare alike. It is therefore difficult to give hard andfast rules about the required intensity ofexplorations or the most appropriate types ofexploration. Nonetheless, following can help in theplanning of explorations:

(a) Plan explorations to define the best, worst,and average conditions for the constructionof the underground works; locate anddefine conditions that can pose hazards or

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great difficulty during construction.

(b) Use qualified geologists to produce themost accurate geologic interpretation so asto form a geological model that can beused as a framework to organize data andto extrapolate conditions to the locations ofthe underground structures.

(c) Determine and use the most cost-effectivemethods to discover the information sought.

(d) Anticipate methods of construction andobtaining data required to selectconstruction methods and estimate costs.

(e) Anticipate potential failure modes for thecompleted structures and required types ofanalysis, and obtain the necessary data toanalyze them (e.g., in situ stress, strength,and modulus data for numerical modeling).

(f) Drill at least one boring at each shaftlocation and at each portal.

(g) Special problems may require additionalexplorations.

(2) Frequently, even the most thoroughexplorations will not provide sufficient informationto anticipate all relevant design and constructionconditions. Here, the variation from point to pointmay be impossible to discover with any reasonableexploration efforts. In such instances, the designstrategy should deal with the average or mostcommonly occurring condition in a cost-effectivemanner and provide means and methods toovercome the worst anticipated condition,regardless of where it is encountered.

(3) The specific scope and extent of theinvestigation must be appropriate for the size ofthe project and the complexity of the existinggeologic conditions; must consider budgetary

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constraints; and must be consistent with the levelof risk considered acceptable.

(4) Since unanticipated ground conditions aremost often the reason for costly delays, claims anddisputes during tunnel construction, a project witha more thorough subsurface investigation programwould likely have fewer problems and lower finalcost.

3. Geotechnical investigation Program for tunnelsshould involve/include:

(a) Active consultation with experiencedgeotechnical engineers, geologists &designers.

(b) “What”, “Why”, “Where”, “How” & “Howmuch” for each Geotechnical parameter to betested/investigated.

(c) Phasing the investigations (Para-1 above).

(d) Keeping the investigation program andcontract flexible enough so as to enabletaking up of additional investigations as perunexpected requirements that emerge duringcourse of work.

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TUNNEL SURVEYING

Surveying plays an important role in construction oftunnels, right from the planning stage to the finalcompletion. Surveying techniques are applied in tunnelsfor:

(i) Proper initial planning.(ii) Integration of geo-technical and geographical

data with topographical mapping (and utilitymapping, if located in urban area).

(iii) Actual alignment and guidance of tunnel, aditand shaft construction.

1. Type of Surveys: Following surveys are required inany typical tunnel project:

1.1 Preliminary Surveys: Topographic maps,maintained by Survey of India (some of them indigital form), are generally sufficient for initialplanning. However, in most cases, supplementarydata are required, either due to inaccuracies inthe data available or due to changes in land useor topography. Therefore, a horizontal and verticalcontrol survey is required to obtain general sitedata for route selection and for design. Thissurvey should be expanded from existing recordsand monuments, that are based on the samehorizontal and vertical datum, that will be used forfinal design of the structures. Additional temporarymonuments and benchmarks are placed as neededto support field investigations, mapping,environmental studies and route selection.

Typically, reasonably detailed mapping in corridors100 to 1000m wide are required along allcontemplated alignments. This mapping should be

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sufficiently detailed to show natural and man-madeconstraints to the project. In urban areas,mapping of major utilities, that may affect theproject, must also be done.

When the project corridor has been defined, newaerial photographs should be obtained andphotogrammetric maps should be prepared tofacilitate/obtain data on portal design, access,drainage, depth of cover, geology, seismic historyetc.

1.1.1 Equipment and Techniques: Modern mappingequipment and techniques provide a wide range ofproducts and services to support planning anddesign, and ongoing construction management,including:

(a) Digital Ortho Mapping, wherein the aerialphotographic image is digitized in true plan positionand scale (Example: Fig. 5.01), and can be insertedinto the project Geographic Information System(GIS) or database.

Fig. 5.01: Digital Ortho Mapping

(b) Digital topographic mapping, whereincontours and planimetric features are directlydigitized during the map compilation process(Example: Fig. 5.02) and can be CAD-plotted and/or inserted into the project GIS.

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Fig. 5.02: Digital Topographic Mapping

(c) Software enabling manipulation of digital mapand survey data to extract profiles, cross sections,spot elevations, etc., and to superimpose this dataselectively with design, right of way, geologic,and other data sets that have been digitized intothe GIS/database.

1.2 Utility Surveys: Utility surveys are mainlyrequired for urban tunnels for information onpreliminary and final route selection and todetermine the type and extent of utilityprotection, relocation, reconstruction or monitoringneeded. Deep tunnel construction may not passthrough any utility systems, but vibration, blastingshock, and settlement may affect surface andunderground utilities in the project corridor. Cut-and-cover construction, particularly in urban areas,extensively affects overlying and adjacent utilities.The information from utility surveys supplementsexisting utility maps and records.

Utility surveys, like all other surveys on theproject, must be based on the primary horizontaland vertical control network, and must besufficiently accurate to ensure that all utilityfeatures are located within required tolerances.

1.2.1 Equipment and Techniques: Instruments andsystems available for locating utilities include:

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(a) Photogrammetric mapping: Routinely usedto document the location of pre-painted surfacefeatures such as manholes, valves, inlets,hydrants, etc. This is normally done during thephotogrammetric mapping phase of the surveywork.

(b) Magnetic surveys: Ferrous bodies such asiron and steel pipes, barrels, piles, etc., induceanomalies in the earth's magnetic f ield.Magnetometers detect the anomalies, whoseamplitude is a function of the ferrous mass andthe distance from the surface.

(c) Electromagnetic toning: A low-frequency ACcurrent is conducted into linear metal featuressuch as pipelines, cables, cable jackets etc., byconnecting an AC tone generator to an exposedsection of the feature. A handheld receiver detectsthe feature by electromagnetic signals whosemagnitudes are a function of the strength ofinduced AC current, distance between tonegenerator and mobile receiver, depth of cover overthe feature, electrical conductivity of the feature,and electrical insulation between the feature andits burial medium (earth, water). Operating ACelectrical cables may also be detected byelectromagnetic toning.

(d) Ground Penetrating Radar (GPR): A portableinstrument that emits radar frequency signalsvertically downward and plots energy pulsesreflected by buried objects.

1.3 Primary Survey Network: Primary surveys arethe basic positional reference for the project.These surveys must be founded on stable andaccessible monuments, and they must beconducted to a high degree of accuracy to meetproject needs. The survey work, computations,adjustment, and data recording must be accurateand reliable so that design and construction can

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proceed with absolute confidence in the credibilityof the survey data.

1.3.1 Survey Control: Primary horizontal surveysare conducted using Triangulation, ElectronicDistance Meter (EDM) traverse, Global PositioningSystem (GPS) surveys or a combination of thesemethods. The GPS is helpful in providing precisereferences at low cost over long distances. Whenused in differential mode in establishing controlnetworks, GPS gives relative positioning accuraciesas good as two ppm. GPS is also flexible, becauseline-of-sight is not required between points.

After completion of route selection, a horizontal andvertical survey of high accuracy is conducted, withpermanent monuments installed near portals, adits,and other selected locations in the project corridor.Design and execution of the survey must be donewith the objective of establishing a singular andauthoritative survey system that is based onsecurely founded monuments and meets the accuracystandards required for the project. All subsequentsurveys and construction work must be based solelyon the control survey network, and the project plansand specifications should contain specific statementsaffirming this.

(a) Electronic Distance Measuring: Modern EDMinstruments (TotalStation) combine accuratemeasurement of angles and distances, computerprocessing of data and storage of observed angle& distance data. Range of distance measurementdepends upon type of EDM used, number ofreflective prisms and clarity of the air. Typicalrange is 2000-3000m, with some specializedinstruments ranging in excess of 7000m. Standarddeviation of angles and distance measurementsvary with the various models and makes of EDMsavailable. EDMs with data collectors can downloadsurvey data for processing and plotting usingspecialized "field-to-finish" software.

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Theodolites, TotalStation and EDMs cannot beadjusted or calibrated in the field. This work mustbe done in a competent service facility asrecommended by the OEM. Level instruments,however, require regular testing to assure thatthe horizontal crosshair defines a true level plane.

(b) Global Positioning System (GPS):Coordinate positioning of widely spaced controlmonuments is usually accomplished by GPSsurveys, which utilize the signal transit time fromground station to satellites to determine therelative position of monuments in a control network.The accuracy of measurement is dependent uponthe number of satellites observed, configuration ofthe satellite group observed, elapsed time ofobservation, quality of transmission, type of GPSreceiver, and other factors including network designand techniques used to process data. GPS surveyingrequires the simultaneous operation of severalreceiving instruments located at different stationsthroughout the survey network, and the successof an observing session depends upon eachinstrument being in place and operating at apredetermined time. This requires detailed advanceplanning.

Although GPS surveying is now increasinglybecoming common, high-order GPS surveys entailextremely sophisticated procedures for both fieldand office work. Accordingly, the work should beplanned and executed under the direction of aqualified GPS specialist with strong credentials inthe application of advanced geodesy to designand construction.

2. Surveying steps in alignment control of Tunnels:Setting out centre line of tunnel at exact location andelevation is done in following steps:

(i) Establishment of temporary benchmarks andalignment posts, as required for work, and

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ensuring their stability. In case stability is indanger or they have been disturbed, promptmeasures shall be taken to transfer & re-establish these temporary benchmarks andalignment posts.

(ii) Surface Survey (setting out tunnel on groundsurface).

(iii) Transferring the alignment underground(transfer of centre line from surface tounderground).

(iv) Underground setting out, taking care toeliminate cumulative error.

(v) Transferring levels underground (undergroundleveling), taking care to eliminate cumulativeerror.

Modern Tunnel Boring Machines are normally equippedwith semi-automated or fully automated guidanceinstrumentation that offers good advance rates withgreat precision.

3. Recommendations for framing contractdocuments: Except in rare instances, the contractorshould be entrusted with responsibilities for all surveying,including control of line and grade and layout of allfacilities and structures. Railway officials must conductverification surveys at regular intervals and also ensurethat the work is properly tied to adjacent existing ornew construction. Contract documents should be framedto clearly stipulate the responsibilities of Railways &Contractor. Also, the contract documents must containall reference material necessary to conduct surveyingcontrol during construction including specificationsstating the accuracy requirements and the requiredquality control, quality assurance and surveyorqualification requirements. Minimum requirements to thetypes and general stability of construction benchmarksand alignment posts should also be stated.

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CHOICE OF TUNNEL SYSTEM, ALIGNMENT AND SHAPE/SIZE OF CROSS-SECTION

The tunnel system, alignment of tunnel and shape/sizeof the tunnel cross-section are decided in early planningstage, based on the requisite geological/geotechnicalexploration and a thorough understanding of the ground.

1. Choice of Tunnel System: The tunnel systemcomprises all underground works that are necessary toachieve the planned use of tunnel and ensure safety ofpersons & material assets. Besides main tunnel(s), thetunnel system may comprise of cross-passages, aditsand shafts as escape routes or other ancillary structuressuch as ventilation shafts or caverns for technicalequipment. The choice of the tunnel system is basedmainly on operational, organizational and safetyconsiderations. The ground conditions and thetopography may also have an influence on the selectionof the tunnel system.

Following factors should be considered for decidingprovision of Escape Distances, Lateral Exits/Access andParallel Service & Safety Tunnel:

1.1 Escape Distance: To reach a safe place inthe event of fire is the central aspect of allrescue concepts including escape distances.Additionaly, arrangements used for providingshorter escape distances can also be used bymaintenance staff for performance of their work.Maximum distance between two safe places(portal, emergency exit or cross passage) in thetunnel is defined in order to enable self rescue.UIC standard specifies it to be not more than1000m as a general guideline, i.e. mean escapedistance of 500m for self rescue. For double-bore

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CHOICE OF TUNNEL SYSTEM,ALIGNMENT AND SHAPE/SIZE OF

CROSS-SECTION

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single-track tubes and parallel safety tunnels, thedistance between safe places can be reduced to500m. This distance can vary depending on thelocal conditions, operating parameters and totalsafety concept. Maximum escape distance shouldbe decided after considering following factors:• Cost-effectiveness.• Expected situation in the tunnel: smoke

spread etc.• Topography, particularly availability of

opportunities like construction shafts/adits ora place very close to the surface.

• Local situation (including tunnel length, dailytraffic, rescue concept, availability of paralleltunnel etc.).

• Security & maintainability.

1.2 Lateral Exits/Access: Provision of Lateralexits / access in tunnels offers following adva-ntages:• Ensures escape to a safe place in the event

of fire and smoke.• Leads directly out of tunnel or to a safe

place.• Access for emergency services.• Can also be used for maintenance purposes.

The disadvantage of providing Lateral exits/accessis possibility of their misuse by miscreants forsabotage purpose and additional cost ofmaintenance.

A decision on providing Lateral exits/access shouldbe taken based on following:(i) Opportunities like Construction adits or a

place close to surface may be utilized forconstruction of lateral exits/access withnecessary precautions for their maintenance/security.

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(ii) Relevant factors including cost effectiveness,daily traffic, tunnel length, rescue concept,local situation, security and maintainability.

In case Lateral exits are provided, cross sectionof 2.25mx2.25m may be adopted as a guidelineupto a length of about 150m. For longer lengths,the exits should be accessible with road vehicles.Lateral exits should have:• Design or installation that prevents smoke

from spreading into the safe place.• Adequate lighting and communication means.• Design or installation for preventing

unauthorized access from outside.

1.3 Parallel Service & Safety Tunnel: Provision ofParallel service & safety tunnel offers followingadvantages:• Provides a safe place in the event of any

accident.• Possibility of reducing the escape distance in

the main tunnel with cross passages.• Independent access for emergency services

and possibility of arriving close to place ofaccident.

• Can be used for maintenance purposes also.

Disadvantages of providing parallel service &safety tunnel are:• Passengers are not yet outside the tunnel.• Possibility of use by miscreants for sabotage

purpose.

A decision on providing parallel service & safetytunnel should take following aspects intoconsideration:(i) It is not recommended as a general solution.

A decision in this regard should be madeafter evaluation of the optimal system basedon consideration of relevant factors like cost

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effectiveness, daily traffic, tunnel length,rescue concept, geo-technical factors,security and maintainability etc.

(ii) In case an exploratory tunnel has beenplanned to be constructed, the possibility ofusing it as a service and safety tunnel maybe considered, if it is cost effective.

(iii) It may cover only parts of the tunnel length(in combination with shafts or adits)

In case parallel service & safety tunnel is provided,cross section of 3.5m x 3.5m may be adopted as aguideline. It should have independent ventilation systemto keep it free of smoke. It should be accessible byroad vehicles, with facility to reverse and pass. It shouldbe connected to main tunnel through appropriatelydesigned cross passages of size approximately 2.25m x2.25m spaced at about 500m. These cross passagesshould have:

• Design or installation that prevents smokefrom spreading into the safe place.

• Adequate lighting and communication means.• Design or installation for preventing

unauthorized access from outside.

2. Choice of Alignment: The vertical and horizontalalignment of the tunnel(s) depends on several factorssuch as:

2.1 Maximum ruling gradient: Ruling gradient intunnels is normally kept flatter than that in openair, owing to:(i) Reduced rail-wheel adhesion due to presence

of moisture in tunnels. This causes decreasein the traction force in tunnels.

(ii) Increased air resistance: The magnitude ofair resistance depends on the relativevelocities of wind & train, as well as on therelative cross-section areas of tunnel andtrain. Resistance is especially large in single

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track tunnels which are comparativelynarrow.

(iii) Decrease in efficiency of internal combustionengines, used in diesel traction, due topresence of less oxygen inside tunnels.

Therefore, maximum grades in straight tunnelshould preferably not exceed 75% of the rulinggradient of the track outside tunnel. Grades incurved tunnels should be compensated forcurvature in the same manner as for sectionsoutside the tunnel.

2.2 Permitted maximum degree of curve.

2.3 Drainage considerations during constru-ctionand operation.

2.4 The accessibility of and natural hazards in theportal areas.

2.5 The ground conditions.

If possible, the alignment should be adapted tothe ground conditions in an early phase of theproject, as hazards and the respectiveconstruction time and cost risks can be avoided orreduced by the choice of a different alignment.

Aspects of execution or operation and safety(such as the necessity of intermediate adits,ventilation shafts or escape adits) may alsoinfluence the choice of the alignment, especially inlong tunnels.

3. Shape and Dimensions of the Cross-section: Theyare determined essentially by:

3.1 Serviceability requirements: The requireddimensional/ clearance profile is a key factor indetermination of cross section of tunnel. Otherserviceability criteria relevant for choice of crosssection are:

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(a) Additional space requirements for operatingand safety equipment (cable installations,signaling systems, signage, lighting,ventilation, etc.).

(b) Aerodynamic requirements, for the givenspeed and dimension profile of the vehicleswhich will use the tunnel.

(c) Drainage requirements.(d) Maintenance requirements(e) Requirements arising from the safety and

rescue concepts (escape routes within thetunnel, availability of the facilities inemergencies)

3.2 Geological/geotechnical conditions: Theshape and the size of the cross section alsodepend on the ground conditions, as the laterdetermine the extent of required support measuresin the construction stage (tunnel support) and inthe service stage (permanent lining).Unacceptable reduction in size of the opening dueto ground convergence is avoided by means ofadditional excavation to account for grounddeformations and corresponding support measures.

3.3 Construction aspects:(a) Economic considerations and availability of

the necessary equipment may be decisive forthe construction method and have, therefore,a considerable influence on the shape of thecross section.

(b) In determination of shape and dimensions ofthe cross section, attention must be given totolerances with respect to driving accuracy,construction tolerances and surveyingtolerances.

(c) In contrast to TBM or shield tunneling, thecross section of tunnels excavated byconventional methods can be freely chosen

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within the constraints of the geologicalconditions.

4. Shapes of Railway Tunnels: Following shapes arecommonly used in Railway tunnels:

4.1 D Shaped: This shape (Fig. 6.01) is commonlyseen in railway tunnels constructed upto 15-20years back. From structural/load distribution pintof view, this shape in not efficient. Therefore, itis suitable for “Hard Rock” only.

Most of the railway tunnels constructed earlierwere of “D” shape, and mostly unlined, becausethey were located in central part of the India,passing through competent rocks. In Udhampur –Katra section of Indian Railways, one of thetunnels (T-1) collapsed after being constructedfully and one of the reasons pointed out by aninternational consultant for this collapse was “Oneof the main problem lies within the geometricalcross section of the tunnel. A horizontal side wallas executed basically only allows for transfer ofvertical stresses. In case horizontal loads areoccurring, the system is likely to fail as horizontalloads can only be coped with to a very limitedextent”.

Fig. 6.01: D-Shaped Tunnel

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Structural analysis of a typical railway tunnel, withD shape and Elliptical shape, has given followingresults:

The tabulation above clearly shows the structuralinefficiency of D-shaped vis-a-vis elliptical shapedtunnels, especially when side pressure/horizontal loadsare considered. Now-a-days this shape is very rarelyused.

4.2 Horseshoe Shaped: The horse-shoe shapeconsists of a series of arcs, with no sharp corner(Fig. 6.02). There are horse-shoe shaped andmodified horse-shoe shaped cross sections in use,most of them comprising of semi-circular roofalong with arched sides and curved invert.

Fig. 6.02: Horse Shoe Shaped Tunnel

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Fig. 6.03: Horse Shoe Fig. 6.04: ModifiedShape Horse Shoe Shape

This cross-section offers a good resistance toexternal ground pressure and is suitable for softrocks. This shape is most common now-a-days intunnel constructed with “drill and blast technique”.

4.3 Elliptical/Circular Shape: These shapes arestrong in offering resistance to external pressurecaused by water, water bearing soils or softgrounds. This is the best theoretical section forresisting internal or external forces and it providesthe greatest cross sectional area for the leastperimeter. They are best suited shapes for tunnelsin “soft grounds”. The circular section is oftenuneconomical as more filling will be required forobtaining flat base. However, the tunnelsconstructed by TBM are only circular in shape.

Fig. 6.05: Elliptical Fig. 6.06: CircularShape Shape

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Weak rock zones, squeezing or swelling rock andsoft ground (soils) require a circular cross sectionor at least a horseshoe-shaped cross sectionincluding an invert arch.

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TUNNEL DESIGN AND TUNNEL SUPPORTS

Understanding, designing and constructing anyunderground excavation (e.g. Tunnel) is conceptuallyquite different and complex as compared to other normalsurface constructions (e.g. Building, Bridge etc.). Innormal surface constructions, the loads on the structurecan be estimated with reasonable accuracy (e.g. DeadLoad, Live Load, Wind Load, Earthquake Load etc.) andthe materials (e.g. Steel, Concrete, Timber etc.) ofknown strength/deformation properties are assembledin a known fashion in such a way that the strength/deformation capability of the construction materials/structure are lesser (with certain margin of safety,known as factor of safety) than the stresses/deformations caused by the imposed loads. But in caseof underground excavations, it is neither possible topredict the loads, the way it is done in surfaceconstructions, nor the strength/deformation capabilityof the construction materials/structure can be defineddue to the ground around excavation (rock mass) beinga highly non-homogeneous and anisotropic matter.

1. Concept of Stabilization of a Cavity: Forunderstanding the behaviour of undergroundexcavation, it is necessary to understand this concept,which is many times called as “Arch effect” also.

To understand this, take easily understood example offlow lines/water current in a river, which get deviated/disturbed due to construction of a pier in the waterstream (Fig. 7.01). There is turbulence/increase in speedof water flow around the pier for some extent andbeyond that there is no change in flow lines/watercurrent. Similarly, in case of a rock mass, the

CHAPTER-7

TUNNEL DESIGN AND TUNNELSUPPORTS

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equilibrium state of ground stresses is disturbed due toexcavation of an underground cavity like tunnel (Fig.7.02).

Fig. 7.01 Fig. 7.02

The stresses are channeled/re-distributed around thecavity, creating a zone of higher stresses around theexcavation boundary. This causes convergence of thecavity and some extent of rock mass just behind theexcavation boundary getting badly affected/cracked(and may be going into plastic state), some rock massfurther inwards in the ground getting moderatly affected(may be in elastoplastic state) and the remaining rockmass not getting affected at all (remaining in elasticstate). This re-distribution of stresses is called “archeffect”. The magnitude of convergence and the extentof various states of rock mass, which is a “reaction” ofthe ground to the disturbance caused in it, dependsupon the properties of “medium” (i.e. the ground inwhich cavity is made) and the “action” (i.e. shape/sizeof tunnel, method of tunnel excavation and tunneladvance rate).

Depending on various combinations of “medium” and“action”, the “reaction” can be broadly classified in twocategories. In the first case, the ground around thecavity well withstands the re-distributed stresses withthe convergence being limited (within permissible limits,

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depending on the type of excavation) and the extent ofplastic and elastoplastic zone not being excessive. Inthis case, the excavation will become self-stabilizing ina reasonable time and no support may be required. Insecond case, either the convergence may be progressingtowards unacceptable limits (worst case being collapseof the cavity) or the extent of plastic & elastoplasticzone not getting stabilized (or becoming very high,which in-turn contributes to instability of the cavity)i.e. “arch action” not getting established. In this case,some “intervention” is required from within the cavity,to limit the convergence to permissible limits and alsoto limit extent of plastic/elastoplastic zones (i.e. archeffect getting formed). Such type of “intervention" iscalled “tunnel support” and design of tunnel isessentially design of “tunnel support system”.

Another peculiarity of underground excavations is thatthe stage at which the structure is subjected to mostload is not when the tunnel is finished but it is theintermediate construction stage. This is very delicatesituation because the effects of disturbance caused byexcavation have not yet been completely countered bythe final tunnel support system. Thus, the purpose of“tunnel supports” is to:

(i) Preserve the integrity of rock mass.(ii) Provide safe and secure work area.(iii) Maintain an opening in the rock mass.

Initial or primary ground support is installed shortlyafter excavation, to make the tunnel opening safe untilpermanent support is installed. The initial groundsupport may also function as permanent ground supportor as a part of the permanent ground support system.Initial ground support systems are usually not subjectto rigorous design but are selected on the basis of avariety of rules (viz. Empirical, Analytical & Numerical),and one or more of these approaches can be used.

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2. Properties of Supports: Important properties ofsupports (including their material) are as under:

2.1 Stiffness: The correlation between variousparameters, related to stiffness of supports, can beexplained as under:

Table: 7.01

The vertical and horizontal load on supports isimportant to determine the section and type of thesupport. But since this cannot be predictedaccurately, the supports may have to be designedin such a way that they are suitable for manyscenarios of loading. If a stiff support is installedinitially, it prevents redistribution of stresses inrock mass and therefore, such supports areadvisable to be installed only after stressdistribution has largely taken place (i.e. as finallining). Some examples of locations, where highlystiff supports are provided, are:(i) In cut and cover tunnels.(ii) In or near portal zones, where earthquake

forces are taken into account for design ofsupports.

(iii) Tunnels in heavily faulted zones.(iv) Tunnels with very shallow cover.(v) Tunnels designed with very low deformation

and settlement.

Support systems, which are weak in bending, arenot stable on their own but require interaction withsurrounding rock mass. In such cases, their bondwith the rock mass is very critical.

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2.2 Bond: The bond between the support and rockmass ensures transfer of radial forces over thewhole area and continuous transfer of tangentialforces.

2.3 Time of installation: If the rock mass isunstable, temporary support should be providedduring or before the excavation. In a stable/competent rock mass, supports may not be neededalso. However, the support is required to beinstalled before the convergence of the cavity and/or extent of plastic zone in the rock mass aroundthe cavity becomes more than permissible limits.

3. Type of Tunnel Supports: Following type of tunnelsupports are commonly used:

3.1 Steel ribs/Steel sets: Such supports aremade using rolled steel sections (e.g. ISMB, ISHBetc.) which are bent to the shape of the tunnelcross section (Fig. 7.03). For the purpose ofhandling, the supports are made in two or threeparts and they are joined after erection in position.They are normally used with lagging (made ofprecast concrete, steel plates etc.) and backfill (oflean concrete or mortar or tunnel muck) (Fig.7.04).

Fig. 7.03: Fig. 7.04:Steel ribs/sets Lagging

Many types of yielding supports are also available,in which the support can change its’ length withina given limit, to account for some convergence in

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the cavity. Advantages of such supports are –possible to pre-fabricate and feasibility to installvertically/inclined according to the form of theface. Disadvantages of such supports are - difficultto handle, being very heavy, costly and poorflexibility.

Till few years back, use of steel ribs withbackfilling by lean concrete or tunnel muck wasmost common method of tunnel supports. But dueto their poor flexibility and uncertainties in lack ofinteraction with rock mass, they are very rarelybeing used as primary supports, except at thelocations warranting very rigid supports (listed inPara 2.1 above). In Udhampur – Katra section ofIndian Railways (Fig. 7.05 to Fig. 7.07), one of thetunnels (T-1) where such supports were installedand which collapsed after being constructed fully,one of the reasons pointed out by an internationalconsultant for this collapse was “Due to installationprocess of support (concreting and utilization ofsandbags for excess excavation) a complete soundcontact of primary support with the surroundingground is unlikely. Therefore, gravitational loadscaused by loosening of the rock mass due tocavities will occur which are loading the liningadditionally”.

Fig.7.05: Tunnel T-1 in Udhampur - Katra Section

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Fig. 7.06: Finished Fig. 7.07: TunnelTunnel Collapse

3.2 Lattice Girders: They are fabricated at site,to the required cross section of tunnel, by weldingof reinforcement bars (Fig. 7.08).

These types of supports are relatively light weight(hence economical), flexible and easy to handle.They are normally installed with shotcrete (Fig.7.09). Use of such supports is very common now-a-days and their flexibility is an added advantage,as will be discussed subsequently in “RockStructure Interaction”.

Fig. 7.08: Lattice Girder

3.3 Rock Bolts: Rock reinforcement in tunnels(of which rock bolt is one of the types) is used formany purposes. There are many types of rockbolts, with many of them being patented products,generally made of steel (mostly reinforcement barsof suitable diameter). A borehole of requireddiameter is drilled in the rock mass and the bolt is

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inserted in the borehole. The bolt is anchored nearthe tip using suitable mechanism, then it isstressed and the bore hole mouth is covered byusing a face plate and face nut. Afterwards,grouting is done (in most of the cases) to fill upthe annular space between the bolt and the wallsof the borehole.

Fig. 7.09: Installation of Lattice Girder

In case bore hole does not remain stable untilwithdrawal of drill rod and insertion of bolt, Self-Drilling Rock bolts (SDR) or Self-Drilling Anchors(SDA) are used, wherein the drill bit is located atthe end of bolt and borehole is drilled using thebolt and drill bit. In such a system, the drill bit isnot re-used and one drill bit gets scarified in eachborehole. SDAs or SDRs are costly as comparedto normal rock bolts, but their use is necessary incase of weak ground tunnelling. When therequired length of rock bolts becomes excessive(say more than 10m or so), rock anchors (whichare made of woven high tensile steel ropes) areused in pace of steel bolts.

Rock bolts serve many purposes in the tunnels.First purpose of rock bolts is to support individualrock block(s), which has become loose due tocreation of cavity and may have eventually fallen(Fig. 7.10). The rock bolt anchors/stitches the

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loose block(s) to the competent rock mass awayfrom the excavation boundary, thereby, providingstability to such potentially unstable blocks.

Fig. 7.10: Supporting individual blocks

Fig. 7.11: Building up Slab and column

Strength in bending or as a column, of a numberof layers joined together and working as one layeris multiple fold higher as compared to strengthwhen these layers act individually. Using thisprinciple, the rock bolts are used as a joining orstitching mechanism for various layers of rockmass, to increase their strength in bending as wellas a column (Fig. 7.11).

Use of rock bolts limits plastic zone in a localizedmanner (Fig. 7.12) as it helps in re-distribution ofstresses at that point and when used all aroundcavity they help in controlling yield (developmentof plastic zone around the periphery of theexcavation) by building a structurally active arch/

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ring of the reinforced zone around the periphery ofthe excavation (Fig. 7.13 & Fig. 7.14). After thecavity is created, the stress in rock mass is re-distributed. The tri-axial stress state changes tobi-axial state, since the radial stress is no longerpresent. The tangential stresses increase and canexceed the compressive strength of the rock, if therim is not reinforced. Due to formation of astructurally active ring of the rock, the stresses aretransferred to a competent/unaffected rock massat deeper depths. However, to achieve this, therock bolts/anchors length should be more than theextent of plastic/affected zone and they shouldalways be anchored in competent/unaffected rockmass.

Fig. 7.12: Limiting Plastic Zone locally

Fig. 7.13: Building up Arch/Ring andControlling Yielding

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When pattern bolting is installed quickly withspraying of shotcrete, this acts to provide supportresistance and a near tri-axial stress condition isrestored at the periphery of the excavation. Thisacts against further weakening of rock mass andprevents further build-up of plastic/disturbed zone.

The rock bolts should be positioned at an angle of450 to 900, but never less than 300, to the beddingplanes/joining surface. Rock bolts installed parallelto the joining surfaces can only bear little or nolongitudinal load and are of no use.

Owing to their ease of installation, fasterinstallation after excavation and flexibility providedby them in construction process; rock bolts havebecome essential part in all tunneling works. Theyare normally used to secure the profiled excavationboundary till installation of final lining. Butdepending on the type of rock mass and dimensionof the excavation, rock bolts can be used aspermanent support also.

The load bearing capacity of the rock bolt dependson its’ shaft and its’ head, and furthermore on theanchor force in the rock mass. The load bearingcapacity of the shaft is determined by its’permissible load, which is force at the yield limitdivided by a factor of safety, normally 1.5 to 2.

Common types of rock bolts are as under:

(A) Mechanical/Expanding/End Anchor bolts:In these bolts, an expansion mechanism is providedat the end of shaft, which expands when the shaftis rotated. Due to this expansion, the bolt end getsanchored with the wall of the bore hole (Fig. 7.14).Such kind of anchors are used mostly for temporarysupports only and are not very popular in tunnelling.

(B) Grouted/Friction Bolts: In these types of rockbolts, steel bar (mostly steel reinforcement bars ofrequired diameter) is used in the bore hole. If the

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anchor is to be installed as “unstressed”, the bolt issimply kept in the bore hole and then grouted oralternatively the borehole is filled with grout andthen bolt is pushed in it. If the anchor is to beinstalled as “stressed”, then the farthest end of boltis anchored in borehole using fast-setting mortar or

Fig. 7.14: Mechanical/Expanding Rock bolts

epoxy resin at the end portion (Fig. 7.15). Afterthis portion gets anchored, the bolt is tensioned,borehole is closed using face plate/face nut and thenthe annular space between the bolt shank andborehole boundary is filled by pumping cement groutthrough a hole in the face plate. Some anti-shrinkagent (e.g. aluminum powder) is added to cementgrout to prevent reduction in adhesion with rocksurface due to shrinkage during its’ drying. Thesebolts are most commonly used in tunnelconstructions.

(C) Friction Tube/Swellex Bolts: This is apatented product of Atlas-Copco, consisting of steeltube of diameter of 41mm, which is reduced to adiameter of 28mm by double folding (Fig. 7.16).The hollow folded tube, which is closed at both endswith a bush, is installed in drilled bore hole. Then itis hydraulically expanded by the water pressure of

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up to 300 bars, into the borehole periphery, toprovide the anchoring effect (Fig. 7.17). The

Fig. 7.15: Grouted/Friction bolt

anchoring action is provided by the friction betweenthe hydraulically expanded anchor and the rockmass. These anchors can be installed in a very shorttime, they immediately give full load bearingcapacity over the full length, are not sensitive tovibrations from blasting and have good adaptabilityfor various diameter holes. Thus, they are useful asimmediate supports after excavation, as part of initialsupport, especially in soft rocks/grounds.

Fig. 7.16: Swellex Bolts

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Fig. 7.17: Swellex Bolt Installation

(D) GRP Anchors: They consist of one or morebars of glass-fiber reinforced plastic (GRP),consisting of textile glass and polyester resin (Fig.7.18).

Fig. 7.18: Glass Fiber Tubes

This results in weight reduction of 60-70% comparedwith other steel bolts of same permissible load. Theyalso have High tensile strength, High elasticity, Lowweight, better corrosion resistance and betterelectrical insulation properties vis-a-vis conventionalsteel bolts. Use of such anchors, are commonly usedfor advance face support in “ADECO-RS Tunneling”methodology.

3.3.1 Reinforcement by Rock Bolts: The rockbolts reinforce the rock mass in following ways:

3.3.1.1 Reinforced beam: According to Lang(1961), due to tensile pre-tension stress of thebolt, compressive stress is developed in the rockmass , in the direction normal to bolt axis, dueto Poisson’s effect. This pre-stress can stabilizethe rock beam effectively as in the case of pre-stressed concrete beam.

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A two-dimensional photo-elastic study showedthat the pre-tension of bolts form a zone ofuniform compression between the ends of thebolts (Fig. 7.19). The only condition is that theratio between length (l) and spacing (s) of boltsis more than 2. At this ratio, the zone is relativelynarrow whereas for l/s equal to 3, it isapproximately equal to two-third of the boltlength (i.e. equal to l-s). The normal stress (σ v)within the zone may be estimated as ratio ofpre-tension to the area per bolt. The horizontalstress (σ h) equal to k0σ v would be inducedwithin this zone provided that the bolted beamis clamped laterally. The total horizontal forceis the sum of axial pre-stress (Ph) and the thrust(T) due to the arch action. Higher horizontal forcemeans greater frictional resistance to sliding ofthe beam downwards.

The photo-elastic model further indicated thatzones of tensile stresses develop between boltsand so it may require an additional support inthe form of wire-netting.

3.3.1.2 Reinforced rock arch: As can be seenfrom the Fig. 7.20 that radial bolting patterncreates a reinforced rock arch over the tunnels.The thickness of arch can be increased byemploying supplementary bolts of shorterlength. The most common practice is (Lang,1996; Barton et al., 1974):

(i) Rock bolts be pre-tensioned to giverequired ultimate support capacity (proof orpwall) which is equal to P/b.s where P = pre-tension, b = bolt spacing along tunnel axisand s = bolt spacing perpendicular to tunnelaxis. The pre-tensioned bolts are suitable fortemporary support of openings in the hardrocks.

(ii) Grouted bolt anchors should be designed

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to provide ultimate support pressure (P roofor P wall) equal to P/b.s where P is the tensilestrength of bolts, provided bolts areadequately grouted. The bolt length shouldbe greater than 4

1 to 31 of span of tunnel.

Fig. 7.19: Rock bolt – Photoelastic stress pattern

Fig. 7.20: Arch concept of rock reinforcement

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(iii) The l`ength of bolts (L in meters)should be calculated from the followingsimple relationship given by Barton et al.(1974),

L = 2 + (0.15 B/ESR) for roof= 2 + (0.15 H/ESR) for wall

Where : B = span or width of opening inmeters,

H = height of opening wall inmeters,

ESR= excavation support ratio(Table-7.02).

(iv) The adequate length of grouted anchorsbe obtained similarly as follows:

L = 0.40 B/ESR for roof

= 0.35 H/ESR for wall

Table 7.02: Values of ESR (Barton et al., 1974)

(v) When single (2-3 cm thick) or double (5cm thick) layers of shotcrete are appliedusually in combination with systematicbolting, the function of shotcrete is to prevent

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loosening, especially in the zone betweenbolts. The capacity of shotcrete lining is,therefore, neglected. The application ofshotcrete is essential to make grouted bolt-anchor system as permanent support.

(vi) Clear spacing between bolts should notbe more than three times the averagefracture spacing otherwise use wire mesh andguniting or shotcreting. Further center tocenter spacing must be less than one-half ofthe bolt length.

(vii) Bolts are installed on a selected patternexcept near weak zones that would requirespecial treatment. Spot bolting should bediscouraged.

(viii) Bolts should be oriented to make anangle of 0 to Ø to the normal on the criticaljoint sets in order to develop maximumresistance along joints (Fig. 7.21).

Fig. 7.21: Rock bolting with different dip angles

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(ix) Bolts must be installed as early aspossible within the “stand up time” and closeto the excavated face.

However, a tunnel is always unsupported ina certain length “t” between the last row ofbolting and the newly excavated/blastedface. According to Rabcewicz (1955), thezone of rock mass of thickness t/2 may befractured and loosened due to blasting asshown in Fig. 7.22. Thus, the bolt lengthmust be at least equal to the thickness ofloosened zone (t/2), so that the loose zonemay be suspended by competent rock mass.

Fig. 7.22: Principle of rock bolting

Rock bolts/anchors should be designed toabsorb high longitudinal strains in the casesof weak rock masses. So the bolts of hightensile strength are failure in caverns andtunnels in weak rocks under high tectonicstresses.

3.3.2 Rock bolting pattern: It is generallyagreed that pattern bolting should be preferredover spot bolting because unknown conditionsbehind the surface of an excavation may be morecritical than those visible at the surface. Inaddition, pattern bolting is advantageous fromconstruction point of view.

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3.3.3 Floor bolting: Floor bolting is required toprevent floor of a tunnel heaving in order tomaintain the track properly. Attempts to chop offsqueezed rock mass are fruitless and may damagethe wall support. However, there is no standardpractice.

3.4 Concrete Lining: The concrete lining can bepre-cast or cast in-situ. The lining can be in singlelayer or double layer. Even sprayed concrete(shotcrete), sprayed in one layer or multiplelayers, is also used as lining system. In case oftunnelling by Tunnel Boring Machine (TBM)normally pre-cast lining segments (Fig. 7.23) areused. They are cast at a casting yard, transportedto site and then placed in position in the form oflining rings, using erector arm in the TBM. In caseof tunnels excavated by drill and blast or roadheader method, normally one/multiple layer ofshotcrete or cast in-situ concrete lining is used.The in-situ concrete lining is done using a liningshutter (Fig. 7.24) of the shape of the tunnelprofile, which moves on the temporary trackprovided in the tunnel invert. After placing theshutter in the position, the concrete is pumpedinto the annular space between the shutter andthe tunnel periphery. After the concrete sets in,the shutter is moved forward and the process isrepeated till whole length of the tunnel is covered.

Fig. 7.23: Pre-cast Fig. 7.24: LiningLining Gantry

Segments for in-situ casting

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3.4.1 Cast in-situ Concrete Lining: These aregenerally installed sometime after the initial groundsupport. These can be used in both soft groundand hard rock tunnels and can be constructed ofeither reinforced or plain concrete. While the liningmay generally remain unreinforced, structuraldesign considerations and design criteria willdictate the need and amount of reinforcement.

To ensure contact between the initial and finallinings, contact grouting is performed as early asfinal linings have achieved its 28 days strength.This ensures contact between initial and final tunnelsupport and any deterioration or weakening ofinitial support will lead to an increased loading ofthe final support by the increment not beingsupported by initial lining.

Cast in-situ final lining pour length is normallyrestricted to limit surface cracking and ismandatorily followed in unreinforced lining.Adjacent concrete pours feature construction jointsand continuous reinforcement in joints is notdesired to allow relative movement.

Cast in-situ concrete lining is having followingadvantages:

• Suitable for use with any excavationand initial ground support method.

• Corrects irregularities in the excavation.• Can be constructed to any shape.• Provides a regular sound foundation for

tunnel finishes.• Provides a durable, low maintenance

structure.

Cast in-situ concrete lining is having followingdisadvantages:

• Concrete placement, especially aroundrein-forcement may be difficult. Thenature of the construction of the lining

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restricts the ability to vibrate theconcrete. This can result in incompleteconsolidation of the concrete around thereinforcing steel.

• Reinforcement is subject to corrosionresulting into deterioration of concrete.This is a problem common to allconcrete structures, howeverunderground structures can also besubjected to corrosive chemicals in thegroundwater that could potentiallyaccelerate the deterioration ofreinforcing steel.

• Cracking that allows water infiltrationcan reduce the life of the lining.

• Chemical attack in certain soils canreduce lining life.

• Construction requires a secondoperation after excavation to completethe lining.

In order to maximize flexibility and ductility, a castin-situ concrete lining should be as thin as possible.There are, however, practical limits on how thin asection can be placed and still obtain properconsolidation. Thickness of 25cm is consideredpractical minimum thickness for cast in-situconcrete lining.

Reinforcing a thin section can also be problematic.If two layers of reinforcement are used, thenstaggering the bars may be required to obtain therequired concrete cover over the bars. This canmake the formwork congested and concreteplacement more difficult.

Cast in-situ concrete is used as the final lining. Inmany cases a waterproofing system is placed overthe initial ground support prior to placing the finalconcrete lining. Placing reinforcing steel over thewaterproofing system increases the potential for

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damaging the water proofing. In all cases where itis practical, cast-in-place concrete linings shouldbe designed and constructed as plain concrete, withno reinforcing steel. The presence of thewaterproofing systems precludes load sharingbetween the final lining and the initial groundsupport. A basic design assumption is that thefinal lining carries long term earth loads with nocontribution from the initial ground support.

Ground water chemistry should be investigated toensure that chemical attack of the concrete willnot occur if the lining is exposed to ground water.If this is an issue on a project, mitigation measuresshould be put in place to mitigate the effects ofchemical attack. The waterproofing membrane canprovide some protection against this problem.Admixtures, sulphate resistant cement and highdensity concrete may all be potential solutions.This problem should be addressed on a case tocase basis and the appropriate solution beimplemented based on best industry practice.

Concrete behaviour in a fire event must also beconsidered. When heated to a high enoughtemperature, concrete will spall explosively. Thisproduces a hazardous condition for trains and foremergency response personnel responding to theincident. Spalling and loss of reinforcing strengthcan cause changes in the shape of the lining,redistribution of stresses in the lining and possiblystructural failure. The lining should be protectedagainst fire.

Mixes for cast in-situ concrete should have highenough slump to make placement practical. Airentrainment should be used. Compressive strengthshould be kept to a minimum. High strengthconcretes require complex mixes with multipleadmixtures and special placing and curingprocedures. Since concrete lining acts primarily incompression, 28 day compressive strengths in the

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range 25 to 30 MPa are generally adequate.

3.4.2 Precast Segmental Lining: Precastsegmental linings are used in circular tunnelsexcavated using Tunnel Boring Machine (TBM). Itcan be used in both soft and hard ground. Severalcurved precast elements or segments areassembled inside the tail of the TBM to form acomplete circle. The segments are relatively thin,20 to 30cm thick, and typically 1 to 1.5m widemeasured along the length of the tunnel.

Precast segmental linings can be used as initialground support followed by a cast in-situ concretelining (the “two-pass” system) or can serve as boththe initial ground support and final lining (the “one-pass” system) straight out of the tail of the TBM.Precast segmental linings, used as both initialsupport and final lining, are built to high tolerancesand quality. They are typically heavily reinforced,fitted with gaskets on all faces for waterproofingand bolted together to compress the gaskets afterthe ring is completed but prior to advancing theTBM. As the completed ring leaves the tail of theshield of the TBM, contact grouting is performedto fill the annular space that was occupied by theshield. This provides continuous contact betweenthe ring and the surrounding ground and preventsthe ring from dropping into the annular space.Bolt ing is often performed only in thecircumferential direction. The shove of the TBM isusually sufficient to compress the gaskets in thelongitudinal direction. Friction between the groundand the segments hold the segment in place andmaintain compression on the gasket. Segmentallinings were initially fabricated in a honeycombshape that allowed for bolting in both thelongitudinal and circumferential directions. Recentlining designs have eliminated the longitudinalbolting and the complex forming and reinforcingpatterns that were required to accommodate the

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longitudinal bolts. Segments now have a flat insidesurface. Once adequate strength is achieved, thesegments are inverted to the position they mustbe in for erection inside the tunnel. Horizontal andvertical tunnel alignment is achieved through theuse of tapered segments.

Precast segmental lining have following advantages:

• Provides complete stable ground support that isready for follow-on work.• Materials are easily transported and handledinside the tunnel.• No additional work, such as forming and curing,is required prior to use.• Provides a regular sound foundation for tunnelfinishes.• Provides a durable low maintenance structure.Precast segmental lining have fol lowingadvantages:• Segments must be fabricated to very tighttolerances.• Reinforcing steel must be fabricated and placedto very tight tolerances.• Storage space for segments is required at thejob site.• Segments can be damaged, if mishandled.• Spalls, cracked and damaged edges can resultfrom mishandling and over jacking.• Gasketed segments must be installed to hightolerances to assure that gaskets perform asdesigned.

Segments used as an initial support lining arefrequently designed as structural plain concrete.Reinforcing steel is placed in the segments to assistin resisting the handling and storage stressesimposed on the segments. Reinforcement is oftenwelded wire fabric or small reinforcing steel bars.

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Ends of the segments that form the joints shouldbe reinforced to facilitate the transfer of load fromone segment to another without cracking andspalling. The primary load carried by the precastsegments is axial load induced by ground forcesacting on the circumference of the ring. However,loads imposed during construction must also beaccounted for in the design. Loads from the jackingforces of the TBM are significant and can causesegments to damage and require replacement.These forces are unique to each tunnel and arefunction of the ground type and the operationalcharacteristics of the TBM. Reinforcement alongthe jacking edges of the segments is usuallyrequired to resist this force. The segments shouldbe checked for bearing, compression and buckling fromTBM thrust loads. Initial lining segments are consideredto be temporary support, therefore, long term durabilityis not considered in the design of the linings.

Segments used as final lining are designed asreinforced concrete. The reinforcement assists inresisting the loads and limits cracking in thesegment. Limiting cracking helps make thesegments waterproof. Final lining segments canbe fabricated with straight or skewed joints. Theorientation of the joint should be considered inthe design of the lining to account for themechanism of load transfer across the jointbetween segments. Skewed joints will inducestrong axial bending in the ring and this should beaccounted for in the design of the ring. Whetherusing straight or skewed joints, segments arerotated from ring to ring so that the joints do notline up along the longitudinal axis of the tunnel.Joints should be adequately reinforced to transferload across the joints without damage. The primaryload carried by the precast segments is axial loadinduced by ground, hydrostatic and other forcesacting on the circumference of the ring. Thepresence of the waterproofing systems precludes

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load sharing between the final lining and the initialground support. A basic design assumption is thatthe final lining carries long term earth loads withno contribution from the initial ground support.

Lifting and erecting the segments also imposeloads. The segments should be designed andreinforced to resist these loads. Grouting pressurecan also impose loads on the lining. Groutingpressures should be limited to reduce the possibilityof damage to the ring by these loads. Theanticipated grouting pressure should be added tothe load effects of the earliest ground loads appliedto the lining.

Concrete mixes for precast segments for initiallinings do not require special designs and cangenerally conform to the structural concrete mixesprovided in most standard constructionspecifications. Strengths in the range of 25 to 35MPa are generally adequate. Air entrainment isdesirable since segments may be stored outdoorsfor extended periods of time and final liningsegments may be exposed to freezingtemperatures inside the tunnel.

3.5 Shotcrete: It is also known as torcreteconcrete, sprayed concrete or gunite. Essentially itis a concrete mix which is sprayed withcompressed air, it hydrates on the substrate andhardens. It has become an essential part ofsupport in almost all tunneling projects.Admixtures can be used to improve the propertiesof strength and adhesion. Shotcrete providesimmediate support after excavation by filling smallopenings, cracks and fissures. This reducespotential relative movement of rock bodies or soilparticles and limits loosening of exposed groundsurrounding tunnel.

Adhesion of shotcrete depends on condition ofground surface, dampness/presence of water,

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composition of shotcrete and pressure of shotcrete.Generally rougher ground surface provides betteradhesion. Dry rock surface have to be adequatelydampened prior to application. Dusty or flakysurfaces, water inflow or water film on surfacereduce adhesion. Modern admixtures can improveadhesion of shotcrete significantly to reducerebound, however, these should be usedjudiciously to avoid its ill effect e.g. accumulationaround reinforcement bars, voids etc.

Owing to its’ flexibility, faster placement time andcontinuous contact with excavated surface, as ittakes the shape of excavated surface even after itsconvergence, it is very effective as a supportsystem in mobilizing rock support interaction. Inmost of the tunneling projects, a layer of shotcreteis sprayed immediately after excavation, which inaddition to providing initial support prevents andseals the cracks on the surface of the excavatedboundary and prevents further degradation of thenewly formed rock mass surface.

Fig. 7.25: Shotcrete Machine

Fig. 7.26: Shotcreting

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In dry shotcreting, the dry mix and water aremixed at the head of spraying nozzle and itrequires very high degree of skill by the operator.Dry shotcrete in not used very commonly now-a-days. In wet shotcrete, which is very commonlyused, ready mixed concrete is poured into theshotcrete machine (Fig. 7.25) and then it issprayed using manual nozzle or robotic arm (Fig.7.26). In a single layer, thickness of 100-400 mmcan be deposited, depending on the type of mixused, type of equipment used and the surface onwhich it is sprayed.

Shotcrete can be unreinforced or reinforced byputting a welded wire mesh in the shotcrete layer.In case, toughness and ductility are desirable,shotcrete reinforced with randomly oriented steelfibers, known as Steel Fiber Reinforced Shotcrete(SFRS), is used as alternative. The steel fiberscome in bundles which can be crumbled intoindividual pieces by hand (Fig. 7.27). The typicaldimensions of these fibers is 0.75 to 1.0mmdiameter and 60mm length. They are added to themixing drum of the shotcrete machine and theyget spayed with the concrete.

Occasionally, structural plastic fibers are used inlieu of steel fibers when shotcrete is expected toundergo high deformation and ductility post-cracking is of importance.

Fig. 7.27: Steel Fibers

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Sometimes shotcrete is also used as permanentlining, which is normally reinforced to provide longterm ductility.

3.5.1 Initial Shotcrete Lining: Initial shotcretelining typically consists of 100 to 400mm thick layermainly depending on the ground conditions and sizeof the tunnel opening, and provides support pressureto the ground. It is also referred to as shotcretelining. A shotcrete ring can carry significant groundloads although the shotcrete lining forms a ratherflexible support system. By deforming, it enablesthe inherent strength and self-supporting propertiesof the ground to be mobilized as well to share andre-distribute stresses between the lining and ground.From the ground support point of view the design ofthe shotcrete lining is governed by the supportrequirements, i .e., the amount of grounddeformations allowed and ground loads expectedas well as economical aspects. The earlier thesprayed concrete gains strength the more thesupport restrains ground deformation. However, byincreasing stiffness the support system increasinglyattracts loads. It depends on the ground conditionsand local requirements how stiff or flexible thesupport system has to be and thus what earlystrength requirements, thickness and reinforcementshould be specified.

In shallow tunnels and deformation sensitivestructures (e.g. buildings) being located on groundsurface, ground deformations and consequentlysurface settlements have to be kept withinacceptable limits. The advantage of the mobilizationof the self-supporting capacity of the ground can,therefore, be taken to a very limited extent. Here,early strength of the shotcrete is required to gainearly stiffness of the support to limit grounddeflections. Under these conditions, the shotcretelining takes on significant ground loads at an early

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stage. Early strength can be achieved withadmixtures and modern types of cement.

In contrast, for tunnels under high overburden, theprevention of ground deformation and surfacesettlement plays a secondary role. By allowing theground to deflect (without over-straining it) theground’s self-supporting capability, mainly shearstrength, is mobilized. Consequently, the groundloads acting upon the shotcrete lining can be limitedsignificantly because the ground assumes a part ofthe support function and a portion of the groundloads is dissipated before the initial support is loaded.For rock tunnels under high cover, early strength isnot a necessity but final strength of the entire system(including rock) is of importance.

Shotcrete support and rock reinforcement aredesigned to form an integrated support system inview of the excavation and support sequence. Thedesign engineer must define the requirements forthe support system based on thorough review ofthe ground response anticipated.

Friction between the ground and the sprayedconcrete lining (tangential subgrade reaction) isparamount for the support system. This frictionreduces differential movement of ground particlesat the ground surface and contributes to the ground-structure interaction.

An important aspect of shotcrete linings is the designand execution of construction joints. These jointsare located at the contact between shotcreteapplications in longitudinal and circumferentialdirections between the initial lining shells of theindividual excavation rounds and drifts. Anappropriate location and shape as well as connectionof the reinforcement through the longitudinal jointsis of utmost importance for the integrity and capacityof the support system. Longitudinal joints have tobe oriented radially, whereas circumferential joints

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should be kept as rough as possible. Splice bars/clips and sufficient lapping of reinforcement weldedwire fabric maintain the continuity of thereinforcement across the joints. Rebound, excesswater, dust or other foreign material must beremoved from shotcrete surface against which freshconcrete will be sprayed. The number of constructionjoints should be kept to a minimum.

In case of ground water ingress, the ground waterhas to be collected and drained away. Any build-upof groundwater pressure behind the shotcrete liningshould be avoided because increased ground waterpressure in joints and pores reduces the shearstrength in the ground, undue loads may be shedonto the shotcrete lining (unless it is designed forthat, which is unusual for initial shotcrete linings);softening of the ground behind the lining; increasedleaching of shotcrete and shotcrete shell detachmentfrom the ground.

3.5.2 Final Shotcrete Lining: Shotcrete as a finallining is typically utilized in combination with theinitial shotcrete supports in NATM applications whenthe following conditions are encountered:

• The tunnels are relatively short inlength and the cross section is relativelylarge and therefore investment informwork is not warranted, i.e. tunnelof less than 250m in length and largerthan about 12m in spring line diameter.

• The access is difficult and staging offormwork installation and concretedelivery is problematic.

• The tunnel geometry is complex andcustomized formwork would be required.Tunnel intersections, as well asbifurcations qualify in this area.

When shotcrete is utilized as a final lining in dualshotcrete lining applications, it will be applied against

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a waterproofing membrane. The lining thickness willbe generally 200 to 300mm or more and itsapplication must be carried out in layers with a timelag between layer applications to allow for shotcretesetting and hardening. To ensure a final lining tobehave close to monolithic from structural point ofview, it is important to limit the time lag betweenlayer applications and ensure that the shotcretesurface to which the next layer is applied is cleanand free of any dust or dirt films that could create ade-bonding feature between the individual layers.It is typical to limit the application time lag betweenthe layers to 24 hours. Shotcrete final linings areapplied onto a carrier system that is composed oflattice girders and welded wire fabric mounted tolattice girders toward the waterproofing membraneside. This carrier system also acts fully or partiallyas structural reinforcement of the finished lining.The remainder of the required structural reinforcingmay be accomplished by rebars or mats or by steelor plastic fibers.

Unlike the hydrostatic pressure of cast-in-placeconcrete during installation the shotcrete applicationdoes not develop pressures against thewaterproofing membrane and the initial lining andtherefore one must ensure that any gaps betweenwater proofing system and initial shotcrete liningand final shotcrete lining be filled with contact grout.To ensure a proper grouting around the entire liningcircumference it is customary to use longitudinalgrout hoses arranged radially around the perimeter.

General trends in tunneling indicate that theapplication of shotcrete for final linings presents aviable alternative to traditional cast-in-placeconcrete construction. The shotcrete lining systemfulf i l ls cast-in-place concrete structuralrequirements. Design and engineering, as well asapplication procedures, can be planned such as toprovide a high quality product. Excellence is needed

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in the application itself and must go hand-in-handwith quality assurance during application.

4. Design of tunnel lining: IS:15026-2002(Reaffirmed-2012) gives the formula for calculating thecapacity of shotcrete lining or Steel Fiber Reinforcedshotcrete (SFRS) lining. The load to be taken by theconcrete lining, after taking into account the load takenby the rock bolts or other supports, depends type ofrock mass as well type of support. Therefore, design oflining is done on case to case basis, by the designconsultant, based on the methodology being followedfor tunnel construction.

5. Need for Design Consultant: Reliable and sufficientknowledge of geological and geotechnical properties ofthe ground encountered during tunneling is essentialto assess the loads for which tunnel supports are to bedesigned. Hence detailed geological and geotechnicalinvestigation need to be carried in consultation withconstruction engineer and design engineers/consultant.For this, it is recommended that Detailed DesignConsultant (DDC) is engaged, to provide inputs ondesign and construction related issues in the beginningas well during execution of work.

6. Initial and Final Support System: Tunnel supportsystems mainly include initial ground support as wellas final/permanent support system.

Initial or Primary support is installed immediately afterexcavation, within the standup time, to make the tunnelopening safe until permanent support is installed. Theinitial support may also function as the permanent orfinal support or as a part of the permanent supportsystem.

Initial support system is usually not subject to rigorousdesign, but it is selected on the basis of one or more ofthree basic approaches (viz. Empirical, Analytical andNumerical) for support design.

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EMPIRICAL METHODS OF TUNNEL SUPPORT DESIGN

The Empirical Methods of tunnel support design haveevolved based on the extensive experience of variousresearchers from numerous tunneling projects. The maincharacteristic of these methods is breakdown of a widerange of rock mass quality by matching each of thesecategories with various support measures. Moreover,in these support measures, the stages of excavation,presence of underground water and stresses developedare also taken into account. Most commonly usedempirical approaches for design of initial support systemare covered as under:

1. Terzaghi’s Rock Load Factors Terzaghi (1946)defined rock load factor Hp as the height of looseningzone over tunnel roof which is likely to load the steelarches (Fig. 8.01).

C = B + Ht

Fig. 8.01: Terzaghi’s Rock Load Factor

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EMPIRICAL METHODS OF TUNNELSUPPORT DESIGN

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Terzaghi stipulated Rock Load Factor (Hp) based ontunnel width (B) and tunnel height (Ht) for each of thenine rock mass classes defined by Terzaghi (Ref.: Para-2 and Table-3.02 in Chapter-3) as given in Table 8.01below:

Table 8.01: Terzaghi’s Rock Load Factors

Note: The roof of the tunnel is assumed to be locatedbelow the water table. If it is locatedpermanently above the water table, the valuesgiven for types IV to VI can be reduced by50%.

(*) Some of the most common rock formations containlayers of shale. In un-weathered state real shalesare no worse than other stratified rocks. However,the term shale is often applied to firmly compactedclay sediments which have not yet acquired theproperties of rock. Such so called shale may behavein the tunnel like squeezing or even swelling rock. Ifa rock formation consists of a sequence of horizontallayers of sandstone or limestone and of immature

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shale, the excavation of the tunnel is commonlyassociated with a gradual compression of the rock onboth sides of the tunnel involving a downwardmovement of the roof. Furthermore the relatively lowresistance against slippage at the boundaries betweenthe so-called shale and rock is likely to reduce veryconsiderably the capacity of the rock located abovethe roof of bridge. Hence in such rock formations theroof pressure may be as heavy as in a very blockyand seamy rock.

Terzaghi’s rock load estimates were derived from anexperience of tunnels excavated by blasting methodsand supported by steel ribs or timbers. Grounddisturbance and loosening occur due to the blastingprior to installation of initial ground support, and thetimber blocking used with ribs permits somedisplacement of the rock mass. As such, Terzaghi’s rockloads generally should not be used in conjunction withmethods of excavation and support that tend tominimize rock mass disturbance and loosening, suchas excavation with TBM and immediate ground supportusing shotcrete and rock bolts. Moreover, theseestimates were not found to be reliable for tunnel widerthan 6m.

2. Modified Terzaghi’s Theory: Singh et al. (1995)compared support pressure measured with estimatedpressure from Terzaghi’s rock load theory and foundthat support pressure in rock tunnels does not increasedirectly with excavation size (as assumed by Terzaghi).They recommended vertical and horizontal supportpressure (pv and ph) for various rock mass conditions,are given in Table 8.02 below:

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Table 8.02: Clarification of Singh et al. (1995)

Notations:pv = Vertical support pressure, ph = Horizontalsupport pressure,ua = Radial tunnel closure, a = B/2,

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Thin shear zone = Shear Zone up to 2m thick

3. Supports based on Rock Quality Designation(RQD): Deere et al. (1970) modified Terzaghi’sclassification system by introducing the RQD (Ref.: Para-1 in Chapter-3). They also distinguish between blastedand machine excavated tunnels, and proposed supportsystem for 6 to 12m diameter rock tunnels, as given inTable 8.03 below:

Table 8.03: Support

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Notes:1 In good and excellent rock, the support

requirement will be, in general, minimal butwill be dependent upon joint geometry,tunnel diameter and relative orientations ofjoints & tunnel.

2 Lagging requirements will usually be zero inexcellent and will range from up to 25percent in good rock to 100 percent in verypoor rock.

3 Mesh requirements usually will be zero inexcellent rock and will range from occasionalmesh (or strip) in good rock to 100 percentmesh in very poor rock.

4 B = tunnel width.TBM: Tunnel Boring MachineD&B: Drilling and Blasting

4. Supports based on RMR Classification:Bieniawski (1989) provided guidelines for selection oftunnel supports based on the RMR classification, asgiven in Table 8.04 below:

4.1 Support pressure based on RMR value:Using measured support values from 30instrumented Indian tunnels, Goel and Jethwa(1991) have proposed following equation forestimating the short-term support pressure forunderground openings in both squeezing and non-squeezing ground conditions in the case oftunnelling by conventional blasting method usingsteel rib support (but not in rock burst condition):

pv = (7.5 B0.1 H0.5 – RMR)/ (20 RMR)Where:

B = span of opening (in m),H = overburden or tunnel depth (in m)

50-600m,pv = short-term roof pressure (in MPa), and

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RMR = post-excavation rock mass rating justbefore supporting

Table 8.04: Supports based on RMRClassification

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Note:(i) The recommendations are applicable only totunnels excavated with conventional drilling andblasting method.(ii) The support measures recommended aboveare permanent and not temporary or primarysupports.(iii) Double accounting for a parameter shouldnot be done in the analysis of rock structures andin estimating the rating of a rock mass. Forexample, if pore water pressure is beingconsidered in the analysis of rock structures, itshould not be accounted for in RMR. Similarly, iforientation of joint sets is considered in stabilityanalysis of rock slopes, the same should not beaccounted for in RMR.

5. Standup time v/s Unsupported span: For witharched roof, Bieniawski (1989) gave correlation betweenstandup time and unsupported span, for various RMRvalues (Fig. 8.02).

Fig. 8.02: Standup Time v/s Unsupported Span

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Note:(i) There should not be unnecessarily delay in

supporting the roof in case of a rock masswith high standup time as this may lead todeterioration in the rock mass whichultimately reduces the standup time.

(ii) Standup time improves by one class of RMRvalue in case of excavations by TBM (Lauffer,1988).

(iii) RMR system is found to be unreliable in verypoor rock masses. Care should therefore beexercised to apply the RMR system in suchrock mass. Q system is more reliable fortunnelling in the weak rock masses.

6. Supports based on Q System of rock massclassification: Estimated support categories based onthe Rock mass quality Q, as given by NorwegianGeotechnical Institute (NGI), are indicated in thenomogram in Fig. 8.03. Values of ESR (ExcavationSupport Ratio), for various types of excavation are givenin Table-7.02 in Chapter-7. The explanatory notes forthe fig 8.03 are given in fig 8.04

Fig. 8.03: Supports as per Q System

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Fig. 8.04: Explanatory Notes for Fig. 8.03

“E” is energy Absorbed in the Fibre reinforced SprayedConcrete (in Unit Joules) and it indicates the thicknessof concrete (E=500J, E=700J and E=1000J meansconcrete thickness of 6-9 cm, 9-12 cm and >15 cmrespectively).

6.1 Estimating support requirement: In Fig.8.02, draw a vertical line from the given value ofQ and draw a horizontal line for the given value of“effective span De” (Span or Length/ESR). Find outthe zone where these two lines intersect (fromZone 1 to 9). The support requirement for eachzone in given in Fig. 8.04.If systematic bolting is stipulated, in Fig. 8.03,then find out the spacing by extending the verticalline upwards (in case of bolting with fiberreinforced sprayed concrete) or downwards (incase of bolting without fiber reinforced sprayedconcrete) till it cuts the upper or lower envelopeline. Read the bolts spacing value at thisintersection point.If fiber reinforced sprayed concrete is stipulated, inFig. 8.03, the range of concrete thickness at thezone boundaries can be read from Fig. 8.03 or Fig.8.04. Exact value of the concrete thickness can beestimated by interpolating the concrete thicknessvalues at the zone boundaries, based on locationof the intersection point of vertical and horizontallines between the zone boundaries.

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In case the intersection point of vertical andhorizontal lines falls in Zone-6, 7 or 8 (weak rockmass with low Q values), then support system RRS(Rib Reinforced Shotcrete) of Type-I, II or III is tobe provided. When Q values are below 0.1, it canbe expected that there will be possibility of largeover-break, low standup time and significant earlydeformations. The use of steel sets should beavoided in such situations, due to the actualrelatively large rock-block loosening that theyallow, unless followed immediately by bolting orshotcrete, or both. It is for this category ofproblems that RRS has been developed (Fig. 8.05).

Fig. 8.05: Rib Reinforced Shotcrete

The process used for building up RRS is: (1) Sprayfirst layer of fiber reinforced shotcrete, (2) Builduplocal, smooth but not necessarily circular arch orarches of fiber reinforced shotcrete, (3) Drill boltholes and install end anchored bolts with pre-fabricated welded cross bars, (4) Attach (wire andweld) reinforcing bar steel arches to each bolthead cross-bar (pre-fabricate in bundles for easierattachment). These bars can be bent into over-break zone, therefore requiring less shotcretevolumes than with lattice girder, (5) Spray fiberreinforced shotcrete over reinforcing bars tocomplete arch, (6) Bolts and washers tensioned(bolt thread protected with plastic caps), and (7)Spray fiber reinforced shotcrete over bolt head tocomplete the arch.

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The requirements of RRS for the given zone are givenin Fig. 8.04, in terms of single/double layer of rebars,numbers of rebars thickness of shotcrete, diameter ofrebars etc., for any given span. For example, “D40/6+2 Ø 16-20 (Span 20m)” in RRS-I means “40cmsprayed concrete with Double layer of rebars, 6 and 2nos. rebars in each layer, rebar dia. 16 to 20mm andthis arrangement is suitable for span of 20m”.

For road tunnel of 10m dia, the ESR value is 1.0 and,therefore, the effective span (De) will be 10m. For watertunnel of 10m dia, the ESR value is 1.6 and, therefore,the effective span (De) will be 10/1.6 = 6.3m. For thesetunnels, the support requirements for some given valueof Q have been worked out using Fig. 8.03 and 8.04:

Table 8.05: Estimation of support requirements

6.2 Length of Rock bolts: The length of rockbolts can be estimated from the following formulae(Barton et al., 1974):

L = 2 + (0.15B/ESR)where:

L - Length of rock boltsB - Excavation widthESR - Excavation Support Ratio

6.3 Maximum unsupported span: The maximumunsupported span can be estimated from followingformulae:Maximum Unsupported Span = 2 ESR Q0.4

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7. Elements of commonly used Excavation andSupport Classes (ESC) in Rock and Soft Ground,as given by Hung et al. (2009) are tabulated inAppendix-8.1 and Appendix-8.2 respectively.

8. Example of estimating support requirements:Design tunnel supports for a road tunnel by variousempirical approaches, for a tunnel with following details:

• Dia. of Tunnel:By Drill and Blast - 7.4m + 0.6m (Overbreak) = 8mBy TBM = 7.4m

• Located 20m below ground surface• Rock : Shale• γ = 2.66 t/m3

• RQD : 55 – 85% (Weighted Average = 72%)• UCS = 45 MPa• Joint Spacing : 50 – 900 mm• 0.8-1.1 mm Separation, Slightly Weathered,

Rough• Large inflow or High Water Pressure• Strike perpendicular to Tunnel Axis, Dip 200

• Two Joint Sets, Random• Rough Planar Joints• Unaltered Joint Walls, Surface Staining• Medium Stress σ c /σ 1= 45/(0.027*20) = 83

(A) Based on RQD RQD 72%

(Ref. Para-3 and Table 8.03 above)For TBM –

Light to Medium Steel Sets @5 to 6’(Say 1.5m c/c)Rock bolts @ 4 to 6’ (Say 1.2m c/c)Shotcrete – 2 to 4’ (Say 100mm) thickin Crown

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For D&B –Light to Medium Steel Sets @4 to 5’(Say 1.2m c/c)Rock bolts @ 3 to 5’ (Say 0.9m c/c)Shotcrete – 4’ Say 100mm) thick inCrown and Sides

(B) Based on RMR ClassificationCalculation of RMR Value: (Ref. Appendix 3.1 inChapter-3)

Class – III “Fair” Rock Mass

(Ref. Para-4 & Table 8.04 above):

For Class-III “Fair” Rock massBy Top Heading and Benching.1.5 to 3m advance in top heading.Commence support after each blast.Complete support 10m from face.Rock bolts 4m Long, @1.5-2m c/c, in crownand Walls with Wire mesh in CrownShotcrete 50-100mm (Crown) and 30mm(Sides)No Steel sets required.

mailto:@1.5-2m

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(Ref. Para-5 & Fig. 8.02 above):

For RMR=50 and Roof Span= 8mStandup time = 70 Hours

(C) Based on Q SystemCalculation of Q Value: (Ref. Appendix 3.2 inChapter-3)

Q = (RQD/Jn) * (Jr/Ja) * (Jw/SRF) = (72/6) * (1.5/1.0) * (0.5/1.0) = 9

“Fair” Rock Mass – Group “2”

(Ref. Para-6 and Fig. 8.02 & 8.03 above)For Road tunnel, ESR = 1.0 (Ref. Table 7.02 inChapter-7)Hence, De = 8.0/1.0 = 8.0For De = 8.0m and Q = 9 (From Fig. 8.03) - Fallsin Zone-3

For Zone-3 (From Fig. 8.04)Systematic Rock bolts & Fiber reinforcedshotcrete is required.Bolt spacing (From Fig. 8.03) @ 2.3 m c/cShotcrete thickness (From Fig. 8.03) 5.5 cm.

Length of Rock bolt: (Ref. Para 6.2 above)L = 2 + (0.15B/ESR) = 2 + (0.15*8/1.0) = 3.2 m

Maximum unsupported span: (Ref. Para 6.3 above)= 2 ESR Q0.4 = 2 * 1.0 * 90.4 = 4.8m

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9. Limitations of empirical approach: Following arethe limitations of the empirical approach of supportdesign:

(i) These methods provide ground supportscheme based on parameters that can bedetermined from explorations, observationsand testing. They are far from perfect andcan sometimes lead to the selection ofinadequate ground support. It is, therefore,necessary to examine the available rock massinformation to determine if there are anyapplicable failure modes not addressed by theempirical systems.

(ii) These methods lead the user directly fromthe geologic characterization of the rock massto a recommended ground support withoutthe consideration of possible failure modes.Potential modes of failure are not covered bysome or all of the empirical methods andmust be considered independently, includingthe following:

• Failure due to weathering or deterioration ofthe rock mass.

• Failure caused by moving water (erosion,dissolution, excessive leakage, etc.).

• Failure due to corrosion of ground supportcomponents.

• Failure due to squeezing and swellingconditions.

• Failure due to overstress in massive rock.(iii) These methods are largely based on blasted

tunnels. System recommendations should bereinterpreted based on current methods ofexcavation. Similarly, new ground supportmethods and components must beconsidered.

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ANALYTICAL AND NUMERICAL METHODS OF TUNNEL SUPPORT DESIGN

1. Analytical Methods: The support pressures,moments, displacements etc. can be estimated by usingclosed form structural analysis methods, withdimensions of tunnel, engineering properties of rockmass and other boundary conditions as inputs. Carryingout such analysis manually is quite time consumingand, therefore, many custom made software for tunnelanalysis/design can be used for this purpose. Describedbelow are some such software, but this is not completeor exhaustive list of such software.

1.1 Examine (formerly Examine3D) is software fortwo/three dimensional engineering analysis ofunderground excavations in rock. It generatesgeometry and boundary element discretization forunderground openings; computes stresses anddisplacements using the direct boundary elementmethod; and visualizes the analysis results. Usingthis program, the stresses and deformations canbe calculated at any location and can be presentedin various formats.1.2 RocSupport is software tool for estimatingdeformation in circular or near circular excavationsin weak rock and visualization of the tunnelinteraction with various support systems. It can beused as a tool for the preliminary design oftunnels and support systems. It is not applicablefor excavations in hard rock, where failure iscontrolled by structural discontinuities or brittlefailure.

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ANALYTICAL AND NUMERICALMETHODS OF TUNNEL SUPPORT

DESIGN

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2. Numerical Methods: For tunnels, it is not possibleto carry out engineering analysis of the undergroundexcavation due to following reasons:

(i) Rocks may behave in complex ways (e.g.Elastic, Elasto-plastic/Visco-plastic, strainSoftening etc.). Analyzing all such behaviouris not possible by normal engineeringanalysis.

(ii) Rock mass is non-homogeneous andanisotropic, difficult to analyze by normalengineering analysis.

(iii) Complex boundary conditions.(iv) Difference in behaviour of intact rock vis-a-

vis rock mass.(v) It is difficult to account for excavation

sequence in normal engineering analysis.

In such cases, numerical methods are used whereinthe undergrounds excavations are analyzed byconstructing models catering to relevant loadingconditions, boundary conditions and engineeringproperties of the rock mass. The Numerical methodsuse some of the following techniques:

• Finite Difference Method (FDM)• Finite Element Method (FEM)• Boundary Element Method (BEM)• Coupled Finite Element Boundary Element

Method (FEBEM)• Distinct Element Method (DEM)

Out of the above techniques, the Finite Element Method(FEM) is most commonly used technique. Listed beloware some of the FEM based software for analysis anddesign of tunnels, but this is not complete or exhaustivelist of such software:

• ABAQUS• Plaxis 2D and Plaxis 3D

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ANALYTICAL AND NUMERICAL METHODS OF TUNNEL SUPPORT DESIGN

• RS2 (Formerly Phase2) and RS3 (FormerlyRS3)

• MIDAS GTS• SOFiSTiK• The FEM – Tunnel (GEO5)

The numerical methods always provide approximatesolutions. Hence, these results need to be validated byrecording actual displacements/stresses etc. byinstrumentation and constitutive model needs to be fine-tuned accordingly.

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TUNNEL EXCAVATION METHODS

1. Selection of Tunnel Excavation Method dependson various factors, such as:

(i) Geological conditions(ii) Cross Sectional area and Shape of tunnel(iii) Length of tunnel(iv) Ground water condition and expected water

inflow(v) Vibration restrictions(vi) Allowable ground settlements(vii) Availability of resources (machinery/equip-

ment, funds & time)

Commonly used Tunnel excavation methods can begrouped under two categories:

(a) Excavation Methods for Rock Tunnels(b) Excavation Methods for Soft Ground Tunnels

2. Excavation Methods for Rock Tunnels: Threecommonly used excavation methods for rock tunnelsare:

(i) Drill and Blast (Full Face or Partial FaceExcavation)

(ii) Road Header (Full Face or Partial FaceExcavation)

(iii) Mechanized Tunnelling

3. Drill and Blast Method: The Drill and Blast (D&B)method is the most common method for medium tohard rock conditions. Some of its major advantagesare fast start-up and relatively low capital cost for theequipment. On the other hand, the cyclic nature of thedrill and blast method requires good work siteorganization. This excavation method has been used

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throughout the world for a long time and still remainsthe conventional method for rock tunnels.

This tunneling method requires use of explosives.Compared with tunneling by Tunnel Boring Machine,this generally results in higher duration of vibrationlevels and noise, which restricts use of this method inurban areas. The excavation rate is also less than TBM(usually 3 to 5m per day).

Drill and blast method is best suited for medium tohard rocks hard rocks and relatively short tunnels (whereuse of TBM/Road header is uneconomical) and can alsobe used when encountering too great a variety ofgeologies or other specific conditions such as mixedface, squeezing ground, etc. It is suited to any type oftunnel cross section. Appropriate controlled blastingtechnique needs to be implemented at site to reduceover breaks and minimize damage outside the minimumexcavation line.

Fig. 10.01: Drill and Blast Cycle

Various activities involved in a cycle of drill and blastmethod are shown in Fig. 10.01 and they are discussedin following paras.

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3.1 Drilling: Based on the blasting plan,determined in advance, holes are drilled in theforemost front wall of the tunnel (working face).The holes can be drilled manually (Fig. 10.02) orby using a Jumbo (Fig. 10.03). The Jumbo is alsoknown by name Boomer.

Fig. 10.02: Manual Fig.10.03: Jumbodrilling

Now-a-days most of the drilling work is done usingjumbos having multiple drilling arms and anoperator tower. It is run by electric cable, a hosebrings water to the drills and drills are pneumatic.That means that the drill bits both hammer androtate. Broken bits of rock are flushed out bywater. The length of drill holes depends upon thelength to be blasted in one cycle (called as Step,Advance, Pull etc.) which typically varies from 0.8to 3.5m depending on the type of rock, themethod used for tunnelling (i.e. full face or partialface blasting) and support system used.

The drilling pattern to be adopted shall ensureminimum over breaks and shall consume leastquantity of explosives. The drilling pattern dependsupon the texture and formation of rock whichdetermine rock drillability & blastability, size/shapeof tunnel, drilling equipment used, type ofexplosives & means of detonation available,expected water leaks, blast vibration restrictionsand accuracy requirements of the blasted wall etc.

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Following are the most commonly used drillingpatterns for tunnels:

(i) Wedge Cut or “V” Cut Drilling Pattern: Inthis pattern, horizontal cut holes are driven aroundthe middle of the face, in an inclined angle of 600 tothe face towards the centre. Maximum explosivescharge concentration is required at the apex end ofthe blast holes as they are to be blasted at the firstinstant for creation of the free face.

(ii) Cone/Pyramid/Diamond Cut Drilling Pat-tern: Four or six holes are cut at the middle of theface, which converge at the end, to form either acone or pyramid or diamond shape. This cut issuitable for laminated rocks, sedimentary in nature.It also helps in drivage of smaller cross-sectionalarea tunnels so as to break the rock along thecleavage planes.

(iii) Burn (Parallel) Cut Drilling Pattern: Theburn cut holes are drilled parallel to the tunneladvance and perpendicular to the face of the tunnel.Some of the drilled holes (mostly in middle sectionof the face) are holes of large diameter (say 100mm)or a set of closely spaced holes of conventionaldiameter (46 to 56mm) and are left as dummy holeswithout any explosive charge, so that they act asfree face for the heavily charged blast holes around.Specific geometrical relationship between thediameter of dummy holes and spacing in betweendummy holes and charged blast holes is required tobe maintained for the given rock in order to createthe free face.

With Burn or Parallel cut, possible advance per roundis longer as compared to that with Angular cut holes.Success of Burn or Parallel Cut depends uponaccuracy in keeping the holes parallel. Thisrequirement and requirement of holes of largediameter or a set of closely spaced holes ofconventional diameter calls for use of drilling

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Jumbos.

Drilling patterns can be decided manually, butadvanced computer programs are available, whichmake it easier to modify the patterns and predictfairly accurately the effects of changes in drilling,charging, loading and production.

3.2 Loading: During this activity, also known ascharging, the drill holes are now filled withexplosives, detonators are attached to theexplosive devices and the individual explosivedevices are connected to one another (Fig. 10.04).

Fig. 10.04: Loading

3.2.1 Choice of Explosives: The best choices areemulsion explosives, produced by intimate andhomogeneous mix of oxidizer and fuel. Basicallyemulsion explosives consist of micro droplets ofsuper saturated oxidizer solution in oil matrix. Theyare in the form of water-in-oil emulsion. Variousadvantages associated with the use of emulsionexplosives are as below:(i) Emulsion explosives are much better water

resistant than water gel slurry or ANFO(Ammonium Nitrate and Fuel Oil). This isbecause the oil-phase envelopes the waterphase.

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(ii) They are safe to handle, store and usebecause of their relative insensitivity todetonation by friction, impact or fire.

(iii) Due to the oxidizer droplet size (0.2 to 10micron) they have higher value of Velocityof Detonation (VOD) which can tackle thetoughest rock conditions very effectivelyand that too without comprising on safetystandards.

(iv) Since emulsion explosives are more oxygenbalanced they generate minimum noxiousfumes and very less smoke. This in-turncan reduce the ventilation time after theblasts and further can shorten the cycletime of operations. Liquid emulsionexplosives are pumpable and charging timecan be cut down.

3.2.2 Requirement of Explosives: Normally, theconsumption of explosives in tunnel blasting is muchmore than that required for open cut. For road/railtunnels, the Specific Charge or the Powder Factor isnormally 1.2 kg/cum of in-situ rock on an average.However, the amount of explosives required (in kg/cum of in-situ rock) may vary from site to sitedepending upon the geological strata involved, crosssection of the opening and advance length (Fig.10.05).

Fig. 10.05: Requirement of Explosives

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3.3 Blasting: The holes, charged with explosives,are blasted in a proper sequence, from the centeroutward, one after the other. Although more than100 explosions may be set off, one after the other,the blast sequence is completed in several seconds.The devices should not explode at the same time,but rather one after the other at specified intervals.Only when the blast master has ensured thatnobody is left in the danger zone, the explosion canbe triggered by the blasting machine.

3.3.1 Choice of Initiation System and Selectionof Delay Sequence: Selection of delays in initiationsystem and timing shall be such that the rock volumeafter swell from subsequent blasts must beaccommodated. For this, the fracturing and breakingtime of the rock and time required for spreading outcracks in the rocks is to be studied. It, basically, alldepends on condition of rock. In hard and brittlerocks, the speed of developing cracks is faster thanthat in softer rocks. This rock breaking speed canvary from 1 to 3 milliseconds per meter. But theejection speed of rock after blast may vary from 20-30m per second i.e. 20-30mm per millisecond andit can be understood that for a 4m long blast holesthe broken rock takes about 300-400 millisecondsfor complete removal from the face. Precisely, forthis reason long delays (half second, 300 to 500milliseconds) are preferred for tunnel blasting. Theadvantages of delay detonators are:

• Better fragmentation,• Reduced secondary blasting,• More uniformity in size of fragmentation,• Possible to fire more holes in a single blast with

less vibrations and noise.

3.3.2 Controlled Blasting is adopted forminimizing over breaks and has following additionaladvantages:• Less damage to peripheral rock,

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• Reduced requirement of support system,• Safer tunnel operation in general, and• Reduced requirement of scaling.

Controlled blasting generally involves closer spacingof contour (perimeter) blast holes which are alsocalled as trimmers. They are charged with fewerexplosives than that of the production blast holes.The spacing thumb rule is about 10-12 times of blasthole diameter in hard rocks and about 5-6 times ofblast hole diameter in soft rocks.

3.3.3 Air Column Method: In order to reduce thecracking or damage to the strata at the periphery itis advisable to use “Air Column Method” which canminimize the radial vector component of blastinduced vibrations. This method consists of insertinginto the blast holes an inert spacing device of a lengthabout four times the diameter of the blast hole, priorto charging of first explosive cartridge. This leavesrequisite air gap in between the explosive cartridgeand the end of the blast hole. With this method, aplain straight tunnel face can be secured for nextdrilling cycle without any cracks.

3.3.4 Efficiency of Blasting: Efficiency of blastingis routinely assessed by tabulating Pull, SpecificCharge, Specific Drilling, Detonator or Hole Factor,Blast-induced damage and Over break/Under breaksagainst the values assumed during planning.

3.4 Ventilation (De-fuming): Due to blasting,lot of rock fragments are flung around, dispersingclouds of dust which gets mixed with combustiongases of the explosion. To enable resumption ofwork in the tunnel, the dust and fumes areremoved by using air-ducting system, long steel orplastic pipes which are attached to the roof of thetunnel and blow fresh air onto the working face(Fig. 10.06). This gives rise to localized excesspressure and the bad air is pushed towards thetunnel exit.

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Fig. 10.06: Ventilation Fig. 10.07: Mucking

3.5 Mucking (removing rubble): The blastedmaterial, rubble or spoil, is carried out of thetunnel, by either loading onto dump trucks withwheel loaders or on conveyer belts (Fig. 10.07).

3.6 Scaling (Dislodging): This refers tostripping away and removal of loose pieces ofrock, which were not completely released from therock during the blasting procedure. This is doneusing a tunnel excavator or road header withsuitable scaling tool attached to it.

3.7 Supporting (Securing): At this stage, thetemporary or initial supports are provided. A layerof shotcrete is used very commonly, as it enablesa cavity-free connection of support to the rock.Depending on the design of support systems,various types of supports can be implemented.After shotcreting, rock bolts are used verycommonly (Fig. 10.08). A jumbo is used to carryout all the operations need for installing rock bolts.If the support system stipulates, the steel ribs orlattice girder are placed in position.

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Fig. 10.08: Rock Bolting

3.8 Geological Mapping: The working face isnow freely accessible and the geologist has a fewminutes to map it. In the process, he determineswhat type of rock is present and how the rocks lie,i.e. whether they dip in a flat or steep manner,whether they are folded or even broken. At thesame time, the strength of the rock, the reactionof the rock mass to the excavation process andany mountain water infiltration are alsodocumented. The mapping report created from this– with sketches and photos – serves as the basisfor the selection of appropriate supportingmeasures.

4. Full Face v/s Partial Face Excavation: A keydecision to be taken by engineers is whether to go for“Full Face Excavation” or “Partial Face Excavation”.

4.1 Full Face Excavation: Excavating thecomplete tunnel section in one operation is termedas Full Face Excavation. Wherever practical, this ispreferred for higher rate of progress and ease inconstruction. The decision for excavating full facehas to be taken after careful consideration of thegeology, the span) of opening and the stand-uptime.

4.2 Partial Face Excavation: Large openings oropenings in weak rocks are less stable. Therefore,in many cases the tunnel cross section is not

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excavated at once, but in parts. This type ofexcavation is called Partial Face Excavation.

Various types of partial face excavations are asunder:

(A) Heading and Benching. This method involvesexcavation of top portion (called heading or calotte)first and excavation of the bottom portion (calledbench) is done only after securing the top heading(Fig. 10.09). With the stand-up time problemeliminated (unless there will be a problem with wallstability), longer incre¬ments of bench can beexcavated.

Fig. 10.09

Heading Benching

A ramp needs to be constructed for accessing theheading face, if the gap between faces of top headingand bench warrants so. If the heading and the benchare excavated simultaneously, then the ramp mustbe moved forward every now and then (Fig. 10.10).

Fig. 10.10: Ramp in Heading and Benching

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Installation of rock reinforcement in top headingpre¬sents no special problems. However, when steelribs are used, the necessity for a full-strength steelrib arch creates a problem at the abutments. Thearch must have a temporary foundation while thebench beneath it is excavated. Wall plates areinstalled longitudinally beneath the ribs at the topheading invert. A wall plate is a horizontal structuralsteel member placed under the arch to act as anabutment and spread the reaction while the benchis being excavated. For smaller or lightly loaded ribs,the wall plate is a single wide flange beam with itsweb horizontal; arch and post segments fit insideits flanges. For heavier loads or larger spans, a pairof beams joined together, with webs vertical, islocated directly beneath or over the arch/postflanges.

(B) Multiple Drift Advance: If the stand-up timeis insufficient for advance by heading and benchingmethod, because of either the geology or largespans, the top heading and/or benches must bedivided into two or more drifts. This is advantageousbecause the reduced span increases stand-up time,the reduced volume decreases mucking time andtime required to install support or reinforcement isalso reduced. When using steel sets, the appropriatefinal arch segment is used and supported temporarilyon one or more steel posts. When the adjacent driftis excavated, the next arch segment is erected,connected, posted and so on. Once the wall platesare in place and the full arch erected, the temporaryposts are removed.

While tunnel excavation from top down is preferred,in exceptionally poor ground it may be necessary towork from the bottom up. Driving bottom sidewalldrifts first permits concreting abutments andeliminates need to establish, undercut and re-establish supports.

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Some well-established schemes of multiple driftexcavation are as below:

Core heading: This is also known as the Germanheading method. It consists of excavating andsupporting first the side and top parts of the crosssection and subsequently the central part (core)(Fig. 10.11). The ring closure at the invert comesat the end. The first gallery also serves forexploration. The crown arch is founded on the sidegalleries thereby keeping the related settlementssmall.

Fig. 10.11: Fig. 10.12: Sidewall Core Heading Drift

Sidewall drift: The side galleries are excavated andsupported first (Fig. 10.12). They serve as abutmentfor the support of the crown, which is subsequentlyexcavated. This method is preferred in soils/rocksof low strength. Note that a change from top headingto sidewall drift is difficult to accomplish.

In addition to above, many other variations ofmultiple drift sequence are also in practice.

5. Excavation by Road header: Road header is a self-propelled equipment, moving on wheels or crawlertravelling track, consisting of a telescopic boom whichcan be rotated in any direction, boom mounted cuttinghead and a loading device usually involving a conveyor(Fig. 10.13).

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Fig. 10.13: Road Headers

The cutting head can be a general purpose rotatingdrum mounted in line or perpendicular to the boom, orcan be special function heads such as jack hammer likespikes, compression fracture micro-wheel heads likethose on larger tunnel boring machines, a slicer headlike a gigantic chain saw for dicing up rock, or simplejaw-like buckets of traditional excavators.

Road headers are used for moderate rock strengths andfor laminated or joined rock. The cutter is mounted onan extension arm (boom) of the excavator and cuts therock into small pieces. Thus, over profiling can be limitedand also the loosening of the surrounding rock is widelyavoided. Measures against dust (suction or waterspraying) have to be provided. The required power ofthe motors increases with rock strength.

The width of tunnel excavated varies from slightly morethan the width of the machine body plus treads to twicethat width. Much less heading equipment is requiredand start-up costs are only a fraction of that for TBMexcavation. The compactness, mobility, and relatively

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small size of the road header combined withsimultaneous mucking makes it practical to install rockbolts and/or shotcrete quickly and easily.

The principal constraint in use of road headers is thatthey curently are usable only in rock of less than about80 MPa compressive strength. Somewhat stronger rockcan be cut, or chipped away, if it is sufficiently fractured.

This method could also be called partially mechanizedtunneling. Whereas TBMs are generally purpose-built,road headers are nearly "off-the-shelf” equipmentrequiring relatively little lead time. Excavation by roadheader is suited for any type of tunnel cross sections &may be done either partial face or full face.

6. Mechanized Tunnelling: Mechanized tunnelingoffers many advantages, some of them are:

(i) Industrialization of the tunneling process,with reductions in costs and construction tim-es.

(ii) Possibility of crossing complex geological andhydrogeological conditions safely and eco-nomically.

(iii) Good quality of the finished product.(iv) Enhanced health and safety conditions for the

workforce

However, there are still risks associated with mechanizedtunneling, for the choice of technique is often irreversibleand it is often impossible to change from the techniquefirst applied or only at the cost of immense upheaval tothe design and/or the economics of the project.

The selection of tunnelling technique to use must bemade on the basis of the known and suspected groundconditions, in combination with other aspects such asaccess, experience and skill of the officials/workers, aswell as costs. Adaptability of the technique to variabilityof the ground could also be an important factor.

“Tunneling Shields” & “Tunnel Boring Machines” are the

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two principal types of machines employed formechanized tunneling. But quite often these two termsare used interchangeably as distinction between two isgetting blurred.

7. Tunnelling Shields: In its simplest form a tunnelingshield is a steel frame with a cutting edge on the forwardface (Fig. 10.14). For circular tunnels this is usually acircular steel shell under the protection of which theground is excavated and the tunnel support is erected.A shield also includes back-up infrastructure to erectthe tunnel support (lining) and to remove the excavatedspoil.

Fig. 10.14: Simple Tunnelling Shield

There are two main types of tunnelling shield, one withpartial and other with full face excavation. In partially-mechanized shields, an excavator or a partial cutterhead/ road header works on the face. Partially-mechanized shields (also called boom-in-shieldtunneling machines) are used where the cost of fullface tunnel boring machines cannot be justified. Manualexcavation, i.e. by “hand”, is considered for very specialapplications only, e.g. very short advances, due to thelow advance rate. This type of tunneling is called themanual shield technique. Full Face Shields are discussedunder Shielded TBMs & Soft Ground TBMs.

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Tunneling shields do not have an “engine” to propelthemselves forwards, but push themselves forwardusing hydraulic jacks. In order to create the necessaryforce to push the tunnel shield forwards, jacks are placedaround the circumference of the shield. These jackspush against the last erected tunnel segment ring andalso push the shield against the tunnel face in thedirection of the tunnel construction. Of course, thisprinciple does not work at the start of the tunnelconstruction and therefore in the starting a reactionframe is necessary to take the jacking forces.The jackscan be operated individually or in groups, allowing theshield to be steered in order to make adjustments inline and level and to be driven in a curve if required.When the shield has advanced by the width of a tunnelsegment ring, the jacks are retracted leaving enoughroom in the tail of the shield to erect the next tunnelsegment ring.

The support usually adopted with shield tunnelling thesedays is circular segments. These segments form, whenconnected together, a closed support ring. As the tunnelsegments are connected together inside the shield tail,the diameter of the completed tunnel segment ring issmaller than that of the shield. This creates a gapbetween the ground and the tunnel lining. When theshield is jacked further into the ground the size of thisgap is approximately 50 and 250mm. In less supportingsoft ground, it has to be expected that the ground settlesby this value. This can result in the softening of theground and, especially with shallow tunnels, in thesettlements reaching the ground surface and havingundesirable consequences on surface or near surfacestructures. In order to avoid these settlements, the gapis generally injected with mortar.

8. Tunnel Boring Machine (TBM) also known as a“mole“, is a machine used to excavate tunnels with acircular cross section through a variety of soil and rockstrata. They can bore through hard rock, sand andalmost anything in between. Tunnel diameters can range

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from a meter (done with micro TBMs) to almost 17-18m to date. Tunnels of less than a meter or so indiameter are typical ly done using trenchlessconstruction methods or horizontal directional drillingrather than TBMs.

TBMs have the advantages of limiting the disturbanceto the surrounding ground and producing a smoothtunnel wall. This significantly reduces the cost of liningthe tunnel and makes them suitable to use in heavilyurbanized areas. The major disadvantage is the upfrontcost and difficulty in transportation. TBM tunnels havevery high start-up (pre-excavation) costs andaccompanying long lead time, though the high rate ofadvance reduces the per m excavation cost. The decisionon undertaking excavation by TBM requires carefulconsideration of techno-economic factors.

A Herrenknecht mega-Mixshield TBM of 17.6m diameterbecame the largest machine in the world when it waslaunched in 2015 for Tuen Mun-Chek Lap Kok underseahighway link in Hong Kong. The Mixshield is marginallylarger than what is now relegated to being only thesecond-largest machine in the world, the 17.48mdiameter Hitachi Zosen EPBM on the Alaskan Wayviaduct replacement highway tunnel project in Seattle.

Although TBMs are often designed for specific projects,i.e. with a specific diameter and to cope with certainground conditions, these days refurbished machines arebecoming more common and projects are actuallydesigned around the machines available. An exampleof this is when the diameter of the new project is chosento suit the old machine, with just the cutter head beingredesigned for the specific ground conditions expected.

One of the general requirements for the use of a TBM isconsistent geology along the route of the tunnel as thedifferent cutting tools are only suitable for a smallvariation in material characteristics. A universalmachine for all types of ground and soil conditions doesnot exist (although TBMs with multiple modes of

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operation such as Mix-shields are there). Thecombination of different cutting tools on the cutter headcan increase the application of machines to a greaterrange of ground conditions.

Although TBMs can have different mechanisms formoving through the ground, most have to start outsideand hence need a reaction frame to start the drive.

9. Stages of TBM Construction: Following stages areinvolved, before and after the main activity of tunnellingby TBM:

(i) Excavation for launching Shaft and Retrieval Shaft(Fig. 10.15)

Fig. 10.15

(ii) Assembly of TBM at the launching Shaft (Fig. 10.16)

Fig. 10.16

(iii) Tunnel excavation by TBM (Fig. 10.17)

Fig. 10.17

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(iv) Arrival of TBM in retrieval Shaft, to be dismantledand taken out (Fig. 10.18)

Fig. 10.18

10. Types of TBM: TBMs are often grouped undercategories of “Hard Rock TBMs” & “Soft Ground TBMs”.

10.1 Hard Rock TBMs: In hard rock, either shie-lded or open-type TBMs can be used. All types ofhard rock TBMs excavate rock using disc cuttersmounted in the cutter head. The disc cutterscreate compressive stress fractures in the rock,causing it to chip away from the rock in front ofthe machine, called the tunnel face. Theexcavated rock, known as muck, is transferredthrough openings in the cutter head to a beltconveyor, where it runs through the machine to asystem of conveyors or muck cars for removalfrom the tunnel.

Fig. 10.19: Typical Hard Rock TBM

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Hard rock TBMs comprise of following four keysections (Fig. 10.19):(i) Boring section, consisting of the cutter head,(ii) Thrust and clamping section, which is

responsible for advancing the machine,(iii) Muck removal section, which takes care of

collecting and removing the excavatedmaterial, and

(iv) Support section, where the tunnel support iserected.

Open-type TBMs have no shield, leaving the areabehind the cutter head open for rock support. Toadvance, the machine uses a gripper system thatpushes against the side walls of the tunnel. Themachine can be continuously steered while grippershoes push on the side-walls to react themachine’s forward thrust. At the end of a stroke,the rear legs of the machine are lowered, thegrippers and propel cylinders are retracted. Theretraction of the propel cylinders repositions thegripper assembly for the next boring cycle. Thegrippers are extended, the rear legs lifted andboring begins again. The open-type TBM does notinstall concrete segments behind it as othermachines do. Instead, the rock is supported usingground support methods such as ring beams, rockbolts, shotcrete, steel straps and wire mesh(Stack, 1995).

In fractured rock, shielded hard rock TBMs can beused, which erect concrete segments to supportunstable tunnel walls behind the machine. DoubleShield TBMs are so called because they have twomodes; in stable ground they can grip against thetunnel walls to advance forward and in unstablefractured ground, the thrust is shifted to thrustcylinders that push off against the tunnelsegments behind the machine. This keeps thesignificant thrust forces from impacting fragile

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tunnel walls. Single Shield TBMs operate in thesame way, but are used only in fractured ground,as they can only push off against the concretesegments (Stack, 1995).

Hard Rock TBMs primarily fall under followingcategories:• Gripper TBM: Open hard rock TBM - suited

for boring in stable rock.• Shielded TBM: Shielded hard rock TBM -

suited for tunneling in varying rockformations that alternate between stable andunstable formations.

Working principles of these TBMs are brieflydescribed below:

(I) Gripper TBM: The Gripper TBM is bracedradially with grippers against the tunnel wall, withhydraulic cylinders pressing the cutter head againstthe tunnel face to enable a further section of tunnelto be excavated. The maximum boring stroke isgoverned by the length of the pistons in the thrustcylinder. The cutter head is fitted with cutter rings(disks). The rotating cutter head forces the disksagainst the tunnel face under high pressure. In thisprocess, the disks roll over the tunnel face, therebyloosening the native rock.

The excavated rock, or "chips" as it is commonlyknown, is collected in muck bucket lips (openingsin the cutter head) and discharged via hoppers ontoa conveyor belt. The excavated material is broughtoutside the tunnel by conveyers. Typical advance ofcutter head is approximately 0.7 to 1.2m. Aftercompletion of a boring stroke, the drilling process isinterrupted and the machine moved forwards, withthe Gripper TBM being stabilized by an additionalsupport system. A new working cycle can begin whenthe gripper shoes of the machine are once moreengaged.

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Unlike shielded TBMs, where tunnel support, e.g.segmental lining, is fixed and does not change duringtunnel construction, the tunnel support system,when using a Gripper TBM, can vary depending onthe ground quality. The appropriate rock supportdevices can be installed immediately behind thecutter head. These devices can include anchors, steelarches, shotcrete and even segmental linings.

The tunneling performance of a Gripper TBM dependsessentially on the time required to install rocksupporting devices. The Gripper machine enablescomprehensive rock support measures to be takeneven right behind the cutter head.

(II) Shielded TBM: In contrast to Gripper TBMs,the body of the shield TBM has an extended shieldover the front section of the machine. This shieldhas the function of supporting the ground andprotecting the personnel, thus allowing safe erectionof the tunnel lining. There are two basic types ofshield TBMs for hard rock available; the single-shieldand double-shield.

The single-shield TBM in hard rock is mainly used inunstable conditions where there is a risk of groundcollapse. With these machines, the pushing forcesare maintained axially against the installed liningsegments. The Single Shield TBM belongs to acategory of machines which are fitted with an openshield. Tunneling machines described as open shieldsare machines without a closed system for pressurecompensation at the tunnel face. Protected by theshield, the Single Shield machine extends and drivesforward the tunnel practically automatically. In orderto drive the tunnel forwards, the Single Shield TBMis supported by means of hydraulic thrust cylinderson the last segment ring installed. The cutter headis fitted with hard rock disks, which roll across thetunnel face, cutting notches in it. These notchesdislodge large chips of rock. Muck bucket lips, which

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are positioned at some distance behind the disks,carry the extracted rock behind the cutting wheel.The excavated material is brought outside the tunnelby conveyers.

The double-shield machine (or telescopic shield)combines the ideas of the gripper and single-shieldtechniques and can therefore be applied to a varietyof geological conditions. The double-shield machineconsists of a front shield with cutter head, as wellas a gripper section with gripper shoes, a tail shieldand auxiliary thrust jacks. Both parts of the machineare connected by a section called the telescopicshield. The operating principle is based on the grippershoes pressing against the tunnel wall whileexcavation and segment installation are performedat the same time. The system adds some flexibilityto allow the machine to work either in gripper modeor as a shield TBM.

10.2 Soft Ground TBMs: The application of aTBM technique in less stable soft groundcommonly requires the face to be supported. Thisis in contrast to the open face TBMs (often used inhard rocks) where the ground is able to supportitself during excavation by virtue of its significantstrength and stand-up time.

In soft ground, with little or no standup time, theground would simply collapse into the machine andattempts to control the excavation of this materialand to prevent large displacements occurringwithin extensive amounts of the ground around thetunnel heading would be very difficult. In addition,for tunnels constructed below the groundwatertable in permeable materials, water flow into thetunnel must be controlled in order to prevent themachine and tunnel from flooding.

Soft ground TBMs are designed to simultaneouslyprovide immediate peripheral and frontal supportand as such they belong to the closed-faced groupof TBMs.

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Except for mechanical-support TBMs, they all havea cutter head chamber at the front, isolated fromthe rear part of the machine by a bulkhead, inwhich a confinement pressure is maintained inorder to actively support the excavation and/orbalance the hydrostatic pressure of the groundwater.

The face is excavated by a cutter head working inthe chamber. The TBM is jacked forward by ramspushing off the segmental lining erected using anerector integrated into the machine. Soft groundTBMs are classified into following types dependingupon frontal support technique they employ:

(I) Mechanical support TBM: A mechanical-sup-port TBM has a full-face cutter head which providesface support by constantly pushing the excavatedmaterial ahead of the cutter head against thesurrounding ground. Its specific field of applicationis, therefore, in soft rock and consolidated softground with little or no water pressure.

(II) Compressed air TBM: A compressed-air TBMcan have either a full face cutter head or excavatingarms like those of the different boom-type units.Confinement is achieved by pressurizing the air inthe cutting chamber. Compressed-air TBMs areparticularly suitable for ground of low permeabilitywith no major discontinuities (i.e. no risk of suddenloss of air pressure). The ground tunneled mustnecessarily have an impermeable layer in theoverburden.

Compressed-air TBMs tend to be used to excavatesmall-diameter tunnels. Their use is notrecommended in circumstances where the groundat the face is heterogeneous. They should not beused in organic soil where there is a risk of fire.

(III) Slurry Shield TBM: In soft ground with veryhigh water pressure and large amounts of groundwater, Slurry Shield TBMs are needed. These

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machines offer a completely enclosed workingenvironment (Fig. 10.20). Soils are mixed withbentonite slurry, which must be removed from thetunnel through a system of slurry tubes that exitthe tunnel. Large slurry separation plants are neededon the surface for this process, which separate thedirt from the slurry so it can be recycled back intothe tunnel.

1. Cutter Head 2. Shield 3. Bentonite Injection

4. Air regulation 5. Air Bubble 6. Extraction ofslurry with Soil

Fig. 10.20: Slurry Shield TBM

While the use of TBMs relieves the need for largenumber of workers at high pressures, a caissonsystem is sometimes formed at the cutting head forslurry shield TBMs. Workers entering this space forinspection, maintenance and repair need to bemedically cleared as “fit to dive” and trained in theoperation of the locks.

Slurry shield TBMs are particularly suitable for usein granular soi l (sand, gravel etc.) andheterogeneous soft ground, though they can alsobe used in other terrain, even if it includes hardrock sections. There might be clogging and difficultyseparating the spoil from the slurry if there is clayin the soil. These TBMs can be used in ground withhigh permeability, but if there is high water pressurespecial slurry has to be used to form a watertightcake on the excavation walls.

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(IV) Earth Pressure Balance TBM: Earth PressureBalance Machines are used in soft ground with lessthan 7 bar of pressure. An earth pressure balancemachine has a full face cutter head. The cutter headdoes not use disc cutters only, but instead acombination of tungsten carbide cutting bits, carbidedisc cutters and/or hard rock disc cutters. The EPBgets its name because it is capable of holding upsoft ground by maintaining a balance between earthand pressure (Fig. 10.21). The TBM operator andautomated systems keep the rate of soil removalequal to the rate of machine advance. Thus, a stableenvironment is maintained.

Support of the face is achieved by pressurizing themud, formed of the excavated soil in the cutter headchamber. In most cases, water and some otheradditives (e.g. polymer foams) are added to renderthe excavated soil supple.

1. Cutter head 2. Shield 3. Screw conveyor

4. Belt Conveyor

Fig. 10.21: Earth Pressure Balance Machine

EPBMs are particularly suitable for soils which, afterchurning, are likely to be of a consistency capableof transmitting the pressure in the cutter headchamber. They can handle ground of quite highpermeability, and are also capable of working inground with occasional discontinuities requiringlocalized confinement.

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Both types (EPB and SS) are capable of reducingthe risk of surface subsidence and voids if operatedproperly and if the ground conditions are welldocumented. This makes them very suitable forurban tunnelling.

10.3 Special purpose TBMs: In addition toabove, there are Special Purpose TBMs also.Some of these are:(I) Reaming Boring Machines - allows a tunnel

made using a TBM (pilot tunnel) to bewidened (reaming).

(II) Raise Borer - used for shaft excavation whichenables the top-to-bottom reaming of a smalldiameter pilot tunnel created using a drillingrig.

(III) Mixed Face TBMs – allows tunneling undermixed face conditions.

(IV) Multi-Mode TBMs – can operate in differentmodes with appropriate modifications toconfiguration & support techniques.

11. Back-up systems for TBM: Behind all types oftunnel boring machines, inside the finished part of thetunnel, are trailing support decks known as the back-up system. Support mechanisms located on the back-up can include conveyors or other systems for muckremoval, slurry pipelines (if applicable), control rooms,electrical systems, dust removal, ventilation andmechanisms for transport of pre-cast segments.

12. Selection of TBM: Careful and comprehensiveanalysis should be made to select proper machine fortunneling taking into considerations its reliability, safety,cost efficiency and the working conditions. In particular,the following factors should be analyzed:

• Suitability to the anticipated geologicalconditions.

• Applicability of supplementary supportingmethods, if necessary.

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• Tunnel alignment and length.• Availability of space necessary for auxiliary

facilities behind the machine and around theaccess tunnels.

• Safety of tunneling and other related works.

13. Excavation Methods for Soft Ground Tunnels:Two principal Methods for tunneling in soft ground are:

13.1 Multiple Drift Method: This method hasalready been described under “Excavation methodsfor rock tunnels”. Forepoling (Ref.: Chapter-14 fordetails) is normally done before doing excavation,particularly in heading portion.

13.2 Excavation by Tunnel Shields: Thismethod has also been discussed in Para-7 above.

The control of ground water is of utmost importance insoft ground tunneling. To control groundwater,dewatering & grouting are the most common methods.Methods using “compressed air” & “freezing” are alsosometimes used. Further details on this aspect may beseen in Chapter-14.

14. Stipulation about machinery in contractdocument: The type and number of machines requiredfor construction of a particular tunnel will primarilydepend on excavation method used for tunnelling andlength of tunnel. Availability of sufficient number ofmachines, commensurate with desired rate of progress,of appropriate type at work site is prerequisite forsuccessful and timely completion of work. As such,type (viz. minimum specifications/ capacity/output rate)and minimum number of machines required forconstruction of a particular tunnel should be stipulatedin the tender document.

The list of such machineries may include excavators,road headers, drilling jumbos/boomers, loaders, dumptrucks, dozers, shortcrete machines (with/withoutrobotic arm), transit mixers, concrete pump, totalstations etc.

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blank

Tunneling Method or Philosophy does not mean thetunnel excavation method only, but it covers all activitiesrelevant to tunnel construction in totality i.e. datacollection (geotechnical investigation, surveying etc.),design of support systems, excavation methods oftunnel, instrumentation & monitoring etc. VariousTunneling Philosophies (generally called as TunnelingMethods) are discussed in this chapter.

1. Conventional Tunnelling: The definition of“Conventional Tunneling” is rather arbitrary, and subjectto variations, depending on the context. For IndianRailway context, the term conventional tunneling is usedfor any method other than Mechanized or Observationalmethods of Tunneling.

Design of support system in conventional tunneling canbe done using any of the approach i.e. Empirical,Analytical or Numerical (Ref. Chapter-8 & 9), using anytype of support (Ref. Chapter-7) as per the design. Theexcavation can be done by Drill & Blast Method (withfull face or partial face excavation) or By Road header(with full face or partial face excavation). The supportscan be initial/primary supports with or withoutsecondary/final supports. The basic principle used inthis method is that the load coming on the supports isestimated and the supports are designed to resist thisload.

In case exceptional ground conditions are encountered,the Conventional tunneling Method can react with avariety of auxiliary construction technologies like:

• Grouting: Consolidation grouting, fissuregrouting, pressure grouting, compensation

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TUNNELLING METHODS/PHILOSOPHIES

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grouting etc.• Technologies to stabilize and improve the

ground ahead of the actual tunnel face likeforepoling, pipe umbrella, horizontal jetgrouting, ground freezing etc.

2. Ground (Rock) Support Interaction: The term“tunnel lining” or “tunnel support” encompasses a broadrange of concepts, materials, construction methods anddetails (Ref. 27). Several common characteristicspervade all systems:(i) Tunnel lining is not an independent structure

acted upon by well-defined loads, and itsdeformation is not governed by its own internalelastic resistance. The loads acting on a tunnelare ill defined, and its behaviour is governed bythe properties of the surrounding ground. Designof a tunnel lining is not a structural problem,but a ground-structure interaction problem, withthe emphasis on ground.

(ii) Tunnel lining behaviour is a four-dimensionalproblem. During construction, ground conditionsat the tunnel heading involve both transversearching and longitudinal arching or cantileveringfrom the unexcavated face. All ground propertiesare time dependent, particularly in short term,which leads to the commonly observedphenomenon of stand-up time, without whichmost practical tunnel construction methodswould be impossible. The timing of lininginstallation is an important variable.

(iii) The most serious structural problemsencountered with actual lining behaviour arerelated to absence of support – inadvertentvoids left behind the lining – rather than tointensity and distribution of load.

(iv) In virtually all cases, the bending strength andstiffness of structural linings are small comparedwith those of the surrounding ground. The

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properties of the ground control the deformationof the lining, and changing the properties of thelining will not significantly change thisdeformation. The proper criterion for judginglining behaviour is therefore not adequatestrength to resist bending stresses, butadequate ductility to conform to imposeddeformations.

2.1 Ground Reaction Curve (GRC) and SupportReaction Curve (SRC): The radial deformation/displacement, which occurs in the vicinity of anadvancing tunnel face (Fig. 11.01) (Hoek, 1999):• Begins a certain distance ahead of the tunnel

face (about two and one-half tunneldiameters).

• Reaches about one third of its final value atthe tunnel face.

• Reaches its maximum value at about fourand one-half tunnel diameters behind theface.

It is important to note that even for anunsupported tunnel, the tunnel face provides an“apparent support pressure”. It is this apparentsupport pressure that provides the stability to givesufficient stand-up time for the actual support tobe installed.

The rock mass around excavated tunnel enteringinto plastic state does not necessarily mean thatthe tunnel collapses. This material can still haveconsiderable strength, provided that thickness ofthe plastic zone is small compared with the tunnelradius, and only evidence of failure may be a fewfresh cracks and a minor amount of raveling. Onthe other hand, when a large plastic zone isformed and large convergence of tunnel occurs,the loosening of the failed rock mass can lead tosevere spalling and eventual collapse of anunsupported tunnel. The primary function of

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support is to control the convergence of excavatedtunnel and to prevent the loosening, which canlead to collapse of the tunnel. The supports play amajor role in controlling tunnel deformation (Hoeket al. 1995).

Fig. 11.01: Support Pressure (pi) at differentpositions

Fig. 11.02: Ground Reaction Curve

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Assume a circular tunnel of radius ro is subjectedto in-situ stress po and a uniform internal supportpressure pi. Failure of the rock mass surroundingthe tunnel occurs when the pi is less than a criticalsupport pressure pcr. If pi is greater than pcr, nofailure occurs and the rock mass surrounding thetunnel is in elastic stage.

The plot between the Support Pressure (pi) andradial convergence/ squeezing (ui) is called GroundReaction Curve (GRC), or Fenner-Pacher Curves(Fig. 11.02). This plot shows that:• Zero displacement when the support pressure

equals the hydrostatic stress (pi = po)• Elastic displacement uie for po > pi > pcr

• Plastic displacement uip for pi < pcr

• Maximum displacement when the supportpressure equals zero

For a given tunnel radius and in-situ stress, theshape of the ground reaction curve depends on therock mass failure criterion which is assumed andthe specific rock mass characteristics.

In order to complete the rock support interactionanalysis, the reaction curve for the rock supportmust be determined. This is a function of threecomponents:

(i) The convergence that has occurred beforesupport is installed.

(ii) The stiffness of the support system.(iii) The capacity of the support system.

A certain amount of deformation takes place aheadof the advancing face of the tunnel. In addition,there is a gap between the excavation face andthe closest installed support element. Therefore,further deformation occurs before the supportbecomes effective. This total initial displacement isuso (Fig. 11.03).

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Once support is installed and it is in full/effectivecontact with rock mass, the support starts todeform elastically. The plot of support pressureand elastic deformation of support is calledSupport Reaction Curve (SRC). Slope of this curvegives stiffness of the support. The maximumelastic displacement which can be accommodatedby the support system is usm and the maximumsupport pressure psm is defined by the yield of thesupport system.

Equilibrium is achieved if Support Reaction Curveintersects the Ground Reaction Curve before eitherof these curves has progressed too far. If thesupport is installed too late, the rock mass mayhave already deformed to the extent thatloosening of the failed material is irreversible. Onthe other hand, if the capacity of the support isinadequate then yield of the support may occurbefore the rock mass deformation curve isintersected. In either of these cases, the supportsystem will be ineffective.

Fig. 11.03: Support Reaction Curve

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Various types of supports, depending upon their“Stiffness” and “Installation Timing”, and theirimplications are as under (Fig. 11.04) (Brady andBrown, 2006):

Support-1: Too Stiff; Attracts excessive load; Mayfail causing catastrophic failure of rock mass.

Support-2: Lower stiffness; Good soultion;Support not loaded excessively.

Support-3: Much lower Stiffncess; Acceptable onlytemporarily; Situation dangerous.

Support-4: Same Stiffness as Support-2; Installedlate permitting excessive convergence; Supportmay become overstressed before equilibrium.

Radial displacement, uiFig. 11.04: Right type of Support

The Ground (Rock) Support Interaction isrepresented by “Ground Reaction Curves” and“Support Reaction Curves”. These curves can beplotted by using closed form solutions given byDeere et al. (1970), Hoek & Brown (1980) etc., for

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circular tunnels with hydrostatic in-situ stresses.They can also be plotted by using some softwarelike “RocSupport”. For complex problems,Numerical Methods (e.g. FLAC software) are used.

3. Observational Method of Tunnelling: In theconventional tunnelling, the support is normally aimedto be provided at zero displacement/convergence,though some displacement always takes place beforethe support is put in position and it starts resisting thedisplacement. Thus, in this method, the stiffnessrequired from the support is relatively high (Ref. Fig.11.03). If some amount of displacement/convergenceis permitted (within the permissible value from point ofview of serviceability of the tunnel and the extent ofplastic zone remaining within the critical limit), therequired stiffness of the support system will reduce.This in-turn facilitates lighter (and more flexible) supportsystem design, leading to overall reduction/ economyin the support system. But for this, the displacement/convergence of tunnel and support (and some otherparameters also) need to be monitored to ensure thatsupport is provided at the right time and it is of rightstiffness. This approach is called “Observational Methodof Tunnelling”, wherein support is designed based onobservation of displacement and by controlling thedeformation. In this approach:

(i) Rock support is applied based on preliminaryassessment (using empirical/analytical/numerical approach or some software).

(ii) Ground displacement is monitored with timeand the support is installed at the specifieddisplacement.

(iii) If the support is insufficient, displacementcontinues. In such case, additional support isapplied to ensure that displacement isstabilized.

(iv) The tunnel displacements and supportpressures are monitored to develop a suitablesupport system to stabilize the tunnel.

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When the tunnel behaviour is not known withconfidence, this approach is very useful. This is alsocalled “build-as-you-go approach”. In Eurocode-7, the“Observational Approach” is defined as under (Clause2.7, Ref. 28).

(1) When prediction of geotechnical behaviour isdifficult, it can be appropriate to apply theapproach known as “the observationalmethod”, in which the design is reviewedduring construction.

(2) The following requirements shall be metbefore construction is started:• acceptable limits of behaviour shall be

established;• the range of possible behaviour shall be

assessed and it shall be shown thatthere is an acceptable probability thatthe actual behaviour will be within theacceptable limits;

• a plan of monitoring shall be devised,which will reveal whether the actualbehaviour lies within the acceptablelimits. The monitoring shall make thisclear at a sufficiently early stage, andwith sufficiently short intervals to allowcontingency actions to be undertakensuccessfully;

• the response time of the instrumentsand the procedure for analyzing theresults shall be sufficiently rapid inrelation to the possible evolution of thesystem;

• a plan of contingency actions shall bedevised which may be adopted if themonitoring reveals behaviour outsideacceptable limits.

(3) During construction the monitoring shall becarried out as planned.

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(4) The results of monitoring shall be assessed atappropriate stages and the plannedcontingency actions shall be put in operationif the limits of behaviour are exceeded.

(5) Monitoring equipment shall either be replacedor extended if it fails to supply reliable dataof appropriate type or in sufficient quantity.

4. New Austrian Tunnelling Method (NATM): NewAustrian Tunnelling Method (NATM) was developed bythe Austrians Ladislaus von Rabcewicz, Leopold Müllerand Franz Pacher in the 1950s. The name was introducedin 1962 (Rabcewicz, 1963) to distinguish it from the“Austrian Tunnelling Method”, today referred to as the“Old Austrian Tunnelling Method”. It is one of the mostpopular observational method of tunnelling.

The method has often been mentioned as “valueengineered version of tunneling”, due to its use of lightsupports. It has long been understood that the ground,if allowed to deform slightly, is capable of contributingto its own support. The NATM, with its use of modernmeans of monitoring and surface stabilization, such asshotcrete and rock bolts, uti l izes this effectsystematically. The main idea is to use the geologicalstress of the surrounding rock mass to stabilize thetunnel itself.

4.1 Definition of NATM: Austrian NationalCommittee defines NATM as “a concept whichmakes the ground (rock or soil) surrounding thevoid a supporting construction element through theactivation of a ground supporting arch”.

Another useful definition as given by H. Lauffer is“NATM is a tunnelling method in which excavationand support procedures, as well as measures toimprove the ground -which should be distorted aslow as possible - depend on observations ofdeformation and are continuously adjusted to theencountered conditions”.

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4.2 Principles of NATM: The NATM is not a set ofspecific excavation and support techniques and hasoften been referred to as a “design-as-you-go”approach to tunnelling providing an optimizedsupport based on observed ground conditions butmore correctly it is a “design-as-you-monitor”approach based on observed convergence anddivergence in the lining as well as prevailing rockconditions. There are seven features on whichNATM is based:(i) Mobilization of the strength of rock mass –

The method relies on the inherent strength ofthe surrounding rock mass being conservedas the main component of tunnel support.

(ii) Shotcrete protection – Loosening andexcessive rock deformation must beminimized. This is achieved by applying athin layer of shotcrete immediately after faceadvance. Shotcrete and rock bolts appliedclose to the excavation face help to maintainthe integrity of the rock mass.

(iii) Measurements – Every deformation of theexcavation must be measured. NATM requiresinstallation of sophisticated measurementinstrumentation. It is embedded in lining,ground, and boreholes. This ensures thatsupport is not installed too early or too late.

(iv) Flexible support – The primary lining is thinand reflects recent strata conditions. Activerather than passive support is used and thetunnel is strengthened not by a thickerconcrete lining but by a flexible combinationof rock bolts, wire mesh and steel ribs. Thelining should not be too stiff or too weak.

(v) Closing of invert – Quickly closing the invertand creating a load-bearing ring is important.It is crucial in soft ground tunnels where nosection of the tunnel should be left open eventemporarily.

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(vi) Contractual arrangements – Since the NATMis based on monitoring measurements,changes in support and construction methodare possible. This is possible only if thecontractual system enables those changes.

(vii) Rock mass classification determines supportmeasures – There are several main rockclasses for tunnels and corresponding supportsystems for each. These serve as theguidelines for tunnel reinforcement.

4.3 Construction Sequence: The constr-uctionprocess in NATM is normally as follows:

• Excavation: The tunnel advance can beachieved using blasting, a partial face boringmachine or simply using an excavator,depending on the ground conditions. Generally,the advancement is spatially and timelystaggered in the heading, benching and invert.

• Sealing the exposed ground if necessary.• Mucking.• Installation of lattice girders or mesh

reinforcement, and application of shotcrete.Depending on the quality of the ground thesupport might be installed first before thespoil is removed.

• Potential installation of a second layer ofreinforcement and application of shotcrete.

• Installation of anchors.• Construction of inner/secondary lining.

4.4 Limitations of NATM: In order to use NATM,the ground has to be capable of supporting itselfover the length of each advance section, whichmeans that the ground must have a stand-uptime. The limit of this construction technique isreached when the stand-up time of the ground hasto be improved by artificial measures, such asfreezing or grout injection.

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4.5 Pre-requisites for NATM• Balanced Contract Structure- Providing for

equitable risk sharing and flexible approach asper varying technical requirements.

• Qualified Personnel-Particularly for ensuringquality of shotcrete, rock bolts.

• Better Site Management for:• Coping with unseen events.• Proper application of the observational

method.• Elimination of human errors.• Continuous Monitoring of geology- By

qualified & experienced geologists, forinterpretation of exploration data & keepingthorough geological record.

4.6 Advantages of NATM• Flexibility to adopt different excavation

geometries and very large cross sections.• Lower cost requirements for the tunnel

equipment at the beginning of the project.• Flexibility to install additional support

measures, rock bolts, dowels, steel ribs, ifrequired.

• Easy to install a waterproof membrane.• Flexibility to monitor deformation and stress

redistribution so that necessary precautionscan be taken.

5. Norwegian Method of Tunneling (NMT): This isalso an observational method, quite similar to NATM.The difference between NATM and NMT are mainly asunder:

(i) NATM is most suitable for soft ground whichcan be machine or hand excavated, wherejointing and over break are not dominant,where a smooth profile can often be formedand where a complete load bearing ring can be

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(and often should be) established. NMT ismost suitable for harder ground, where jointingand over break are dominant, and where drilland blasting or hard rock TBMs are the mostusual methods of excavation.

(ii) In NMT, bolting is the dominant form of rocksupport since it mobilizes the strength ofsurrounding rock mass in the best possibleway. Rigid steel sets or lattice girders areinappropriate in Norway’s harder rocks due tothe potential over break. Bolting and SFRS(Steel Fiber reinforced shotcrete) are the twomost versatile tunnel support methods. Athick load bearing ring (RRS – reinforced ribof shotcrete) can be formed as needed, andmatches an uneven profile better than latticegirders or steel sets.

(iii) In NMT, Q-system is used for regulating thedescription of rock mass conditions andsupport recommendations. Instrumentationand monitoring is done only in critical cases.

(iv) The shotcrete used in NMT is only SFRS.Concrete lining is used only in extremeconditions such as when tunneling throughfault zones, swelling clay and very weak rockthat may squeeze.

6. ADECO-RS Method of Tunneling: ADECO-RS isabbreviation of Italian word which is translated in Englishas “Analysis of Controlled Deformation in Rocks andSoils”. This method was developed by Pietro Lunardi in1980 in Italy. It has been used in many tunnels in Italyand in some tunnels outside Italy.

6.1 Important Components in Tunneling: Thethree important components in tunneling operationwhich influence the stabilization of the ground,after creation of the cavity for tunnelling, areMedium (properties of the ground through whichtunneling is being done), Action (whole set of

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operations to excavate the tunnel) and Reaction(deformation response of the medium toexcavation) (Fig. 11.05).

Fig. 11.05: Important Components in Tunnelling

6.2 Deformation Response of the Medium: Anypoint in rock mass just ahead of tunnel excavationface is in tri-axial state of stress. With the tunnelexcavation passing from this point, the rock massremoval from this point modifies the stress field atthis point (with minor principal stress along thetunnel axis becoming zero). Thus, the stress fieldat this point changes from tri-axial to plane stressstate, due to tunnel excavation advance, with theconfinement pressure at the excavation faceprogressively reducing to zero. Depending uponthe medium, the stress state and the way in whichface is advanced, the deformation response of themedium may be of three types (Fig. 11.06).

Fig. 11.06: Types of Deformation Response

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If the stresses around the periphery of the cavity,near the face, are in elastic range and thedeformation is limited/absolutely negligible, theface remains “Stable” and the redistribution ofstresses around the cavity (the “Arch Effect”) isproduced by natural means close to the profile ofthe excavation (Fig. 11.07).

Fig. 11.07: Stable Face

If the stresses around the periphery of the cavity,near the face, are in elasto-plastic range, then theface will deform in an elasto-plastic mannertowards the interior of the cavity and it gives riseto a condition of “short term stability”. This meansthat in the absence of intervention, plasticization istriggered, which by propagating radially andlongitudinally from the walls of excavation,produces a shift of the “arch effect” away from thetunnel periphery further into the rock mass.Development of elasto-plastic zone can only becontrolled by intervention to stabilize the ground(Fig. 11.08).

If the stresses around the periphery of the cavity,near the face, are in failure (plastic) range, thenthe deformation response is unacceptable and acondition of instability exists in the ground aheadof the face, which makes the formation of “archeffect” impossible (Fig. 11.09).

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Fig. 11.08: Stable Face in Short Term

Fig. 11.09: Unstable Face

6.3 Some terms used in this methodology:• Advance Core: The volume of ground that

lies ahead of the face, virtually cylindrical inshape, with the height and diameter of thecylinder the same as the diameter of thetunnel.

• Extrusion: The primary component of thedeformation response of the medium to theaction of excavation that develops largelyinside the advance core. It manifests on thesurface of the face along the longitudinal axisof the tunnel and its geometry is either moreor less axial-symmetrical (bellying of theface) or that of gravitational churning(rotation of the face).

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• Pre-convergence of the cavity:Convergence of the tunnel profile ahead ofthe face (the area yet to be excavated),strictly dependent on the relationshipbetween the strength and deformationproperties of the advance core and its originalstress state.

6.4 Experimental and Theoretical Research:This research in Italy, over a period of 20-30years, has brought out following points:(i) It is important to keep “Excavation Rates”

high and constant. This prevents “extrusion”and “pre-convergence”, which are startingpoint for subsequent “Convergence” of Cavity.Pre-convergence can be calculated, usingsimple volumetric calculations, though notpossible to measure it directly.

(ii) Chronologically, “deformation in the cavity(convergence)” normally follows and isdependent on “deformation in the core atface (extrusion)”. There is close correlationbetween the magnitude of “extrusion allowedin advance core” and the “convergencemanifested after passage of the face”. Bothof these decrease as the “rigidity of core” isincreased.

(iii) The advance core extrudes with 3 types ofdeformations (Fig. 11.10), depending on thematerial involved and the stress state.

Fig. 11.10: Types of Core Extrusion

(iv) There is close connection between the “failureof the core face” and “collapse of the cavity”,even if it has already been stabilized.

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“Deformation of advance core” constitutes thefactor most capable of conditioning the“deformation response of the cavity” andmust therefore be true cause of it.

(v) It seems logical to make use of the “core” asa new instrument for controlling it, by actingon its rigidity with appropriate“interventions”.

6.5 Type of Interventions: The “interventions”for increasing rigidity of the “core” can be broadlyof two types:

6.5.1 Protective Intervention: Theseinterventions channelize the stresses around theadvance core to perform protective function, therebyensuring that natural strength and deformationproperties of the core are conserved. Some suchinterventions are:(A)Putting drainage pipes in the advance core, to

drain out the water from it and preventreduction in its’ strength due to presence ofwater.

(B)Full face sub-horizontal jet grouting, whichcreates a grout ring around the advance core,before excavation (Fig. 11.11).

Fig. 11.11: Full face sub-horizontal Jet Grouting

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Fig. 11.12: Mechanical Pre-cutting Machine

(C)Full face mechanical pre-cutting, using amachine (Fig. 11.12) to create a slit aroundthe advance core, which is filled usingconcrete or grout.

6.5.2 Reinforcement Interventions: Theseinterventions act directly on the consistency of theadvance core to improve its’ natural strength anddeformation properties by means of appropriateground reinforcement techniques. Most commonlyused such reinforcement is fiber glass structuralelements (Fig. 11.13), which are placed in holesdrilled in advance core. Advantage of using theseelements is that after excavation, it is very easy tobreak/snap the elements hanging from the face.

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Fig. 11.13: Fiber Glass Reinforcement Elements

6.6 Behaviour Categories of Core Face: In fullface excavation, the behaviour of core face afterexcavation can be categorised in followingcategories:

(A) “Category–A” Stable Core Face (Rock TypeBehaviour): The face as a whole is stable andonly local instability is found due to the fall of isolatedblocks (Fig. 11.14). In this case, stabilization isneeded for preventing deterioration of rock and tomaintain profile of the excavation.

Fig. 11.14: Stable Core Face

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(B) “Category–B” Stable Core Face in ShortTerm (Cohesive Type Behaviour): Instabilitymanifests in the form of widespread spalling at theface and around the cavity (Fig. 11.15). Sufficienttime is available to employ traditional radialconfinement measures, after passage of face. Insome circumstances, it may be necessary to resortto pre-confinement of cavity, to contain deformationwithin acceptable limits. Presence of water shall beprevented by suitable drainage arrangements.

Fig. 11.15: Stable Core Face in Short Term

(C) “Category–C” Unstable Core Face (LooseGround Type Behaviour): Deformation isunacceptable because it develops immediately infailure range (Fig. 11.16). Ground reinforcement isneeded ahead of face to develop pre-confinementaction. Presence of water is absolutely unacceptableand shall be prevented by drainage arrangements.

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Fig. 11.16: U nstable Core Face

6.7 Stages of ADECO-RS Approach: There aretwo stages in this methodology:

(A) Design Stage: This stage consists of followingthree phases:

(I) Survey Phase, to determine characteristicsof the medium. It involves collecting fullinformation about geological / geo morp-hological/ hydro-geological data of the area, location &definition of terrain through which the alignmentpasses, tectonics/geological structure & the stressstate of rock mass, hydro-geological regime of rockmass and geo-mechanical characteristics of thematerials.

(II) Diagnosis Phase to divide the tunnelsections in Category A/B/C, defining the detailsof deformation development and types of loadsmobilized by excavation. Analytical and/orNumerical Methods are used to predict thebehavior category and deformation response forvarious sections of the tunnel alignment.

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(III) Therapy Phase to decide type ofconfinement and/or pre-confin-ement and testtheir effectiveness. It involves following steps:(a) Decide the pre-confinement and/orconfinement action (in the context of behaviourCategory A/B/C).(b) Select suitable pre-confinement and/orconfinement intervention based on recentadvances in the technology,(c) Design & test the proposed intervention usingmathematical modelling.

(B) Construction Stage: This stage consists offollowing three phases:

(I) Operational Phase in which stabilizationmeasures are employed, followed by excavationoperation. This phase proceeds hand-in-hand with“Monitoring Phase”. The tunnel section, specifiedby design engineer, guides in this phase abouttype of intervention to be performed. Success ofthis phase depends on the accuracy of thepredictions made in “diagnosis phase” and on the“design decisions” made as a consequence. Fullface advance shall be employed, whereverpossible, giving concave shape to the face andavoiding over-breaks. All stabilization works shallbe performed rapidly.

(II) Monitoring Phase in which deformationsare measured to verify accuracy of predictionsmade and to fine tune the design further. Duringservice life, monitoring is continued for safety ofthe tunnel. Reliability of prediction made in“diagnosis” and “therapy” phases are tested here.This phase starts as soon as construction beginsand sometimes even before it. Scale ofinstrumentation to be done depends upon thecategory of core face also (Category A, B or C).

(III)Final Design Adjustment Phase based onthe results of the monitoring.

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6.8 Tunnelling with ADECO-RS Approach:Typical stages involved in tunnelling with ADECO-RSwill be as under (any particular stage may or maynot be involved depending upon type of ground):

• Phase-1: Reinforcement of the core faceusing suitable reinforcement elements (e.g.Fiber glass tubular elements).

• Phase-2: Carry out full face excavation.• Phase-3: Placement of a layer of shotcrete

on newly excavated face.• Phase-4: Cavity pre-confinement by a

suitable method (e.g. Mechanical pre-cutting).• Phase-5: Providing other support measures,

behind the excavation face.• Phase-6: The cycle continues repeating

Phases- 2, 3, 4 & 5, until required advancelength is reached.

• Phase-7: Excavation and casting of invert.• Phase-8: Placing waterproofing system and

casting of final lining.

6.9 Difference between NATM and ADECO-RS:The major differences between NATM (and derivedmethods) and ADECO-RS approach are as under:

(i) In ADECO-RS methodology, it is always fullface excavation whereas in NATM this it is notso necessarily.

(ii) In NATM, only “excavation” behind theexcavation face is reckoned for design andmonitored later on. But in ADECO-RSmethod, “extrusion” of core face as well as“pre-convergence” of rock mass ahead ofexcavation face are considered.

(iii) In NATM, the “intervention” in the form ofsupports is provided only in the cavity behindthe excavation face. But in ADECO-RSmethod, pre-confinement and reinforcement ofadvance I also done, if needed.

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6.10 Case History with ADECO-RS Method:This case history pertains to “Construction ofTartaiguille Tunnel for TGV Mediteranee HighSpeed Rail Line from Marseilles to Lyon in France”.

Total length of tunnel was 2330m, with crosssection area of 180 m2. Tunnel Advance began inFeb’1996 with NATM Principle. ConstructionSequence followed was: Top heading with Roadheader, Bench excavation with Excavator Hammer,Shotcrete (25 cm thick), Steel Ribs (HEB 240),End Anchored Radial Bolts (4m long), micro pilesbeneath base of ribs, casting of side wall, castingof invert, water proofing layer and Concrete Lining(70 cm thick). In Sept’1996, it became practicallyimpossible to continue, due to heavy swelling inMontmorillonite formation.

Convergence observed was 60mm in Heading and150mm in Benching, leading to cracks in shotcreteand its spalling. SNCF constituted a Study Groupand major European experts were consulted. Nonecould offer Safety and Reliability, with requiredcompletion times; except Rocksoil S.p.A. Italy(with whom Pietro Lunardi was associated).

Work of balance 860m length was awarded toRocksoil in March’1997. Tunnel advance resumedin July’1997, with ADECO-RS methodology. Thetunneling work was completed in July’1998, withgood advance rate. The progress rate chart isshown in Fig. 11.17.

The photos of this tunnel work during progress areshown in Fig. 11.18 and the view of finishedtunnel is shown in Fig. 11.19.

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Fig. 11.17: Progress Rate Chart

Fig. 11.18: Work in Progress

Fig. 11.19: Finished Tunnel

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INSTRUMENTATION AND MONITORING OF TUNNELS

In past, there was little emphasis on instrumentation,although ad-hoc measurements were sometimes madeif the failure appeared imminent. However, because ofproblems faced in tunnelling like rock bursts, supportfailures, water inflow etc., the field instrumentationgained popularity among both designers andconstruction engineers. In recent times, the geo-technical instrumentation and monitoring is generallyan integral part of the tunneling project.

1. Need for Geotechnical Instrumentation: Thereis a limit to geotechnical data acquisition and no amountof expenditure can result in 100% data acquisition,leaving at least 20% uncertainty at best (Fig. 12.01).In many large Indian projects, this is supposed to beupto 80% also.

Fig. 12.01: Extent of Geotechnical Investigation

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INSTRUMENTATION ANDMONITORING OF TUNNELS

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Lack of geotechnical investigation and consequent datainaccuracies have potential of leading to poor designs,additional construction costs and long term maintenanceliabilities. This brings into picture the need forinstrumentation and monitoring. In addition, theinstrumentation and monitoring becomes essential, if“observation approach” of tunneling is followed.

2. Purpose of Instrumentation and Monitoring:In tunnelling projects, it serves following purposes:

2.1 Design and Design Verification:• Can be used to obtain data from pilot tunnels

or shafts, which can be used for design oftunnel.

• Helps in deciding design of final supportsystem.

• In “observational approach”, monitoring ofdisplacements and loads is essential part ofthe construction process.

• To verify the assumptions made in the initialdesign and to verify that performance is “aspredicted’ or not.

• Data from initial phase is used to improvisethe support design in later phases.

2.2 Construction Control:• Instruments are used to monitor the effects

of construction.• Instrumented data can help in deciding how

fast construction can proceed without risk offailure.

• Can be used to diagnose flaws in thecontractor’s construction methodology andindicate the required improvements.

2.3 Safety/Stability:• Monitoring the stability of excavation or

adequacy of ground support also serves asafety function, by warning the potential for

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ground failure during construction, in somecases during service life.

• Checking adjacent structures and facilities fortheir safety and serviceability due to tunnelconstruction.

2.4 Regulatory/Environmental requirements:To ensure compliance with regulatory/environmental requirements (e.g. groundwaterlowering, ground settlements, vibrations etc.).

2.5 Performance Monitoring: To monitor in-service performance of structure (e.g. monitoringloads on rock bolts and movements within a tunnelcan provide an indication of its performance)

2.6 Contractual Documentation: Monitoring datacan also be used for avoiding/settling disputeswith the contractor.

The geotechnical instrumentation and monitoringby a geotechnical engineer, for a tunnel, is quitesimilar to the way a doctor treats his patient (Fig.12.02).

Fig. 12.02: Geotechnical Monitoring Cycle

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3. Items to consider in Instrumentation andMonitoring Program:

3.1 Project conditions: The official(s) planningand designing should be familiar with projectconditions including type & layout of tunnel,subsurface stratigraphy, engineering properties ofsubsurface materials, groundwater conditions,status of nearby structures or other facilities,environmental conditions and planned constructionmethod.

3.2 Mechanisms that control behavior: Beforedefining a program of instrumentation andmonitoring, one or more working hypotheses mustbe established for mechanisms that are likely tocontrol behavior. Instrumentation should then beplanned around these hypotheses.

3.3 Purpose of Instrumentation: Everyinstrument should be selected and placed to assistin answering a specific question. If there is noquestion, there should be no instrumentation.

3.4 Parameters to be Monitored: Following arethe typically monitored parameters:• Convergence• Crown settlement• Floor heave• Load in rock bolts/anchors• Stress in shotcrete/ concrete• Groundwater pressure• Water pressure acting on lining• Surface settlement• Vertical and horizontal deformation of

buildings and other structures• Vertical and horizontal deformation of the

ground at depth

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3.5 Expected values of parameters recorded:An estimate of maximum possible value willdetermine the instrument range. The minimumvalue of interest determines the instrumentsensitivity/accuracy.

3.6 Instrument selection: Reliability is the mostdesirable feature when selecting monitoringinstruments. First lowest cost should not dominatethe selection of an instrument. A comparison ofthe overall cost of procurement, calibration,installation, maintenance, reading and dataprocessing should be made. The least expensiveinstrument necessarily may not result in leastoverall, as cost of instruments is usually a minorpart of the overall cost.

3.7 Location for Installation: Selection oflocations for the instruments should be based onpredicted behavior of the tunnel or shaft. Apractical approach to select instrument locationsinvolves first identifying areas of particular concern(e.g. structurally weak zones or areas that aremost heavily loaded) and locating requisiteinstruments there. Then select zones wherepredicted behavior is considered representative ofbehavior as a whole (primary instrumentedsections). Lastly, install simple instrumentation ata number of secondary instrumented sections toserve as indices of comparative behavior. If thebehavior at one or more of the secondary sectionsappears to be significantly different from theprimary sections, additional instruments can beinstalled at the secondary section as constructionprogresses.

3.8 Threshold Values: A predeterminationshould be made of instrumentation readings thatwill indicate need for remedial action. The conceptof green (all is well), yellow (need for cautionarymeasures including an increase in monitoringfrequency) and red (need for timely remedial

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action) response values should be adopted.

3.9 Remedial action and Implementation:Assign duties and responsibilities for all phases,including planning, instrument procurement,calibration, installation, maintenance, reading, dataprocessing/presentation/ interpretation, datareporting and deciding on implementation of theresults. When duties are assigned for monitoring,the party with the greatest interest in the datashould be given direct responsibility for producingit accurately.

3.10 Factors affecting measurements: Forproper interpretation of site instrumentation data,it is essential to monitor and record site activitiesand climatic conditions that can affect themeasurements obtained.

3.11 Ensuring data correctness: In criticalsituations, more than one of the same type ofinstrument may be used to provide a backupsystem even when its accuracy is significantly lessthan that of the primary system. Repeatabilitycan also give a clue to data correctness. It is oftenworthwhile to take many readings over a shorttime span to determine whether a lack of normalrepeatability indicates suspect data. Also plan forregular calibration and maintenance of instrumentsto ensure correctness of date recorded by them.

4. Instruments used: The instruments typically usedfor monitoring various parameters in tunnels are asunder:

4.1 Surface Settlement: Surface settlement/movement is measured to forewarn about surfacesettlement and/or to monitor stability of open cutexcavations. The instruments used for this are asunder:

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Fig. 12.03: Borehole Extensometer

(A) Borehole Extensometer: It is used to monitordisplacement or deformation in soil and rock massat various depths (Fig. 12.03). It normally consistsof anchors, rods, protective tubes and a VW(vibrating wire) displacement sensor. The anchorscoupled to the rod are installed in the borehole. Theanchors and anchor rods referenced to stable ground,move up or down as movement in borehole occurs.This changes tension of the vibrating wire insidethe VW transducer, which is transmitted through acable to the readout unit.

(B) Multiple Point Borehole Extensometer: Theyare installed in borehole to monitor displacementsat various depths. Relative movements betweenthe anchors (which are fixed at different depths)and the reference head (which is common for allanchors) are measured (Fig. 12.04).

Fig. 12.04: Multiple Point Borehole Extensometer

4.2 Sub-surface Horizontal Movement: This ismeasured to forewarn about tunnel instability bymonitoring ground movement towards excavation orheading and/or to verify adequacy of rock bolting/

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other supports. The instruments used for this are asunder:

Fig. 12.05

(A) Borehole Extensometer: The BoreholeExtensometer, details given in Para 4.1(A) above,can be used for measuring horizontal movementsof rock mass around tunnel cavity, by fixing it inhorizontal direction or the direction in whichmovement is to be measured (Fig. 12.05).

(B) Inclinometer: The inclinometer systemconsists of casing and measurement system (Fig.12.06).

Fig. 12.06: Components of Inclinometer

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Fig. 12.07: Working Principle of Inclinometer

The casing provides contact for sub-surfacemeasurements, while its grooves control theorientation of the inclinometer sensor and providea uniform surface for measurements. The casing isusually installed in a borehole; however, it canalso be buried in a trench, cast into concrete orattached to a structure. It is used for measuringangles of slope/tilt and deviation from true vertical(Fig. 12.07).

Plot of inclinometer readings over a period of timecan show trend of sub-surface movements alongthe depth of borehole (Fig. 12.08).

There are many types of inclinometers likeElectronic Inclinometer, Mercury Inclinometer,Manual Inclinometer and Gravity Inclinometer. TheElectronic Inclinometer enables precise readingsand is used very commonly.

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Fig. 12.08: Plot of Inclinometer Readings

4.3 Diameter or Width Change: This is done tocheck the convergence of the cavity afterexcavation. The instruments used for this are asunder:

(A) Tape Extensometer: It is a portable devicedesigned to measure any change in distance betweenthese two points (Fig. 12.09 and Fig. 12.10). Theinstrument consists of a precision punched steel tapeincorporating a repeatable tensioning system and

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dial gauge readout. Both ends of the equipmentare fixed to the anchors provided at the two points,between whom the change in distance is to bemeasured. A precision dial gauge measures thechange in distance between two anchor points. Theyare normally used for maximum length of 25m.

Fig. 12.09: Schematic diagram of TapeExtensometer

Fig. 12.10: Tape Extensometer

Fig. 12.11: Bi-reflex Fig. 12.12: MiniTargets Prism Targets

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(B) 3-D Optical Targets: Use of 3-D OpticalReflectors is a cost effective and it is precise productfor recording deformations in tunnel, over a longperiod of time. These targets can be of Bi-Reflex orMini Prism type (Fig. 12.11 and Fig. 12.12).

They consist of two parts, the bottom part (anchorbolt) is fixed permanently at the location wheredeformation is to be measured. When not in use,the anchor bolt is covered by a cap.

When the deformations are to be measured, the toppart (the reflective target) is fixed to the anchorbolt and three-dimensional co-ordinates of the targetis determined in an absolute reference system byoptic-trigonometric surveying of targets in repeatedmeasurement cycles, using a TotalStation or similarsurveying equipment. This is achieved by alsoincluding a number of reference points which areconsidered stable in the survey. The measuringinstrument is positioned to provide the best possiblelines of sight to the targets and reference pointsand can otherwise be freely positioned (Fig. 12.13).

Fig. 12.13: Measurement of Deformations

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Fig. 12.14: Sample Deformation Measuring Scheme

These targets are used for measuring deformationsduring construction phase of the tunnel but some ofthe target are continued to be monitored duringservice life of the tunnel also as part of inspectionand maintenance. A sample of deformation measuringscheme used in 10.96 km long Pir Panjal tunnel inUdhampur Srinagar Baramulla Rail Link (USBRL)Project of Indian Railway is shown in Fig. 12.14.

4.4 Tilt: Tilt is measured for the buildings affectedby the tunnelling work. It is measured using Tiltmeter. It is a sensitive equipment designed tomeasure very small changes from the verticallevel, either on the ground or in structures. Tiltmeters may be purely mechanical or incorporatevibrating-wire or electrolytic sensors for electronicmeasurement. A sensitive instrument can detectchanges of as little as one second.

4.5 Load or Stress in Structural Supports: Thisis done to verify adequacy of structural support(rock bolts, ribs, etc.) and increasing knowledge ofsupport behavior as input to improved designprocedures. Various equipment used for this are asunder:

(A) Load Cell: Centre hole type load cells are fixedin the rock bolt itself to measure the load taken bythe rock bolts (Fig. 12.15).

(B) Pressure Cell: Pressure cells (Fig. 12.16) areused to measure the pressure between the twosurfaces where they are fixed.

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Fig. 12.15: Centre Hole Load Cells

Fig. 12.16: Pressure Cell

By fixing pressure cells in the appropriate direction,the tangential or radial stresses in the shotcrete canbe measured (Fig. 12.17).

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Fig. 12.17: typical use of Pressure Cell

Fig. 12.18: Measuring Anchor

(C) Measuring Anchor: They are sort ofinstrumented rock bolts and can be called ascombination of rock bolt and extensometer (Fig.12.18). They are used to record the extension ofrock bolt.

(D) Shotcrete Strain Meter: This is used tomeasure stain in shotcrete (Fig. 12.19). Two parallelsteel bars, embedded in the shotcrete at a defineddistance to each other, distort a central tube whenthey move relative to each other. This distortioncorresponds to the mean compression or elongationof the concrete in between. The distortion ismeasured by strain gauges. The full bridge straingauge signal is transmitted to a data acquisitionsystem via a 6-conductor cable.

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Fig. 12.19: Shotcrete Strain Meter

4.6 Pore water Pressure: This measurement isused for forewarning of distress to buildings due tomovement of soil or water. It is done usingPiezometer. It is a device which measures thepressure (more precisely, the piezometric head) ofgroundwater at a specific point. It is designed tomeasure static pressures. There are many typesof piezometers available in the market like Electric,Hydraulic or Vibrating Wire type (Fig. 12.20).

Fig. 12.20: Vibrating Wire Piezometer

4.7 Groundwater Level: This is recorded tomonitor draw down of groundwater table due totunnelling work. This can be done by measuringgroundwater level in observation wells, using waterlevel sounder.

4.8 Vibrations: These are recorded to verify thatground and building vibrations due to tunnelingactivities do not exceed an acceptable limit. Theyare measured using Engineering Seismograph (Fig.

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12.21), which is a device used for recording earthtremors. Basically, it is a heavily weightedhorizontal rod (pendulum) suspended from a pole.This rod is free to swing from side to side if theearth shakes. One end of the rod rests against thepole, while the other holds a pen or stylus. Thisstylus marks a slowly moving roll of paper. Ifthere is no shaking, the passing paper is markedwith a straight line. If there is a tremor, the paperis marked with a squiggly line.

Fig. 12.21: Seismograph

A typical instrumented tunnel section is shown in Fig.12.22.

Fig. 12.22: Typical Instrumented Section

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5. Monitoring Sections at Construction Stage:During the construction stage, measurements can beundertaken at Standard Monitoring Sections, PrincipalMonitoring Sections and on the Surface.

5.1 Standard Monitoring Section:(a) The purpose of standard monitoring section is

to evaluate the structural stability of thetemporary support.

(b) At each monitoring section, the progressivedisplacement is determined by recordingthree dimensional coordinates on differentdays. These measurements can be used toderive settlements, convergences, divergencesand displacements along the tunnel.

(c) The targets are fixed to the tunnel wallthrough Convergence Monitoring bolts. Thelayout of monitoring stations and theirspacing depends primarily on the geologicalconditions. Typically, seven number targetsare fixed in a cross-section – five in heading& two in benches.

(d) In zones of rock susceptible to swelling,levelling points are also provided in theinvert.

(e) Following spacing can be adopted for thestandard monitoring sections:• Stable Rock: maximum 30 m• Unstable Rock: maximum 20 m• Squeezing Rock: maximum 10 m

(f) Survey points are installed at a distance ofless than 1m from the face before the nextround and surveyed (zero position).Subsequently, the points are normallysurveyed at least once a day for the first fewdays. Near fault zones or in the event ofheavy deformation, the interval is shortened.When a number of successive measurementsshow decreasing rates of deformation, the

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measurement interval can be lengthened.(g) The vertical movements of the points at the

feet of the top heading cannot be directlysurveyed since the points have to be about0.8 to 1.0m above the invert in order to berecorded by instruments. If largedisplacements occur at the feet of the topheading, it is often better to measure theforce at the foot of the top heading with loadcells.

5.2 Principal Monitoring Sections: The mainpurpose of principal monitoring sections is to checkthe input data for the design and the suitability ofthe calculation model.

(A) Measurements: Following individual mea-surements are normally carried out:

• Horizontal/diagonal convergences andsettlements of crown/foot points): Throughoptical surveying instruments (Total Station& Targets) or Tape extensometers.

• Displacement of the surrounding rock mass:Through Multipoint Borehole extensometers

• Strain & pressures on the outer supportlayer/steel arches: Shotcrete strain meters& Pressure cells

• Loading of the Rock Bolt: Axial Force Meter(to determine the load development alongthe rock bolt) & Rock Bolt Load Cell (todetermine maximum anchor load and thedegree of utilization of the anchor).

• In areas with clay minerals susceptible toswelling, invert heave is also measured.

(B) Number of instruments in each cross-sectionshould be decided in consultation with designer &geologist.

(C) Number of Principal Monitoring Sections:In addition to the standard monitoring sections, at

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least two principal monitoring sections should beprovided in every tunnel. It may also be appropriateto provide surface monitoring sections at the samelocation if it can be expected that deformation couldextend to the surface. As soon as the workingsequence has become established after driving thefirst 50 to 100m of tunnel, the first principalmonitoring section should be setup. Subsequentspacing may vary from 150 to 500m depending ongeological conditions.

5.3 Surface Monitoring: Surface measurementsare taken to monitor open Excavations near portaland for monitoring surface movements in case ofshallow tunnels, particularly when there arebuildings in the area affected by the tunnel.

This normally entails precision levelling to observethe behaviour of settlement with time,supplemented when necessary with extensometer.All settlement points should be installed as toallow for reliable zero readings without anyinfluence of construction activities.

6. Stages in Geotechnical Monitoring: Following arethe stages in Geotechnical monitoring:

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7. Graphical Presentation of Monitoring Data: Thedata recorded by instrumentation is processed/analyzedand then presented in required for, including graphicalform, for better appreciation. This is done by thecustomized software, which are part of theinstrumentation and monitoring scheme. Some of thecommonly used graphical representations are listedhere.

7.1 Time Displacement Diagram: TimeDisplacement diagrams are used to presentvertical, horizontal and longitudinal displacementcomponents versus time. Typically, the results ofdisplacement measurements of all or one targetsin one monitoring cross section are plotted in asingle diagram. Construction phases are alsopresented on the same plot showing correlationbetween construction activities and displacements.The displacement history is used to assessstabilization process. A sample of time-displacement diagram for crown settlement, in atunnel excavated with heading and benching, isshown in Fig. 12.23.

Fig. 12.23: Time Displacement Diagram

7.2 Vector Diagrams: The vector diagrams showplot of displacement development with time, atselected points, in sections perpendicular andparallel to the tunnel axis (Fig. 12.24 & Fig.12.25). The vector plots can show the influence of

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geological structure(s) and quality of surroundingground, which has a significant influence ondisplacement characteristics.

Fig. 12.24: Vector Diagram

Fig. 12.25: Screen shot of Vector Diagram

7.3 Lines of Influence: Lines of Influence areproduced by connecting displacement values of anumber of monitoring points along the tunnel axis

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at the same time. An example of lines of influencefor “displacement of crown” due to top headingexcavation is shown in Fig. 12.26. As theexcavation approaches the Fault (9), in excavationStep-8, a significant deviation of the previouslyuniform behaviour can be observed. Duringtunnelling through the fault, a further increase insettlements is measured.

Fig. 12.26: Lines of Influence

7.4 Trend Lines: Trend lines are generated byconnecting settlement values of individual lines ofinfluence at a predefined distance behind the face(Fig. 12.27 & Fig. 12.28).

Fig. 12.27: Trend Lines

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Fig. 12.28: Trend Lines

They give a good overview of the displacementdevelopment along the tunnel and are quite useful forextrapolation of the displacement behaviour ahead ofthe excavation face. Trend lines which show increasingdisplacement can indicate critical situations and mustbe considered as a serious warning signal.

8. Control limits in Observational Approach: Inobservational approach, various parameters (e.g.displacement, stress, force etc.) are monitored andbased on the values recorded, various decisions aretaken. For deciding action and type of action, controllimits for various parameters are required to be set.Concept of various control limits and their applicationin observational approach is shown in Fig. 12.29.

Comparison of monitoring data with control limits willgive first indication for identification of potential areaswhich are close to or exceeding design limits. Forjudgment of rock mass behaviour and performance ofthe primary support, control limits are normallyestablished for:

o Primary lining displacements,o Displacement velocities,o Shotcrete strains,o Settlements etc.

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Fig. 12.29: Control Limits

The definition of control limits shall be considered asflexible and adjustable, which means control limits shallbe updated regularly, if necessary, on basis of experiencegained during construction.

8.1 Type of Control Limits: As shown in Fig.12.29, normally three types of control limits areestablished by following three trigger levels:

(A) Alert Level: This relates to to threshold values,on exceedance of which, certain routines will bestarted to impose an increased attention andsurveillance to these specific areas. It indicates thatspecific area is approaching a level where additionalactions and/or contingency measures may benecessary.

(B) Alarm Level: This relates to threshold values,on exceedance of which the element of work maybe approaching a critical state. The GeotechnicalEngineer shall decide for the specific case and theoverall support and rock mass performance.

(C) Action Level: This relates to threshold valueson exceedance of which the element of work isconsidered to be outside the expected range of

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assessed behaviour and may be close to its ultimatelimit capacity. The overall performance shall be re-checked with a related risk assessment. Designreview shall be done together with assessment ofneed for additional support. Additional support and/or contingency measures, to guarantee safety of theworks, shal l be implemented. For ease ofidentification of an unacceptable safety risk, theworks shall be stopped and remedial measures shallbe implemented immediately.

8.2 Threshold Values for Control Limits: Theparameters in tunnelling, on which mostly controllimits are set, and their typical threshold valuesare discussed in following paras.

(A) Control Limits–Displacement Velocity: Itshows rate of displacement with time, at monitoringpoints (Fig. 12.30).

Usually, time interval between successiveobservations is one day, with progress in top headingbeing 2-3m per day. It is an important indicator ofstability development. Immediately after excavation,increase of displacement velocities is expected dueto stress redistribution. After installation of theprimary support, this must decrease and stabilizeafter excavation face has advanced further and stressredistribution is completed. Continuing/increasingdisplacement velocity indicates that rock mass isnot stable and may indicate progressivedestabilization.

As a guideline, the control limits related to measureddisplacement velocities are defined as follows:

Alert Level δ n = 0.8 (n -1)

Alarm Level n = 1.0 (n -1)

Action Level n = 1.1 (n -1)

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Fig. 12.30: Displacement Velocity

(B) Control Limits–Differential Settlement: TheDifferential Settlement between the top heading

crown and the top heading footing ( S = Scrown–Sfooting) shall be monitored to identify potentialinstabilities at the shotcrete lining footing.

As a guideline, the control limits related todifferential settlement are defined as follows:

Alert Level + 5mmAlarm Level + 1mmAction Level - 3mm

(C) Control Limits-Trend Lines: Control Limitsfor trend lines are defined in terms of ratio “δ /Advance”.

Where, “δ ” is increase in displacement, and

“Advance” is corresponding face advance.

As a guideline, the control limits related to the ratioabove are defined as follows:

Alert Level 10-3

Alarm Level 5 x 10-3

Action Level 10-2

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(D) Control Limits–Shotcrete Strain: As aguideline, the control limits for shotcrete strains(in %) are defined as follows:

Alert Level 0.2%Alarm Level 0.4%Action Level 0.6%

9. Ignoring Instrumented Data: If the data obtainedfrom instrumentation is not properly interpreted in atimely fashion, or if no action is taken based on thisdata, the instrumentation program will serve nopurpose. Some of the examples of collapse/unsafeconditions as a consequence of ignoring instrumenteddata, are shown below:

Fig. 12.31 - The deformation started increasingsignificantly from 30th December onwards. Ignoranceof this led to collapse of cavern on 6th January.

Fig. 12.31: Collapse of a Cavern

Fig. 12.32 - The deformation, which had stabilized,started increasing significantly from 16th Marchonwards. Ignorance of this led to collapse of cavern on1st April.

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Fig. 12.32: Collapse of underground excavation

Fig. 12.33 – Sudden increase in deformation, afterabout 150 days, is an impending sign of collapse.

Fig. 12.33: Signs of Collapse

Fig. 12.34 – Sudden increase in load taken by anchors,after about 150 days, is an impending sign of collapse.This is for same site shown in Fig. 12.33.

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Fig. 12.34: Signs of Collapse

10. Contract Document: Responsibilities forinstallation & commissioning, calibration, monitoring,information flow, data interpretation and reporting etc.must be clearly defined in contract documents.

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DRAINAGE AND WATER PROOFING IN TUNNELS

Presence of water inside the tunnel affects the progressof construction work as well as the operation of thecompleted tunnel. A dry tunnel provides a safer andfriendlier environment and significantly reducesconstruction, operation and maintenance costs.Adequate geological and hydrological investigationsshould be done in advance so as to identify the stretcheswhere water ingress is expected to be encountered alongwith a reasonable assessment of the quantity of water.The water entering the tunnel, during the constructionstage, has to be collected and drained away until thefinal measures planned for the completed tunnel areimplemented.

1. Stages of providing Drainage: The design ofpermanent drainage system and control systems beginsduring the geotechnical exploration phase with anassessment of stretches where water ingress is expectedto be encountered along with a reasonable assessmentof the quantity of water. The stages of providing drainageand water proofing arrangements in tunnels are asfollowing:

1.1 Pre-drainage: Water should be preventedfrom entering the tunnel before starting the tunnelconstruction work. Following are the methods/systems employed for this purpose:

(A) Diverting water channels: All existing waterchannels, water from which can find entry into thetunnel, should be diverted away from the tunnel (tothe extent possible) or such channels may be sealed/lined to prevent entry of water from these channelsinto the tunnel. This requires detailed survey of thearea around the tunnel.

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DRAINAGE AND WATER PROOFINGIN TUNNELS

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(B) Ground Water Pumping: This may be doneby means of collecting water in ponding/collectionareas near the tunnel and then pumping it awayfrom the tunnel area.

(C) Grouting: Appropriate type of grouting (ce-ment/ chemical/resin based) besides reducing thepermeability of strata also increases the stability ofthe ground. A grouted body in form of close ring iscreated ahead of tunnel advance (Fig. 13.01). Thisring helps in resisting hydrostatic pressure also.

Fig. 13.01: Grouting

The suitability of the ground and the grout materialfor grouting depend on the geological, hydrologicaland chemical factors including mineralogicalcomposition of the soil, grading, permeability of theground, hydrostatic pressure of the groundwater,speed of groundwater flow and chemical propertiesof the groundwater. Use of grouting is particularlysuitable when water pressure in joints is very highand their volume is small.

(D) Ground Freezing: In this method, the porewater is converted to ice by circulation of a chilledliquid via a system of small diameter pipes placedin drilled holes. Ice creates a frozen mass of soiland/or rock particles, with improved compressive

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strength and impermeability. Brine is typical coolingagent, although fast acting liquid nitrogen can beused for projects where the freeze only needs to bequickly established and maintained for a short periodof time.

Ground freezing can temporarily seal and consolidatethe ground under conditions that are water-bearingbut not suitable for grouting. The availability oftechnology and experience for ground freezing israther limited in India.

1.2 Drainage during Construction: During cons-truction, water may come in tunnel from:

(i) Wash water, which is used for washing drillholes and water from other constructionrelated activities; and

(ii) Ground or sub-soil water (if the base oftunnel is at lower level than the ground waterlevel).

Proper planning should be done to prevent thewater in tunnel creating obstruction in theconstruction work. Precaution should be takenwhile dewatering the area outside the excavationlimits, because lowering of water table could causesettlement of existing structures, impact thevegetation and drying of existing wells. Variousground water collection and dewatering measurescan be used either individually or in combinationdepending on various factors including the groundconditions and the construction process. Theseinclude:

(A) Construction of Cutoff Walls: Imperviousretaining walls, such as steel interlocking sheetingor concrete slurry wall, could be placed into deeperless pervious layers, to reduce ground water inflowduring construction and limit drawdown of existingground water table.

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(B) Use of Plastic Gutters, Channels and Pipes:Plastic Gutters and Channels of various profiles canbe used to collect localized water ingress and drainit out, during tunnel construction (Fig. 13.02).

Fig. 13.02: Use of Pipes

Fig. 13.03: Drainage or Dimpled Mats

(C) Drainage or Dimpled Mats These mats, madeout of plastic (Fig. 13.03), are used when largequantity of groundwater is expected to appear fromlarge areas. The dimpled side should ideally beinstalled towards the ground/rock. They are oftenlaid on the shotcrete support layer followed by in-situ/sprayed concrete inner lining (with or withoutwaterproofing).

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(D) Dewatering Pumps: They are particularlyrequired when tunnel is progressing downhill or whenthe available longitudinal slope is inadequate fordraining out the water. Sumps at appropriatelocations need to be constructed for this purpose.This can be in the form of “Deep Wells”, aroundperimeter or along alignment of undergroundexcavation (Fig. 13.04) or can be in the form of“Well Points”, perforated tubes (covered with screen),sunk in ground (Fig. 13.05).

Fig. 13.04 Deep Wells

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Fig. 13.05: Well Points

(E) Drainage Boreholes before Tunnel Advance:Such boreholes (Horizontal Drains) should be donewhen heavy water inflow and/or sudden (but notpredictable) water inflows can be expected. Advancedrainage is expected to considerably simplify thesubsequent work because flow pressures can havesevere adverse effect on structural stability.

Slotted hard PVC pipes or perforated plastic drainagepipes with glass wool filters are often inserted intothe drainage holes (typically 35 to 100mm diameter)if flow of water can lead to erosion of surroundingstrata. The pipes are normally extended with aflexible plastic hose at the exposed end of the holeto drain the water into the side drainage channelsor into a temporary drainage pipe hung from thewall.

(F) Drainage boreholes after Tunnel Advance:Drainage boreholes drilled after the advance canprevent build-up of pressure behind the shotcretelayer and also help in reducing static pressure onthe support. Drainage boreholes are particularlyuseful for draining out localized water. The extentto which water pressure is relieved by drillingdepends on location of the boreholes, their direction,spacing, length and diameter. The most favourable

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combination of spacing, length and diameter of theholes should be determined experimentally.

In Tunnel T-3 in Udhampur – Katra Section ofUdhampur Srinagar Baramulla Rail Link (USBRL)Project of Indian Railway, there was heavy flow ofwater through the drainage holes constructed (Fig.13.06) and stopping this heavy flow requiredconstruction of a series of grout curtain walls on theside of tunnel walls, along the tunnel axis (Fig.13.07).

Fig. 13.06: Heavy Water Inflow

Fig. 13.07: Grout Curtain Walls

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1.3 Permanent Drainage System: In operationalphase of tunnel, a permanent drainage system isrequired in tunnels to drain out the water thatcould accumulate in tunnel from Rainfall, Tunnelwashing operations, Tunnel seepage and Fire-fighting operations.

Fig. 13.08: Gravity Flow System

Water accumulated in tunnel can be drained outeither by a gravity flow system or a pumpedsystem. Gravity flow system will suffice for tunnelswith continuous grades. Example of drainage undergravity flow system is Pir Panjal tunnel (10.96 kmlong) in Udhampur Srinagar Baramulla Rail Link(USBRL) Project of Indian Railway (Fig. 13.08)which is having peak point in the middle of thetunnel and 6534m and 4426m lengths on bothsides having falling gradient towards tunnel exits,with a vertical curve of radius 2500m joining thetwo grades. If a low point occurs within the tunnel,it may be necessary to have a pumped system todrain out the collected water.

2. Extent of Waterproofing: The degree of extent ofwaterproofing in tunnels can be of following types:

2.1 Watertight/Waterproof Tunnel: In suchtunnels no water is allowed to enter the tunneland, therefore, no drainage system is constructedinside the tunnels (Fig. 13.09). “Tunnel De Viret”

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in Lausanne Metro, in Switzerland, is an exampleof such tunnel.

Fig. 13.09: Watertight/Water Proof Tunnel

A watertight tunnel does not adversely affectnatural groundwater regime. Operation andmaintenance of such tunnels is relatively muchsimpler due to absence of water inside tunnel.Such tunnels are environment friendly and groundsettlements due to water drainage are reduced.The disadvantages of these tunnels include higherconstruction cost and difficulty in ensuring areliable long term water proof system.

2.2 Partially Drained Tunnel: Such a systeminvolves drainage of water until selected limitwater level or selected portion of the tunnel crosssection (Fig. 13.10) or up to permissible waterdrainage quantity.

Fig. 13.10: Partially Drained Tunnel

The advantage of such system is that impact onthe groundwater regime remains within specified

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limits. The cost of construction of such a tunnel ishigh due to the required elaborate drainagesystem. The availability of technology andexperience for such system is presently notavailable in India.

2.3 Fully Drained Tunnel: In such tunnel, thewhole quantity of water coming through the crosssection of the tunnel is collected inside the tunneland then drained out of the tunnel usingPermanent Drainage System (Fig. 13.11).

Fig. 13.11: Drainage System

The construction cost of a fully drained tunnel is relativelylow, since elaborate pressure-tight waterproofing andan inner lining designed to resist pressure are notrequired. In addition, such tunnels function relativelyreliably. However, environmental issues arising out ofdepletion of ground water need to be considered. Suchtunnels entail higher maintenance cost.

The decision on selecting appropriate drainage systemshould be taken keeping in view ground conditions,expected water inflow, technical feasibility and costeffectiveness. Completely drained tunnel shouldgenerally be the preferred option for all new tunnels.

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3. Requirements of Water-tightness in Tunnels

(i) Following classification should be adoptedfor specifying degree of water tightness inthe contracts:

Table 13.01: Classification of Water-tightness

Water- Damp Water proofing requirementsproofing CharacteristicsClass

1 Completely dry The sides of the lining must beso waterproof that no damppatches are detectable on theinner face.

2 Largely dry The sides of the lining must beso waterproof that only a slightdampness (e.g. noticeablethrough discolourat ion) isdetectable on the inner face inisolated locations. When theslightly damp patches aretouched with the hand, no traceof water should be left on thehand. Blotting paper orabsorbent newspaper laidagainst the patch may notdiscolour due to moistureabsorption.

3 Capillary The sides of the lining must bemoisture so waterproof that only isolated

and localized patches, which arewet to touch, occur. Patches,which are wet to touch, aredefined in that moisturepenetration of the tunnel sidesis noticeable and blotting paperor absorbent newspaper laidagainst the patch discolors dueto moisture absorption, but nowater drips occur.

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(ii) Waterproofing scheme should take intoaccount type, quantity and aggressivenessof the water acting on the tunnel structure:

(iii) The water proofing scheme/material shouldprovide:

• Permanent resistance against theprevailing mix of soil and water,including all chemical contents.

• Resistance against all adjacentconstruction materials like shotcreteadmixtures, grouting chemical etc.

• Resistance against the expected staticand dynamic loading.

• Adequate mechanical strength.

• Resistance in case of fire. Attentionshould be paid not only to theflammability but also the release ofpoisonous fumes.

• Durability for the design life.

• Ease of installation, maintenance andrepair.

• Adequate Construction detailing.

• Environmental compatibility- materialsused should not contaminate percolatingwater or groundwater.

• Bedding between protective layers.

• Division of the waterproofing intocompartments in order to be able tolocalize and repair any leaks.

• Multi-layer construction of thewaterproofing, or if there is one layer,reliable feasibility of checking itsfunction.

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• Feasibility of testing with appropriatetesting procedure.

• It must be possible to clearly describethe waterproofing system and itstechnical and material specification inthe contract.

• It must be possible for the contractor tocomply with the guarantee requirementsof the specification.

(iv) Waterproofing system should always be anintegrated system that takes into accountintermediate construction stages, finalconditions of structures and their ultimateusage, including maintenance andoperations.

4. Waterproofing Systems: Available waterproofingsystems fall under following two categories:

4.1 Rigid Systems: These include water-resistantplaster, sealing mortar and resin concrete. Onemajor disadvantage of this system is itsdependency on quality of application. However,experience in use of synthetically modifiedconcretes or mortars as sealing materials fortunnels is rather limited.

4.2 Flexible Systems: Flexible systems includefollowing:

(A) Bitumen Waterproofing Layer: Bituminouswaterproofing materials are now hardly used inunderground tunnelling, because of followingreasons:

• Bituminous waterproofing materials need amostly dry and flat support, requiringprofiled levelling of the tunnel sides.

• Installation of reinforcement for the innerlining can easily damage the waterproofinglayer.

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• Groundwater that cannot be completelycollected leads to waterproofing not adheringto the support with a risk of defects.

• Enhanced fire risk.

(B) Use of Geo-synthetics: Geo-synthetics,manufactured by polymers, are used very commonlyfor waterproofing because of their good mechanicalproperties (high tension strength, high failure strain,good flexible behaviour etc.).

Fig. 13.12: Geo-synthetic Waterproofing System

Tunnels in rocks are waterproofed by a geo-composite sandwich (Fig. 13.12) consisting of:

• A non-woven geotextile layer on the initialshotcrete layer.

• A waterproof HDPE/PVC geomembrane.• Cast-in-situ/sprayed final (inner) lining of

concrete.

Typical construction sequence involved in placingthis waterproofing layer is as following:

(i) Laying a non-woven geotextile layer directlyon the initial shotcrete layer, held in placeby pins/nails driven or shot into the rock(Fig. 13.13).

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(ii) Over this, a continuous waterproof HDPE/PVC geomembrane is installed (Fig. 13.14).

(iii) The membrane has to be cut and fit to allshapes and corners, welding together (byheat) to make a continuous waterproofingmembrane (Fig. 13.15).

(iv) Membrane joints are tested for watertightness (Fig. 13.16).

(v) Finally, cast-in-situ/sprayed lining ofconcrete is placed to provide inner (final)lining, without damaging the membrane.

Fig. 13.13: Fixing Fig. 13.14: LayingNon-woven Geomembrane

Geotextile Layer

Fig. 13.15: Membrane Fig. 13.16: Testing ofWelding Membrane Joints

(C) Sprayed Waterproofing Layer: It involvesspraying of the polymer based waterproofingmaterial in liquid form, of minimum 2mm thickness,directly onto the support. Termed as composite shelllining system, it consists of a double-bonded sprayapplied membrane embedded between two concretelinings (Fig. 13.17). This bonding property makesthe interface between membrane and concrete

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impermeable. The waterproofing material is sprayedin liquid form directly onto the concrete. Commonmaterials include Reaction resins, Cement-plasticcombinations, Polymers, Bitumen-plastic com-binations. The addition of about 20% by weight ofglass fiber 3 to 5cm long can also improve theproperties.

Fig. 13.17: Sprayed Waterproofing Membrane

The main advantages of sprayed waterproofingsystem are absence of seams, easy installation andeasy to repair any defective patch. One majorlimitation is that the application area should not haveany local bumps larger than the layer thickness. Theconditions in a tunnel (dampness etc.) may alsocause problems for the application. High humiditycan delay the setting process of sprayedwaterproofing materials.

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TUNNELLING IN WEAK/SOFT GROUNDS

Tunnelling in weak/soft ground presents differentchallenges because of the fact that such grounds havevery less (or almost zero) standup time and beforecreation of cavity as well as after creation of cavity,they require extensive stabilization measures to stabilizethe ground again. In soft/weak strata, ground looseningbreaches the integrity of natural arch, with aconsequence that without supporting the excavation,soon after it is completed, the walls may squeezetogether and the roof may fall in. Some of the “tunnelexcavation methods in soft ground” have beenelaborated in Chapter-10. In this chapter, the wholeissue of “Tunnelling in Soft/Weak Grounds” is beingelaborated in detail.

1. Conventional Methods: Before the advancementsin tunneling technology, from second part of 20thcentury onwards, the soft ground tunnelling was mostlydone by “multiple drift” methods. There were manyversions of it, mostly originated from Europeancountries.

1.1 Belgian Method: In this method, the tunnelsection was excavated in stages; starting from thepart of the top heading, then widening it sidewaysand finally opening the benching portion in stages(Fig. 14.01). The openings created in stages weresupported on timber struts.

This method was first employed in Chaleroy tunnel(in Belgium) in 1828. But experience of using thismethod was catastrophic during construction ofGotthard Tunnel (1872-1882). The key problemwas that the sequencing following top headingrequired the arch to be underpinned. However, this

CHAPTER-14

TUNNELLING IN WEAK/SOFTGROUNDS

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proved difficult in the yielding ground conditionsencountered, leading to the timbers giving way,followed by the cracking or total collapse of themasonry arch.

Fig. 14.01: Belgian Method

1.2 German Method: The underlying principle ofthis method was to leave a central bench ofground to be excavated last and to use it tosupport roof and wall timbering (Fig. 14.02). Thisallowed the arching to be built in one operation(unlike the Belgium method).

Fig. 14.02: German Method

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Fig. 14.03: English Fig. 14.04: AustrianMethod Method

1.3 English Method: In this method, theexcavation started from a central top heading (Fig.14.03), which allowed two timber crown bars to behoisted into place. Development of the headingthen allowed additional bars to be erected aroundthe perimeter of the face with boards betweeneach pair to exclude the ground. The system iseconomical, permits construction of the arch of thetunnel in full-face excavation, and is tolerant of awide variety of ground conditions, but depends onrelatively low ground pressures.

1.4 Austrian Method: This method requires astrongly constructed central bottom heading uponwhich a crown heading was constructed. Thetimbering for full-face excavation was then heavilybraced against the central headings, withlongitudinal poling boards built on timber barscarried on each frame of timbering (Fig. 14.04). Asthe lining advanced, so was the timbering proppedagainst each length to maintain stability. Themethod was capable of withstanding high groundpressures but had high demand for timber.

2. Ground Improvement: Ground improvement intunneling signifies improvement in mechanical andhydrological properties of the ground, before or during

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or after tunnel excavation, to facilitate tunnel excavationand ground stabilization. Ground improvement may berequired:

• To create an improved zone for theexcavation, prior to excavation.

• To prevent instability during construction,related to poor geotechnical properties (looseand un-cemented ground, flowing ground).

• To control water inflow when excavating.• To control settlements of the structures

above/around excavation.

Ground improvement is a general term for all kinds ofimprovement techniques. Technically it can be offollowing categories:

(i) Ground Consolidation: This term is usedwhen the ground is consolidated to improvethe properties (e.g. draining the water,reducing the voids in the soil matrix whichincreases the density etc.).

(ii) Ground Compaction: Increasing the densityof soil by compressing soil or applyingexternal load on it, to reduce the void spacein the soil matrix.

(iii) Ground Treatment: This term is used whenthe ground is treated with some compounds(chemicals, resins, lime, and etc.) whichresults in increase in strength and decreasein permeability of the ground.

(iv) Ground Reinforcement: This term is usedwhen the ground is reinforced with steel orfiber glass elements or geo-syntheticsincrease its load bearing capacity and shearstrength.

Some of the commonly used ground improvementtechniques, used for tunnelling, are elaborated in Para-3 to 7 below.

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3. Drainage: Control of groundwater is important insoft ground tunnelling. The presence of a small amountof water in granular soils above the water table may bebeneficial in providing an increase in stand-up timebecause of apparent cohesion brought about by negativecapillary forces (until they dissipate), but below watertable the presence of water reduces effective strengthdrastically and seepage pressures cause rapid andcomplete failure in the non-cohesive soil. Presence ofwater in clays is of primary importance in determiningthe strength, sensitivity and swelling properties of thematerial.

In some cases the tunnel construction is only possiblewith the application of special dewatering measures.For various stages for providing drainage (or dewatering)and various methodologies the Chapter-13 on “Drainageand Waterproofing in Tunnels” may be referred.

In case of low overburden, dewatering measures canbe carried out from the ground surface. In other cases,dewatering has to be done from the tunnel cross sectionor from pilot tunnels.

4. Grouting: Several types of grouting are used tomodify and/or stabi l ize soi ls in-situ. Recentimprovements in grouting have made it a valuable toolin both groundwater control and soil stabilization fortunnelling projects. It can be very effective in followingsituations:• To strengthen loose or weak soil and prevent

cave-ins due to disturbance of loose, sensitiveor weak soils by the tunnelling operations.

• To decrease permeability and in-turn grou-ndwater flow.

• To reduce the subsidence effects of dewateringor to prevent the loss of cines from the soil.

• To stabilize sandy soils those have a tendencyto run in a dry state or to flow when below thewater table.

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4.1 Location for Grouting: Grouting can becarried out in the tunnel excavation as facegrouting or as radial grouting from the excavatedtunnel or from a pilot tunnel (Fig. 14.05).

Fig. 14.05: Grouting Locations

4.2 Groutability of Ground is primarily dete-rmined by the permeability of the ground or thepercentage fines (passing 75 micron sieve). Thethumb rule is that soil having less than 10% finescould be successfully grouted and those with morethan 20% fines could not. However, advancementsin grouting technology have raised this limitapproximately by 5%. Groutability is also assessedby using Groutability Ratio (N), as given in Table14.01

Table 14.01: Groutability of different Grounds

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Fig. 14.06: Types of Grouting

4.3 Types of Grouting: Three ways of introducinggrout material into the soil are possible (Fig.14.06):

(A) Permeation Grouting: In this method, thegrout fills the voids in the soil and there is no changein volume or structure of the original ground.Permeation grouting may be done with either cementbased or chemical based grout, with latter beingnecessary for satisfactory penetration of fine soils.This type of grouting can be used for creation of asupport ring around the tunnel excavation boundaryor to create support for foundation of any structurein the vicinity of tunnel.

(B) Compaction or Displacement Grouting: Inthis method, soil is densified during tunnelling byinjection of a stiff grout. The thick mortar mix actsas a radial hydraulic jack, creating bulbs or lensesand thus displacing and compressing the surroundingsoil. This type of grouting is useful in controllingsettlement of foundation of structures located abovethe tunnel or underpinning of foundations ofstructures located in vicinity of the tunnel.

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(C) Jet Grouting : In this method, the ground isfragmented by deliberate hydrofracturing, in orderto increase total stresses by wedging action ofsucessive thin grout lenses, to fill unconnected voidsand possibly consolidate the soil under injectionpressure. Jet grouting can be used for:• Forming an umbrella (canopy) ahead of the

face.• Reinforcing and stabilizing the tunnel face.• Reinforcing the walls of tunnel.• Underpinning the steel ribs.• Creating impermeable diaphragms (e.g.

before starting the excavation with TBM).

Jet grouting is applied mainly horizontally or at aslightly upward or downward angle from within theface of the tunnel. An improvement of the roofarching behaviour is achieved by applying one ormore layers of jet grouting columns in stagescorresponding to the excavation operations.

An improvement of the stability of the face isachieved by placing individual jet columns parallelto the direction of advance in the working face.

Less common in tunneling is vertical or steeplyinclined jet grouting, except in shallow tunnels whereit is applied from the surface. From within the tunnelvertical or steeply inclined jet grouting is mainlyapplied to underpin the bottom of the roof arch.

4.4 Grouting Material: The most commonly usedgrout material is cement. In special cases chemicalproducts such as resins or foams are also applied.In these cases, the environmental and safetyrestrictions have to be considered specially. Fig.14.07 may be referred as a rough guide forassessing material of the grout to be used.

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Fig. 14.07: Grout Material Suitability

5. Ground Freezing: In this method, pore waterpresent in the soil is converted into ice by extraction ofthe latent heat. The ice then acts like a cement to bindthe soils grains together, thereby raising the strengthand lowering the permeability of soil mass. This methodis successful only when sufficient water pore water ispresent in the ground. It may be noted that presenceof organic material or salt water will result in greaterdifficulty in freezing. Another major deterrent is movingground water, which makes the freezing difficult.

Following advantages can be derived by the freezing:• Makes water-bearing strata temporarily

impermeable.• Increases compressive and shear strength of

ground.• Provides structural underpinning (temp-orary

supports).

5.1 Refrigeration Process: The typical freezinginstallation consists of a refrigeration plant thatcools a brine solution, which is then pumped downthe center of an annular freeze pipe to the bottomof the hole, returning via the outer annulus incontact with the soil. The warmed brine is returnedto the refrigeration plant and the cycle continues.In practice, a number of freeze pipes areconnected to a pair of headers for the flow and

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return lines. For tunnel construction, it is notnecessary to maintain continuous freezing andkeep on lowering the temperature of the soil, theonly requirement is that pore water should be keptat a temperature below the freezing point. Forspecial purposes, especially for projects of limitedextent and duration, boiling of liquid nitrogen inthe freezing elements may be appropriate.

5.2 Ground Freezing Techniques: Followingground freezing techniques are known:

(A) Continuous frozen bodies which provide long-term load-bearing. The freezing is achieved by adrilled tube system, through which coolant ispumped.

(B) Short-term local freezing of damp zones closeto the face or in the immediate vicinity outside theexcavated cross section.

6. Face Consolidation: Once the analysis of groundbehaviour indicates possibility of caving phenomenon,the need for face reinforcement (consolidation) may beof great importance, in addition to other supportmeasures.

Face bolts are used for face reinforcement. Dependingon the anticipated ground condition/behaviour, therelevant bolt type and length have to be determined inthe design. As a protection against rock fall, spot boltsmay be sufficient whereas in difficult ground conditions(e.g. squeezing ground condition) systematic anchoringwith a high number of long and overlapping bolts maybe necessary. Face bolts are placed during theexcavation sequence, if necessary in each round or inpredefined steps.

Though steel bolts are also used for face reinforcementbut most commonly used bolts for this purpose are “fiberglass reinforcement elements” (Fig. 14.08). Theseelements can be grouted after inserting in the boreholes.

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Fig. 14.08: Fiber Glass Reinforcement Elements

Advantage of using these elements is that afterexcavation, it is very easy to break/snap the elementshanging from the face.

7. Advance Supporting or Pre-Supporting: The mainproblem with conventional methods, when tunnellingthrough difficult geotechnical conditions, is control ofdeformation. Without support or treatment, the groundweakens and tends to sink into the opening (fall ofground from the upper part of the tunnel face, tunnelface extrusion and tunnel face failure), a phenomenoncalled “decompression”. El imination of thisdecompression may require a “cavity pre-confinementaction” (any active action that favours the formation ofan arch effect in the ground ahead the tunnel face)that can be achieved through reinforcement and/orprotective intervention ahead of the tunnel excavation.Such interventions, done in advance of tunnel faceexcavation, are called as Advance Supports or Pre-supports. Some of the commonly used Advance Supportor Pre-support are as under:

7.1 Mechanical Pre-cutting: In this method, aperipheral cut (like slit) is made slightly outsidethe periphery of the area to be excavated. This isdone using a slit cutting machine (Fig. 14.09).

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Fig. 14.09: Mechanical Pre-cutting Machine

The slit is cut at a small outwards angle from thedirection of tunnel axis and then filled withshotcrete. This forms an annular ring around theperiphery of the area to be excavated (Fig. 14.10),which provides a suppression or reduction invibrations and over-excavation, and keeps the rockmass intact.

Fig. 14.10: Mechanically Pre-cut Ring

7.2 Spiles: Spiles are steel bars inserted in theground, on the boundary of the excavation in roofarea, for local short-term stabilization of the roofsection and at the working face. The spiles reston the first steel rib or lattice girder support infront (Fig. 14.11). The spiles act as advance

289


support at the tunnel face and limit over break.Their length is about 3 times the round/steplength to ensure sufficient overlap and they arespaced around 30cm. Depending on the type ofsoil, the spiles can be jacked, rammed or insertedin drill holes.

Fig. 14.11: Spiles

7.3 Umbrella Arch: Umbrella Arch Method (UAM)or Forepole is an economical method to increasethe excavation front sustainability, minimize theland subsidence, preventing rock debris falling andlandslides. In this method, first bore holes aredrilled in a semi-circular pattern around the crownof proposed tunnel profile. Then steel pipes areplaced into each of these holes and they are filledby grout, forming a strong umbrella arch tubeabove the tunnel crown. Pipes are installed in twoconsecutive steps, with overlapping length, and areprovided at a small outwards angle fromlongitudinal axis of the tunnel (Fig. 14.12). Lengthof pipes is from 15 to 30m and they are spaced atabout 30 to 50cm. The main advantages of thismethod are:• Stress reduction in drilling front.• Increased safety during drilling operations.• Simplicity of drilling operations and in-turn

increase in drilling progress.

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Fig. 14.12: Umbrella Arch Roofing

Umbrella arch roofing is useful at following locations:• For Portal development.• For constructing shallow tunnels in soft

grounds.• For tunneling in ground with mix of boulders

and soil.• For tunnelling in sandy gravelly terrain.

8. DRESS Method: In SJVNL (Sutlej Jal Vidyut NigamLimited) Hydro-power Project of India, a special methodnamed as “DRESS” was employed for construction oftunnel in a wide shear zone, near Rattanpur adit. Thislength of about 360m was very difficult due toencountering of sheared rock, high ingress of water andhigh stress condition. DRESS is acronym for “DrainageReinforcement Excavation Support Solution”. Thoughthis was a hydropower tunnel, but the methodologycan be applied in railway tunnel also, with similar groundconditions.

Three alternatives (A, B & C) of tunneling advance withDRESS methodology were proposed, depending on therock conditions. Alternatives A, B and C were proposedfor the regions having GSI value < 15, 15 to 35 and 35to 45 respectively. The steps involved in each alternativewere as under:

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Step Description lternative

A B C

1 Drainage/Exploratory Drill Hole E E E

2 Face Improvement by Grouting E E W

3 Steel Pipe Umbrella E E -

4 Side Drift E - -

5 Rock Reinforcement by Forepoling

ahead of face E1 - -

6 Radial Rock Reinforcement at the Face W W W

7 Temporary Invert (Top Heading) E E E

8 Enlargement of Heading E - -

9 Integration of Rock Reinforcement

behind the face E W W

10 Improvement of Side Wall Footing E E E

11 Benching & Steel Arch Concrete Invert E E E

12 Rock Reinforcement of Bench Profile E E E

13 Final Lining E E E

E–Essential, W–When & If Necessary, E1-Only inLeft Wall

The sequence of activities was as under:

(i) Draining of Rock ahead of the face: Beforeopening of the face, advance drainage was doneall around and ahead of the face, to eliminate thedetrimental influence of water pressure on the facestability.

Six to eight drainage holes of 77mm diameter upo 24m length, depending on the site strata, in anupwards inclination of 150, were drilled with ahydraulic drill using DTH hammer. M. S. Pipe of50mm diameter 12m grouted and 12m perforated,protected with geo-textile were provided in thedrilled drainage holes to avoid the blockage ofdrainage system. These drainage holes wereprovided in alternate forepoling blocks.

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(ii) Face Improvement: After providing thedrainage system, stability of the face and aheadwas improved by cement grouting with W/C ratioof 1:1. Sometimes when grouting was not possibledue to encountering of gouge sheared materialmixed with clay, the face was stabilized byShotcreting and grouted anchor bars of 25mmdiameter 8m long.

(iii) Steel Pipe Forepole Umbrella Arch:Forepoling (casing) of steel pipes was providedahead of the face before excavation of the faceusing the hydraulic drilling rig. In this the crown ofthe tunnel above springing level was supportedwith 12m long steel pipe forepoles (casing) of114.3mm outer diameter with 6mm thick wall andin an upward direction of 60 over rib R1 of theblock. The forepole were spaced @400mm c/cspacing. After drilling and installing of theforepoles, cement grout in W/C ratio of 0.75 to0.45 was placed at a maximum pressure of 5 kg/cm2.

(iv) Face Advance in Heading Excavation: Afterstabilizing the crown and the zone ahead of thetunnel face by forepoles, drainage holes,shotcreting and grouting, the tunnel advance inone forepoling block of 12m length was carried outup to 8.75m length of a variable diameter ofexcavation from 11.65m to 13.45m before thenext block of forepoling. In this 8.75m length oftunnel advance, a total number of 12 sets of ribsof ISMB 300x140 @ 750mm c/c spacing wereprovided in a sequential advance of 0.75m to1.50m depending upon the stand-up time of rockstrata.

Excavation was done by mechanical means in topheading up to 1.0m below springing level, inrounds of 0.75m to 1.50m in the form of half ring,leaving the central portion to brace the face andthe walls against bulging/collapse.

293


After protecting the crown with shotcrete & wiremesh, the excavated section was supported withribs and the space between the rock surface andthe rib intrados were filled with shotcrete.

In the second round of excavation, the centralportion was first excavated up to the previousround advance and again the excavation for thisround was carried out in an advance of 1.5m inthe same way as explained above.

After excavating and supporting the forepolingblock up to rib R12 (last rib of the block), theexcavation of rib R1 of next block was done andthe rib was installed and supported with wire meshand shotcrete.

The ribs were anchored at springing level with25mm diameter 6m long cement grouted anchorbolts with ISMC 150x75 runners joining three tofour sets of ribs. The face was then sealed off withshotcrete for improvement of face and aheadbefore start of excavation of next forepoling block.

The excavated reach was further supported withradial rock reinforcement in the form of 32mmdiameter 6m long hollow core self-drilling cementgrouted rock bolts. Grout was then pumpedthrough the bolts itself forcing out water, debrisetc. and filling of all fissures voids and completegrouting of the bolt was ensured.

A temporary invert arch of 350mm thick shotcretewas also provided to prevent heave of anunsupported invert and punching of steel ribs fromthe arch support into soft rock.

(v) Benching Excavation: The benching excav-ation was taken up about 50m behind the face inorder to have proper drainage system andstabilization of the heading strata. The benchingwas also done with hydraulic hammeringtechnique. After finishing the excavation of

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benching, the side walls were protected with initiallayer of 50mm thick shotcrete, followed by wiremesh fixing, extension of heading ribs in thebenching and providing steel ribs arch invert. Thespaces between the rock surface and intrados ofthe ribs on wall sides were filled with shotcreteand invert steel arch encased in 400mm thick M20concrete. The benching profile was supported with25m diameter 6.0m long cement grouted anchorbolt.

(vi) Monitoring during Construction: The beh-aviour of the ground around the opening wasclosely monitored during the progress ofconstruction with tape extensometer.

In these squeezing rock conditions, it has beenobserved that the behaviour of the convergencewas initially faster with respect to face distanceand it subsided subsequently with the elapsedperiod of opening and face distance.

Deformation in the ribs and cracks in shotcretewere observed in some of the reaches in theblocks but these were within permissible and safelimits. The maximum movement observed in 205mlength of the tunnel excavated at that stage wasto the tune of 14.6cm in an elapsed period of 460days.

Progress of up to 25m in a month was achieved inextremely poor rock conditions. Despite the initialinvestment for hydraulic rig, in view of suchprogress of work coupled with other factors likesafety and stability of operations, DRESS wasfound most appropriate in extremely poor rockmass conditions.

9. Tunnelling in Swelling Grounds: Swellingphenomena is generally associated with argillaceoussoils or rocks derived from such soils. In the field, it isdifficult to distinguish between squeezing and swellingground, especially since both conditions are often

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present at the same time. However, except in extremeconditions, squeezing is almost always self-limiting andwill not recur vigorously, or at all, once the intrudingmaterial has been removed. But swelling may continueas long as free water and swelling minerals are present,especially when the intruding material has beenremoved, thereby exposing fresh, unhydrated rock.

Most swelling is due to the simultaneous presence ofunhydrated swelling clay minerals and free water. Tunnelconstruction commonly creates these conditions.Minerals such as montmorillonite form layered platycrystals; water may be taken up in the crystal latticewith a resultant increase in volume of up to 10 timesthe volume of the unhydrated crystal. The displacementsresulting from this increase in volume give rise to theobserved swelling pressures, whether in soil or in rock.

Swelling grounds cause major problems of supportingboth during construction and during operation life of atunnel due to wall displacements. In certain cases, invertheave of over 25cm per year has been observed. In atunnel in Udhampur-Katra section of Northern Railway,India, floor heaving of 40-60 cm was observed in year2004 because of swell ing claystone havingmontmorillonite, kaolinite and illite. The swell rates inseveral old (75-100 years) Swiss tunnels decreased to0.5 to 1.0 cm per year with the passage of time.However, the total invert heave was of the order ofseveral meters requiring repairs of the inverts.

Sometimes, it is felt that swelling could be prevented ifthe ground is hermetically (completely) sealed byshotcrete against air moisture. But many case historiesshow that this approach has not succeeded. This isbecause of the fact that while shotcreting preventsingress of atmospheric water, the swelling still takesplace due to presence of pore water within the clays.

The swelling pressure on tunnel supports may be veryhigh. Swelling grounds have normally low modulus ofdeformation and are capable of exerting high pressure

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even at moderate depths. Baldovin and Santovito (1973)measured contact pressures up to 3.5 MPa on provisionalconcrete lining (placed about 20 days after excavation)in the Foretore Tunnel in South Italy.

Einstein and Bischef (1975) have proposed an analysis-cum-design method for tunnels in swelling rocks. Theprocedure consists of following seven steps:(i) Determination of the primary state of stress.(ii) Determination of the swell zones around the

opening, based on the primary state of stressand the stress changes caused by the opening.

(iii) Laboratory swell tests in the Oedometer, onsamples taken from swell zones.

(iv)Determination of time-swell properties fromOedometer tests, measuring the timedisplacement relations for several stressincrements.

(v) Derivation of the swell-displacements for thestress difference between the primary state ofstress and the state of stress after excavation.

(vi) Performing swell-time computations.(vii) In-situ measurements of swell-displacements

and swelling pressures.

Further, they recommended following design featuresbased on the above procedure:(i) Use an invert arch instead of horizontal struts.(ii) Bolt the swelling zone with the ground below

it.(iii) Use compressible backfill between the support

and the ground.(iv) Trim the floor.(v) Employ grouting to seal off preferential paths

supplying water to the swell prone zone.(vi) Provide constraint to the swelling ground by

cutting slots and injecting grout under pressurethrough these slots.

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(vii) Avoid exposure of the swelling ground toatmospheric moisture by applying sprayedconcrete (SFRS).

(viii) Provide good drainage inside the tunnel.

10. Tunnelling in Squeezing Grounds: The squeezingor elasto-plastic pressure is mobilized due to failure ofa weak rock mass around a tunnel under influence ofhigh overburden pressure or tectonic stresses. Theoverstressed zone of rock fails where tangential stress(σ ) exceeds the mobilized UCS of the rock mass. Thefailure process will then travel gradually from the tunnelboundary to deeper regions inside the unsupported rockmass. The zone of the failed rock mass is called the“broken zone”. This failed rock mass dilates on accountof the new fractures. A support system after installationrestrains the tunnel closure and gets loaded by thesupport pressure.

“Commission on Squeezing Rocks in Tunnels” of ISRMhas defined squeezing as the time dependent largedeformation, which occurs around a tunnel/otherunderground openings, and is essentially associatedwith creep caused by stress exceeding shear strength.Deformation may terminate during construction orcontinue over a long time period.

High deformability, low shear strength and the high in-situ stress state are the major factors that govern thetunnel wall stability and extent of closure. Prediction ofsqueezing conditions is of great importance to a designerfor designing a stable support system of the tunnel.

It is the time-dependent displacement which dominatesin fragile rock masses under high overburden,particularly when a broken zone is formed around anopening. Therefore, the support attempts to curb thesetime-dependent tunnel closure and in-turn attractshigher loads (Jethwa, 1981: Dube, Singh & Singh,1986).

Terzaghi (1948) advocated that support pressure for

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squeezing rocks is higher for greater overburden and isdirectly proportional to the tunnel width. But theresearch later on proved that the support pressure forsqueezing rocks in all cases is not directly proportionalto overburden. The researchers on the subject havebeen able to predict the degree of squeezing, short term& long term support pressures, allowable tunnel clouresand strategy of supporting squeezing ground.

Steel fiber reinforced shotcrete with embedded ribs hasproved to be successful in supporting tunnels in themild to severe squeezing ground conditions. Followingdetailed strategy has been adopted in squeezinggrounds:(i) Circular or horseshoe shaped tunnel should be

planned in the suqeezing ground condition. Thetunnel width should preferably be less than 6min severe or very severe squeezing grounds. Theexcavated diamater may be 10% more than thedesign diamater.

(ii) The excavation should be by heading andbenching method in minor squeezing ground andby multiple drift method in severe or very severesqueezing grounds. Drill 10m advance probe holeahead of the tunnel face to know the rock massquality and drain out ground water, if any.

(iii) The horizontal drill holes of 3m length aredrilled ahead of the tunnel face and theforepoles of mild steel rods are inserted andwelded to the nearest steel ribs. Then smoothblasting is adopted with short length of blastholes (1m) to cope up with the low stand-uptime.

(iv)A steel fiber reinforced shotcrete (SFRS) layer of2.5cm thickness is sprayed immediately toprevent rock loosening. Full-column groutedbolts are installed all around the tunnelincluding the bottom of tunnel.

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(v) Steel ribs with struts at the bottom are erectedand designed to support the forepole umbrellaand rock support pressure. The struts should bestrong enough to resist high wall supportpressures in the squeezing grounds.

(vi) Additional layers of SFRS are sprayed aftersome delay to embed the steel ribs. It willprovide lateral stability to ribs and also create astructurally robust lining.

(vii) The SFRS should also be sprayed on the floorto cover steel sets and counter heavingtendency of the squeezing ground bywithstanding high bottom support pressures.

(viii) The convergence of the tunnel roof and wallsshould be monitored and plotted with time. Incase rate of convergence/closure is not droppingwith time, additional SFRS layers need to besprayed. It is a good tunnelling practice ifmultple borehole extensometers are installed toknow what is happening within the broken zoneparticularly in severe or very severe squeezingground conditions.

Barla Giovanni (Ref.: 38) has reported some casehistories about full face tunneling usiing fibre glassreinforcement elemets, inlcluding the Saint Martin LaPorte access adit along Lyon-Turin base tunnel (atFrance-Italy border) located in severe squeezing groundconditions. The overburden was about 300-600m. Inthis tunnel, a yield-control support system combied withfull-face excavation was adopted successfully in orderto cope with the large deformations experienced duirngface advance through a highly heterogeneous, disrupted& fractured rock mass of Carboniferous formation, ofteneffected by faulting. A near circular cross section (ofradius 6.10m) was excavated with support system asunder (Fig. 14.13 & Fig. 14.14):(i) Stage 0: Face reinforcement, including a ring of

fiber glass elements around the tunnel

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perimeter, over a 2–3m length in direction oftunnel advance.

(ii) Stage 1: Mechanical excavation carried out insteps of 1m in length, installation of 8m longrock dowels along the perimeter, yielding setswith sliding joints and a 10cm thick shotcretelayer. The tunnel is excavated in the uppercross section to allow for a maximumconvergence of 600mm.

(iii) Stage 2: The tunnel is opened to the full sectionat a 30m distance from the face, with theapplication of 20cm shotcrete lining, yieldingsteel sets with sliding joints fitted with hiDConelements. The tunnel is allowed to deform in acontrolled manner with maximum convergencenot to exceed 400mm.

(iv)Stage 3: Installation of the final concrete liningat a distance of 80m from the face.

Fig. 14.13: Stages of Fig. 14.14: YieldingExcavation Supports

Nine hiDCon elements (one in the invert) are installedin slots in the shotcrete lining between the yielding typesteel sets. These elements (height 40cm, length 80cmand thickness 20cm) yield at approximately 40-50%strain with a yield strees of 8.5 MPa. With 9 elementsinstalled, if each element attains 50% strain themaximum allowed radial displacement is equal to 20cmapproximately, resulting into total convergence of 40cm.Also, with yield stress of 8.5 MPa, the radial confinemetstress on the surrounding rock results to be 0.3 MPaapproximately.

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11. Comparison between Squeezing and Swellingis shown in Table 14.01 below:

Table 14.01: Comparison between Squeezingand Swelling

[Jethwa (1981) and Jethwa & Dhar (1996)]

12. Tunnelling in Seismic Areas: A study of thepublished literature indicates that the tunnels andcaverns in rock medium do not suffer as much damageas the surface structures during major earthquakes (M= 8.5), particularly if they are located at a depth ofmore than 20m and there is no fault zone in theneighbourhood.

The explanation of drastic damage to surface duringshallow major earthquakes is that surface waves (calledRayleigh waves) have major energy than primary andshear waves. The amplitude of Rayleigh waves decaysexponentially with depth and it becomes negligible at adepth of about 15-20m below the ground level in rockmasses (just like surface waves in ocean).

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A dynamic analysis of an underground structure isessential when it is meant to accommodate humanactivities. In addition, transport tunnels may require adynamic analysis when they are located in the area ofhigh seismic activities and the active fault may becrossing it or may be very near to it.

When a dynamic disturbance strikes an undergroundstructure, some deformations are caused. Thesedeformations may be decomposed in three components,namely, radial, axial and tangential. The axialcomponent may be further decomposed into thelongitudinal and transverse (wave) components. Theradial deformation of the underground structures isimportant when the source of the dynamic disturbanceis located within the structure, which is normally notthe case in transport tunnels.

The longitudinal (axial) deformations are representedby alternating regions of compressive and tensile strainsthat travel as a wave train along the tunnel axis. Thetransverse (axial) component creates alternate regionsof negative and positive curvatures propagating alongthe tunnel. A tunnel lining that is stiff compared withthe surrounding soil responds as an elastic beam. For apositive bending associated with the transverse (axial)deformations, the top of the lining is in compressionwhile its bottom is in tension. The same is not true,however, for rock tunnels with flexible or no lining atall. In such cases, the tunnel in positive curvatureexperiences tensile strain on top and compressive strainat bottom. This dynamic effect consisting of alternatingcycles of compressive and tensile strain superimposeon the existing static state of strain in the rock andlining.

The tangential deformations result when wavespropagate normal or nearly normal to the tunnel axis.These may result into distortion of the tunnel crosssection and may lead to additional stress concentration.This effect is not severe as the tunnel diameter is muchless than half the wavelength. Another aspect associated

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with the tangential deformations characteristic of thedynamic disturbance is that of ringing i.e., entrapmentand circulation of dynamic wave energy around thetunnel (Owen et al., 1979). This is not possible as thewavelength of the dynamic disturbance is much morethan the tunnel radius. In general, the seismicwavelengths are very large (25-500m) compared to thenormal tunnel sizes.

Analysis of the maximum longitudinal strains in theconcrete linings during earthquakes, by researchers andby use of software packages, suggest that there is nocause for worry for tunnel stability because ofearthquakes in rock masses below 20m from groundsurface, except in the active fault zones. The Himalayanexperience, about large number of shrines located indeep caves remaining unaffected in spite of beinglocated in seismically active region and experiencingseveral big earthquakes, confirm this observation.

Use of some empirical equations (Ref.: 39) or ComputerSoftware can be made to design the support systemsof tunnels in seismic regions.

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PROBLEMS AND HAZARDS IN TUNNELLING

Knowledge of potential tunnelling problems and hazardsplays an important role in the selection of excavationmethod and designing a support system for undergroundopenings. Different conditions of the ground needdifferent approach to tunnel excavation and support.Common problems and hazards encountered duringtunneling are elaborated in this chapter.

1. Rock Burst: Rock burst is defined as any suddenand violent expulsion of rock pieces from an apparently(temporarily) stable opening. A schematic repres-entation of the phenomenon of rock burst is shown inFig. 15.01 below.

Fig. 15.01: Rock burst

Manifestation of slabbing and release of micro-seismicenergy may be the first sign but suddenly severalthousands of tons of rocks may breakout like anexplosion releasing seismic energy of a mild earthquake.Experience shows that deeper an opening is made inhard rocks, more vulnerable it becomes to rock burst.

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PROBLEMS AND HAZARDS INTUNNELLING

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It is obvious that failure of rock mass will occur wheretangential stress exceeds its biaxial (plane strain)compressive strength. In-situ stresses should bemeasured in drifts, in areas of high tectonic stresses,to know in-situ stress and tangential stress on openingrealistically. It will help in predicting rock burstconditions in massive rock masses. Rock burstspredominantly occur within the first few hours after ablast or an excavation step at tunnel sidewalls, wherethe tangential stresses reach their maximum and atthe tunnel face. For this reason, rock bursts are mostlyrelevant for the primary support design.

Following approach to tunnelling is recommended inrock burst prone areas:• A way of reducing chances of rock burst is to

make openings of small size. This is becauseamount of strain energy released per unit areaof excavation will be reduced considerably.

• Since stress concentration is responsible forinitiation of cracking, it may help to select ashape of excavation which gives minimum stressconcentration.

• Slow down the rate of excavation in the zone ofstress concentration to avoid sudden release ofhigh strain energy.

• The modern trend is to convert the brittle rockmass into a ductile rock mass by using full-column grouted resin bolts. The plastic behaviorof mild steel bars will increase the overallfracture toughness of a rock mass. So theoverstressed rock mass will tend to fail slowly,as the propagation of fractures will be arrestedby the reinforcing bars.

• A sequence of excavation must be so designedthat rock fails in a controlled manner. At leastno rock burst should occur near working faceduring working hours for protection of workers.

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2. Chimney Formation or Daylighting: Collapse oftunnel roof or a part of tunnel, extending up to the topof the tunnel and exposing the tunnel to daylight iscalled as “Chimney Formation” or “Daylighting” (Fig.15.02).

There may be local thick shear zones dipping towards atunnel face. The soil/gouge may fall down rapidly, unlessit is supported carefully and immediately. Thus, a highcavity/chimney may be formed along the thick shearzone. The chimney may be very high in water-chargedrock mass. This cavity should be backfilled by leanconcrete completely (Fig. 15.03).

Fig. 15.02: Chimney Fig. 15.03: ChimneyFormation Backfilling

Chimney formation is normally caused due to reasonslike weakness in the crown of a tunnel, insufficient coverto overlaying permeable water bearing strata andinsufficient cover to surface or insufficient cover tooverlaying deposit materials; as shown in Fig. 15.04,Fig. 15.05, Fig. 15.06 and Fig. 15.07 below.

Fig. 15.04: Due to weakness in crown of Tunnel

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Fig. 15.05: Due to insufficient cover to overlayingpermeable water bearing strata

Fig. 15.06: Due to Insufficient cover to surface

Fig. 15.07: Due to insufficient cover to overlayingdeposit materials

3. Face Collapse: Collapses can not only occur in thetunnel itself, i.e. in the crown, but also at the excavationface. In poor rock conditions, with a possibly instabletunnel face, the appropriate counter measures have tobe taken.

With the exception of tunneling through cohesion-lessgranular soil, the stability of the tunnel face is in generaltime-dependent, i.e. a face that is stable in the short-term may collapse in the long-term. Schematicrepresentation of face collapse is shown in Fig. 15.08below.

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Fig. 15.08: Face Collapse

Face collapse can significantly affect the tunnel advancerate as well as the support requirement. Therefore,stability of the face shall be ensured to avoiduncontrolled collapse. Temporarily or as an emergencymeasure, the face collapse can be controlled by FacePlugging (Fig. 15.09) but as a permanent measureground improvement techniques like grouting, pipe roofumbrella, face dowels etc. can be used to strengthenthe face and avoid its collapse. These methods havebeen elaborated in Chapter-14.

Fig. 15.09: Face Plugging

4. Water Ingress: The inclined beds of imperviousrocks (shale, phyllite, schist etc.) and pervious rocks(crushed quartzite, sandstone, limestone, fault etc.)may be found along the tunnel alignment. The heavyrain/snow charge the beds of pervious rocks with waterlike an aquifer. While tunnelling through the imperviousbed into a pervious bed, seepage water may gush outsuddenly (Fig. 15.10 and Fig. 15.11).

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Fig. 15.10: Water Fig. 15.11: WaterIngress Pooled in Tunnel

Water inflow influences the construction procedure,tunnel stability, and the environment. Groundwater flowin hard rock depends on discontinuities such as jointsand their permeability. Several factors can influenceon the hydraulic conductivity of the rock mass:• Joint characteristics like orientation, continuity/

length, roughness and frequency.• Stress situation.• Faults and adjacent fractures.• Dykes.• Composition and thickness of the overburden.

Sudden flood accompanied by huge washout of sandand boulders may also occur ahead of tunnel face whereseveral shear zones exist. The flooding problem becomesdangerous where the pervious rock mass is squeezingground also due to the excessive overburden. Themachines, including TBMs, get buried partly or fully. InParbati Hydroelectric Project Stage-II, near Kullu inHimachal Pradesh, India, where part length of HeadRace Tunnel was planned to be excavated by TBM, inMay’2007 routine probe drilling ahead of TBM punctureda water bearing horizon which resulted in inflow of waterof over 120 liter/second containing 40% sand & siltdebris. The inflow was sudden and occurred at a highpressure which could not be contained. Eventually over7500 m3 of sand and silt debris buried the TBM. Thisdelayed the commissioning of project by about 10 years.

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Much depends on preparedness, and on whether or notdiscontinuity and fault infillings are washed out in theprocess. This may cause exaggerated over-break andchimney formation, an unsafe working environment.

The seepage should be monitored near the portalregularly. The discharge of water should be plotted alongchainage of the face of tunnel. If the peak discharge isfound to increase with tunnelling, it is very likely thatsudden flooding of the tunnel may take place on furthertunnelling. It is suggested that experts be consultedfor tackling such situations.

Following approach for tunnelling is recommended toguard against water ingress:• Groundwater control by rock mass

impermeabilization using pre-grouting:Groundwater control is achieved by probedrilling ahead of the face followed by pre-excavation grouting (i.e. pre-grouting) of therock mass. The primary purpose of a pre-grouting scheme is to establish an imperviouszone around the tunnel periphery, by reducingthe permeability of the most conductive featuresin the rock mass. The impervious zone ensuresthat the full hydrostatic pressure is removed fromthe tunnel periphery to outside of the pre-groutedzone. The water pressure is gradually reducedthrough the grouted zone and the water pressureacting on the tunnel contour and the tunnel liningcan be close to nil. In addition, pre-grouting willhave the effect of improving the stability in thegrouted zone within the rock mass.

• Constructing a drained structure of the rockmass in combination with rock support: Thismeans that the support measure installed is notconstructed to take external water pressure.Excessive water must therefore not be allowedto build up behind the rock support measure.Excess water must either be piped to the watercollection system in the tunnel or taken care ofby a water protection system.

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5. Portal Collapse: The portal areas frequentlyrepresent some of the most problematic points duringthe excavation of a tunnel. Thus, Portal Collapse (Fig.15.12) is another problem to be tackled in tunnelconstructions.

Several factors, (e.g. the direction of excavation, themorphology of the site, the geo-mechanicalcharacteristics of the terrain etc.) influence the portalproblems. While it is highly desirable that the locationselected for the portal be in fresh rock with cover of thesame order as tunnel width and height, environmentalconstraints or other relevant considerations willsometimes dictate that the portal be located where thereis low cover, weathered rock, or even soil. Where rockis exposed, the preconstruction of a reinforced concreteportal structure will still be of substantial assistance.

Fig. 15.12: Portal Collapse

The philosophy of design used for portals is similar asused for any slope stability analysis. Active support andeffective drainage are key elements to ensure stabilityof the portal slopes. Chapter-18 may be referred forfurther details on Portals.

6. Toxic Gases and Geothermal Gradient: There areserious environmental hazards due to toxic or explosivegases while tunnelling in the argillaceous rocks.Sometimes methane gas is emitted by blasted shales.

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Improper ventilation also increases concentration oftoxic gases like carbon monoxide, carbon dioxide,hydrogen sulphide and sulphur dioxide. Attentionshould be given to the physical properties of the gases,as some gases tend to collect either in high or lowpockets in a tunnel complex. Monitoring of gases andoxygen should be carried out near the face of a tunnelespecially where blast fumes and gas emission aremaximum. Oxygen must be maintained at a level of20% or greater. Dust inside the tunnel should becontrolled for reducing health hazards. Therefore, wetdrilling method is recommended for both blast holesand bolt holes.

As rock engineers are going deeper and deeper, workersface a high temperature. The temperature may increaseat a rate of about 300 C per km. The temperature insidea 1400m deep NJPC (Nathpa Jhakri Power Corporation)tunnel in Himalaya, India, was as high as 450C or more.The efficiency of workers in such a high temperaturewas reduced drastically. They worked for two to threehours only after taking bath frequently with ice-filledwater. If possible, cool fresh air should be used forventilation to maintain a working temperature of around300C at the tunnel face.

7. Timely Decision: Tunnels require deployment ofconsiderable skill and care in their investigation,planning, design, construction and monitoring, if theyare to be safely constructed. Several of the tunnelproblems described above often arise due to failure toproperly plan and design for uncertainties, in particularfor an unfavourable change in ground conditions.Procedures should be developed to overcome theseuncertainties and permit safe tunnel construction. Butsuccessful application of these procedures requirestimely action to either prevent the failure or preventconsequential damages due to the failure. Any failurenormally gives some warning signs in advance. Theremust be procedures and protocols in place to noticethese signs and activate the remedial action immediately

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without wasting time. If the warning signs are ignoredor timely action is not taken on them, it leads tocatastrophic failure.

8. Failure Analysis: Every collapse or failure shall beanalyzed in detail and documented properly. This isrequired not only for settling cost questions fromcontractual and insurance point of view but it also helpsin deciding about continuation of work and need forany additional measures before starting/continuing thework. Documentation and sharing the failure analysis,helps as a repository of knowledge not only for theproject concerned but also for all such/similar projectsin future.

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SAFETY ASPECTS IN TUNNEL CONSTRUCTION

Working in underground structures is an inherently risk-prone activity. In view of this, there is a responsibilityon all stakeholder in tunnel projects– owner, consultantand contractor – to ensure absolute safety, as well asbroader Safety, Health and Environment (SHE) aspectsof construction.

1. Applicable Regulations: Various activities involvedin tunnel construction are covered by number of acts,regulations and codes. Some of them are:

• Indian Explosives Act – 1884• Mines Act – 1952• Mines Rules – 1983• IS:4081 (1986): Safety Code for Blasting &

Related Drilling Operations• IS:4756 (1978): Safety Code for Tunnelling

Works

2. Risk Assessment: Before commencement ofconstruction work, risk assessment should beundertaken after identification of likely hazards and foreach risk occurrence an estimate of the possibleconsequences should be determined. Based on thisassessment, appropriate risk mitigation and controlmeasures should be put in place to control or minimisethe risks. On-going risk assessment will be neededduring the construction phase, particularly if designchanges are made or unforeseen ground conditions areencountered.

3. Project Safety Plan (PSP): Since each undergroundproject has its own peculiarities and special features, itis essential for each tunnel project to carry outcomprehensive Risk Analysis of the particular projectand evolve a Project Safety Plan (PSP) or Health and

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SAFETY ASPECTS IN TUNNELCONSTRUCTION

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Safety Plan (HSP), before commencement of work. Whilethe agencies concerned may adopt standard provisionsof their respective organizations, it is essential to havea project-specific safety plan. The PSP shall be preparedby the concerned construction agency and got approvedfrom the competent authority. The engineer-in-chargefor the work should join with designers and contractorsfor developing the PSP & monitoring its implementation.The plan should also contain details of emergencyprocedures, workers welfare measures and medicalfacilities. A copy of the plan should be made availableto all supervisors/engineers at site. Workers shouldregularly be briefed/educated about provisions of theplan relevant to their work area.

4. General Safety Measures

4.1 Basic Philosophy: For underground, a fund-amental safety measure would be to assess thetype and category of rock and its’ stand-upcharacteristics. It is common practice to divide therock in different classes, depending on the rockmass classification system being used. It isessential to provide adequate rock supportingmeasures, as per the design of the support systembeing adopted, before the expiry of the permissiblestand-up time for each class of rock. In case oftunnelling in weak/soft grounds, suitable measuresfor ground improvement/reinforcement shall betaken, as per the design and as per the provisionsof PSP.

4.2 Personal Protective Equipment: All pers-onnel entering the tunnel during construction shallwear all applicable Personal Protective equipment(PPE). The PPE shall comprise, at minimum, SafetyHelmet, Safety (Hard) Shoes, tight clothing withno loose ends and Jackets/ clothing with reflectivestrips. Additional PPE such as goggles, gloves, dustmasks, helmet lamps, ear plugs/ muffs, safetyharnesses etc. shall also be adopted whereverconditions so warrant.

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4.3 Access Control Systems: A proper accesscontrol system should be in place to have a clearidea at all times on the identity of all personnelwho are inside the underground installations incase any accident takes place and rescueoperations are to be launched. It is also essentialto keep track of all equipment inside the tunnel. Itis common to issue tokens to all concernedpersonnel while entering the tunnel and retrievingthe same on exiting.

4.4 Signage: Well-illuminated boards shall beplaced at required locations to inform people ofsafety hazards inside the tunnel and theprecautions to be taken. Appropriate SafetySignage should be provided as per applicablestandards like IS:9457 (2005)- Code of practice forSafety Colours and Safety Signs and IS:12349(1988)-Fire Protection Safety Signs etc.

Names, contact numbers and addresses of officials/organisations to be contacted in case of emergencyshould be displayed at prominent locations of thesite.

All safety signs shall comply with theinternationally recognized Safety Colours asindicated below:• Yellow – Danger (Fig. 16.01)

• Blue – Mandatory (Fig. 16.02)• Red – Prohibition (Fig. 16.03)• Green - Safe Condition (Fig. 16.04)

Fig. 16.01: Danger (Yellow) Signage

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Fig. 16.02: Mandatory (Blue) Signage

Fig. 16.03: Red (Prohibition) Signage

Fig. 16.04: Green (Safe Condition) Signage

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4.5 Safety Management and Training: Safetypolicies should be established for all the agenciesinvolved in the project, with a clear chain ofcommand/communication and responsibilities forsafety.

All operations inside the tunnel or shaft shall becarried out under supervision of a competentengineer, who shall be responsible for ensuringsafety stipulations and he should brief all workmenon the plan of work before it is started with specialemphasis on all potential hazards and on the waysto eliminate or guard against them. In larger jobs,these responsibilities of safety management maybe delegated to an independent safety officerworking under the overall control of the engineerin-charge. Periodical meetings, preferably onceevery month, shall be conducted to revieweffectiveness of safety measures.

Appropriate training courses should be designedand put in place for people unfamiliar withtunnelling, before they are allowed to workunderground. Where 25 or more employees haveto work underground at any one time, at least onerescue crew of 5 employees per shift must betrained in rescue procedures and resuscitation, useof oxygen breathing apparatus and use offirefighting equipment. Where less than 25employees work underground, not less than 5employees must have such training in rescue work.

All workers should also be trained in use of safetydevices and appliances provided to them. In caseany worker feels that he cannot perform a worksafely he should immediately inform the site in-charge of his inability to carry on with the work.

The training of workers would include at minimum,safety induction (initial training in basics of safety)and training (routine training) exercises, medicalscreening of personnel for working inside tunnels,

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system of permits for simultaneous operations invarious locations, pep talks (regular talks toworkmen before they commence work, onimportance of safety and how necessary it is forthem to observe safety regulations for their ownwelfare) and tool box talks (specific safetyinstructions at site in the specific area of work forthe workmen), talks on specific operations to becarried out on the day, safety walkabouts (generalsafety observance checks carried out by safetystewards by going around the site and checkingobservance of the various safety regulations etc.),safety audits, safety reviews and mock drills etc.

The occurrence of any accident shall be reported tothe supervisory staff/officer and adequateprecautionary measures shall be taken by theengineer in-charge to prevent recurrence. Anaccurate record of such accidents shall be properlymaintained in format approved by Engineer-in-charge. Probable reasons of accidents shall beinvestigated and precautionary measures taken toavoid further recurrence. Accidents occurringduring the fortnight shall be discussed in thesafety meetings and adequate publicity shall begiven to the causes of these accidents and theirpreventive measures.

Mock drills should be conducted periodically(preferably once every six months) to assess thelevel of safety preparedness/awareness. ConcernedState/District administration authorities should alsobe briefed about and involved in conducting ofmock drills. Mock drill can include scenarios forevacuation and rescue/relief in case of differenttypes of accidents/hazards.

4.6 Medical Facilities: Arrangements for rend-ering prompt and adequate first-aid to the injuredpersons shall be maintained at work site. At leastone experienced first-aid attendant with hisdistinguishing badge shall be available on each

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shift to take care of injured persons. Engineer orforeman, who is normally present at each workingface in each shift, can be given adequate trainingon first-aid methods to avoid employment of aseparate attendant. Arrangements shall be madefor calling the medical officer, when such a needarises. Depending upon the magnitude of thework, availability of an ambulance at a very shortnotice (at telephone call) shall be ensured.Stretchers and other equipment necessary toremove injured persons shall be provided at portal.

Where there are more than 50 persons working ina shift, effective artificial respiration arrangementsshall be provided, with trained men capable ofproviding artificial respiration.

4.7 Ventilation: Ventilation shall be carried out intunnels to make the working space safe forworkers by keeping the air fresh and byeliminating harmful and obnoxious dust, explosivefumes, exhaust from operating equipment,particularly diesel operated equipment, and othergases. Mechanical ventilation shall be adoptedwhere necessary to force the “air” in or exhaustthe air out from the working face of the portalthrough ducts. Externally located fans operate inforced ventilation and induced ventilation modes tosupply air through rigid or more commonly,flexible ducts. Immediate booster fans ensurebetter supply for longer ducts. Ventilation shall beproperly designed considering the tunneltopography and emission levels inside. A minimumof 200 cubic feet of fresh air per minute is to besupplied for each employee underground.Mechanical ventilation, with reversible airflow, is tobe provided in all of these work areas, exceptwhere natural ventilation is demonstrablysufficient. Where the temperature is high or heavyblasting is resorted to suitably augmented volumeof air shall be provided. Where blasting or drilling

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is performed or other types of work operationsthat may cause harmful amounts of dust, fumes,vapours, etc., the velocity of airflow must be atleast 30 feet per minute.

It is important to be alert all the time for thepresence of toxic gases in underground works andappropriate instrumentation should be provided tokeep track of the ambient air quality at all times.Proper records shall be maintained of specificmeasurements of air quality at regular intervalsthroughout the day after blasts or major rock falls.Particularly after each blasting for undergroundrock excavation, the ventilation measures shouldbe set in place quickly and effectively for de-fuming and personnel should be allowed to enteronly after establishing that the air quality issufficiently acceptable. In certain regionsgeothermal conditions prevail and cooled airshould be supplied to enable safe and comfortableworking conditions. In any case appropriate andwell-designed ventilation systems should be put inplace to ensure proper ambient conditions.

Air quality test shall be carried out once every 24hours but in any case after every blast or a majorrock-fall. Portable instruments should be providedto test the atmosphere quantitatively. A record ofall tests should be maintained and be keptavailable for inspection. In case any of the gasesare detected to have crossed the threshold valueindicated therein, the workmen shall be withdrawnimmediately till the percentage is brought downwell below the threshold value by improving theventilation or by other effective measures. In casepresence of gases like methane is detected, furthertunnelling work shall be stopped and the advice ofDirector General Mines Safety (DGMS) shall be sought.

4.8 Noise Protection: Sufficient steps should betaken to reduce noise levels to acceptable limitsand workmen and visitors should be asked to wear

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ear plugs/muffs etc. where required. Exposure toa noise level of 85 dB(A) can cause damage tohearing. Steps must therefore be taken to reducethe noise. As proper protection is only possiblewhen the protective devices are properly fitted andworn, an effective assessment, fitting and trainingprogram should be put in place.

4.9 Lighting: Adequate lighting should be prov-ided at the face and at any other point where workis in progress and at equipment installations suchas pumps, fans and transformers. A minimum of50 lux shall be provided at tunnel and shaftheadings during drilling, mucking and scaling.When mucking is done by tipping wagons runningon trolley trucks a minimum of 30 lux shall beprovided for efficient and safe working. Thelighting in general in any area inside the tunnel oroutside an approach etc. shall not be less than 10lux.

Emergency lights (battery operated) shall beinstalled at the working faces and at intervalsalong the tunnel to help escape of workmen incase of accidents. All supervisors and gang-matesshall be provided with cap lamps or hand torches.It shall be ensured that at least one cap lamp orhand torch is provided for every batch of 10people. Any obstruction, such as drill carriages,other jumbos and drilling and mucking zones inthe tunnel shall be well lighted.

4.10 Communication System

(A) Warning Signs and Notice Boards: Irrespe-ctive of length and bends in the tunnel,arrangements shall be made for transmitting ofwarning signals by any one of the following means:(a) By electrically operated bells, operated bybattery/dry cells with the bell placed outside thetunnel and the position of the switch shifting withthe progress of the tunnelling work. The position of

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the operating switch, shall be so chosen as to ensureproper accessibility and easy identification. (b) Bythe use of field (magnet type) telephone. (c) Anyother suitable arran-gement like walkie-talkie.

For tunnels up to 100m, only one of these systemsmay be adequate whereas in tunnels of length morethan 100m at least two systems shall be installedwith the wires running along opposite sides of thetunnel, if practicable. These system(s) shall besubject to daily checks regarding properserviceability, under the supervision of a responsibleperson.

Red and green lights of adequate size and brightnessshall be provided at suitable intervals on straightlengths and curves etc. to regulate the constructiontraffic.

(B) Telephone System: A telephone system shallbe provided to ensure positive and quick method ofcommunication between all control locations insidetunnel and portal of the tunnels when longer than500m and for shafts when longer than 50m.

(C) CCTV System: Closed Circuit TVs are oftendeployed to keep a continuous watch onunderground installations from the Control room.

4.11 Fire Protection

(A) General: All combustible materials like rubbishshall be continuously removed from such areaswhere flammable liquids are stored, handled andprocessed. All spills of flammable liquids shall becleared up immediately. Containers of flam-mableliquid shall be tightly capped. All waste andcombustible rubbish shall be removed at least dailyfrom the tunnel.

(B) Fire System: Fire Incidence Detection Sys-temsshould be able to detect the fire very early in itsdevelopment and also accurately locate the positionof the fire. The degree of accuracy depends on the

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type of active fire safety systems installed in thetunnel. Clearly visible Fire Points shall be establishedinside the tunnel (near any room, recess orcompartment etc.) for use in an emergency. Eachfire point should have available as a minimum DryPowder Extinguisher, Water Type Extinguisher andBucket of Sand. Recharging of fire extinguishers andtheir proper maintenance should be ensured.Supervisors and workmen at the site should betrained in the use of fire-fighting equipment providedat the site.

Water supply for firefighting purposes should beprovided at the construction site. This may be inthe form of static water tank of adequate capacityor a hydrant line with adequate water pressure atoutlet points. Sufficient number of fire hoses withbranch pipes should be provided at site so that thefire can be controlled until the arrival of the FireBrigade. Telephone Number of the local fire brigadeshould be prominently displayed near each telephoneon site. Fire Alarms should be provided atappropriate locations inside the tunnel.

(C) Electrical Installations: The entire electricalinstallation shall be carried out according to theexisting Indian Electricity Rules as modified fromtime to time. Voltage for lighting in a tunnel shouldbe 125V between phases as specified forunderground lighting in terms of Rule 118 (c) ofIndian Electricity Rules, 1956. The electricalinstallations should be designed and executed andregular tests should be carried out to ensure safeconditions and emergency cut-off procedures.Electrical leakage monitoring system should be inplace. All parts of electrical installation shall haveall conductors and contact areas of adequate currentcarrying capacity and characteristics for the workthey may be called upon to do and all joints inconductors shall be properly soldered or otherwiseefficiently made. They shall be so constructed,

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installed and maintained as to prevent danger offire, external exposition and electric shock, be ofadequate mechanical strength to withstand workingconditions underground, be not liable to be damagedby water, dust or electrical, thermal or chemicalaction, to which they may be subjected, be efficientlyinsulated or have all bare live parts enclosed orotherwise protected, and be installed at such locationthat dumpers or wagons do not come in contactwith the same.

On the occurrence of a fire caused by any electricalapparatus or a fire liable to effect any electricalinstallation; the supply of electricity should be cutoff from such apparatus or installation as soon aspracticable, and the fire shall be attacked andreported to the nearest available supervisor. As faras practicable, combustible material shall not beused in the construction of any room or recesscontaining electrical apparatus. No combustiblematerial should be stored in rooms, recesses orcompartments containing electrical apparatus.

A passageway not less than 60 cm wide shall bemaintained in front of switchboards. In no case,space in front or back of a switchboard shall beallowed to be used as a change room, locker orstorage room. Rubber mats shall be provided infront and in back of the switch boards. No one shallbe permitted at the back of switchboards when thecurrent is on.

All electric wires carrying voltage 440 and above,installed underground, shall be in the form ofinsulated, lead covered cables, armoured effectivelyagainst abrasion and effectively grounded.

Most tunnels are wet or damp providing a perfectground for short circuits. Steel forms and drillcarriages shall, therefore, be properly grounded. Theswitches shall be located on a high ground and theseshall be properly grounded. All electrical apparatus

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including portable tools shall be connected only toan electrical supply system, which shall have properearthing and grounding.

Following notice shall be kept exhibited at suitableplaces:

• A notice on the board of 45x30cm prohibitingunauthorized persons from entering electricalequipment rooms.

• A notice on the board of 45x30 cm prohibitingunauthorized persons from handling or interferingwith electrical apparatus.

• A notice on the board of 60x90 cm containingdirections as to the rescue of persons in contact withlive conductors and the restoration of personssuffering from electrical shocks.

• A notice specifying the person to be notified incase of electrical accident or dangerous occurrence,and indicating how to communicate with him.

Adequate fire extinguishing equipment suitable foruse on live parts shall be kept ready for immediateuse in or near any room, recess or compartmentcontaining such parts as will be readily accessiblesafely for use in case of emergency. This equipmentshall be inspected at least once in a month.

4.12 Housekeeping

(A) General: Only the material required for workin progress shall be kept inside the tunnel. All othermaterial shall be removed from inside the tunnel.Sufficient width of formation as even as possibleand without any obstacles shall be created to enablethe workers to get out of the tunnel quickly in casethere is any collapse or any other mishap inside thetunnel.

(B) Traffic Control: Vehicles carrying pipe, rail andtimber shall be properly loaded for safe passagethrough the tunnel. The load shall be kept within

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the side limits for the vehicle as loads projectingover the sides are dangerous to men working in thetunnel. For transportation of wide loads, special careshall be taken in operation of the vehicles, with priorwarning to the workmen along the tunnel to ensurea safe journey.

A safe and smooth walkway system shall be providedfor employees, suitably separated from the vehicularroads by guard railing. For transportation ofemployees by vehicles proper safety precautionsshall be taken.

(C) Pipes and Cables: All water and air pipes aswell as electrical cable shall be arranged along thesides of the tunnel, duly supported at regularintervals and in a systematic and neat fashion.

(D) Water Control: Sudden water ingress can bea catastrophic situation in certain underground areasand emergency dewatering systems should be inplace to tackle such situations.

Many times water seepage is encountered inunderground excavations. Prima-facie this is not adangerous indication by itself. It is an indication offissures in the rock and presence of water streamsnearby, which have to be kept in watch. Excessiveingress of water can give rise to unsuitable conditionsand has to be carefully monitored. Also, for goodworking conditions inside underground enclosures,continuous dewatering to remove the excessiveinflow is essential.

A study of boring data and geological formationsshall be made to have an indication of locations,where water can be expected. Water inflow may bereduced or even entirely stopped by grouting of thewet seams. A wet area covering more than a singleseam shall be sealed off by installing a suitablesection of concrete lining. In case of a steady flowof water from the roof or side of the tunnel the flowshall be deflected down the sides to sumps by metal

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shields. The number of pumps provided at site shallbe 50% more than the requirements calculated onthe basis of the estimated pumping needs, or atleast one number, whichever is more. In case ofsteeply inclined tunnels steps shall be provided forquick exit in case of failure of haulage. Gutters andsumps shall be kept clean. Suitable arrangementsshall be made to indicate the position of sumps incase tunnel invert is flooded.

4.13 Working with Machinery & Equipment:Precautions for safety while working withconstruction machinery & equipment shall befollowed as per Manufacturer’s guidelines &applicable codes. Special care should be takenwhile working with compressed air.

4.14 Insects, Leeches, Vermins and Snakes:Protection against insects, vermins, leeches orsnakes shall include the following:

• Use of boots, hoods, netting, gloves, masksor other necessary.

• Drainage or spraying of breeding areas.• Elimination of unsanitary conditions which

propagate insects or vermins.• Approved first-aid remedies for the affected.

4.15 Emergency Management System: An Eme-rgency Management Plan shall be part of theapproved Project Safety Plan and shall be wellcommunicated to all working personnel and welldisplayed at the site. Emergency Rescue Measuresshould be drawn up to take care of variouspossible contingencies. It would also be advisableto provide safe rooms in deep installations wherepeople can take shelter for a few hours in case ofemergency. Buried large diameter pipe linesleading to outside can be provided to offer amedium for communication and feeding in airsupply in case of any collapse and blockage ofentrance to underground structures.

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5. Safety Requirements for Various Activities

5.1 Drilling and Blasting

(A) Drilling Operations: Only wet drilling shall bepermitted. Drilling shall not be resumed after blastshave been fired until a thorough examination hasbeen made by the blasting foreman to make surethat there are no misfired charges. A drill, pick orbore shall not be inserted in the butts of old holeseven if examination fails to disclose explosives.Separate holes shall be so drilled as to nowhere lessthan 30cm clear distance away from the previoushole. Charging of drilled holes and drilling shall notbe carried out simultaneously in the same area,unless Nonel type of detonators are used andadequate precautions have been taken.

(B) Blasting Operations: Where blastingoperations are to be conducted, sufficient warningshall be given to all staff and workmen prior toblasting. Cell phones are usually prohibited in areaswhere blasting operations are conducted. Sufficientprotective bulkheads etc. shall be provided to enablepersonnel to take shelter behind during blasting.

All explosives shall be handled and used with careeither by or under the direction of competent personsand following the Indian Explosives Act-1884,Explosive Rules-1983 and IS:4081(1967)- Safetycode for blasting and related drilling operations.Explosives and detonators shall be placed in separateinsulated carriers whether carried by persons orconveyed mechanically and an attendant shall ridewith the explosives being conveyed mechanicallyon slopes in shafts or in underground work areas.For carrying explosives mechanically, priorpermission of Chief Inspector of Explosives shall beobtained. Insulated containers, used for carryingexplosives or detonator, shall be of approvedmanufacturer and shall be provided with suitablenon-conductive carrying device, such as rubber,

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leather or canvas handle or a strap. Explosives anddetonators shall be brought to the working placesin separate, tight, well insulated containers, and keptin the containers until removed for placement indrill holes. If drill holes are not ready, they shall bestored in locked box type magazines located at asafe distance of at least 170m from the workingspace. No person other than a shot firer shall carryany priming cartridges into a shaft, in which thesinking is in progress. No such cartridge shall be socarried except in a thick felt bag or other containersufficient to protect it from shock.

Electric firing shall be done by an approved method.All drilling equipment and personnel not engagedon loading shall be removed from the site beforeloading of holes starts. Loading of a round shall becompleted by the crew starting the work of loading.Firing of round shall be the responsibility of theblasting foreman. Only clay sticks or pneumatic airlocks shall be used for separation of charge/stemming of the holes.

Before use, each and every electric detonator shallbe tested with the help of an ohmmeter. Before shotfiring, the circuit shall be tested for insulation andfor continuity. Before a shot is fired in anunderground working place due warning shall begiven to persons within 330m in all directions andevery entrance to the place where a shot is about tobe fired shall be guarded so as to prevent any person,not having received warning from placing himself indangerous proximity to the shot.

In case an exploder is used the revolving handle ofthe exploder shall be in the custody of the blastingforeman to prevent anybody else firing the shot whenthe blasting foreman and other persons are inside.Stray currents may cause accidents while loadingand utmost care shall be taken in removing all faultsfrom electrical circuits. Electric power, light and othercircuits in the vicinity within 70m of the loading

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points shall be switched off after charging theexplosives and before the blasting operation starts.Power supply is to be switched on only after theblasted area has been properly inspected by theblasting foreman for misfires. All tracks, airlines andvent pipes shall be kept properly grounded. Theheading shall be properly lighted with the electricfloodlights before and after blasting.

(C) Inspection after Blasting: Immediately aftera blast has been fired, the firing line shall bedisconnected from the blasting machine or othersource of power. When at least 5 minutes havepassed after the blast was fired, a careful inspectionof the face shall be made by the blaster to determineif all charges have been exploded. Electric blastingmisfires shall not be examined for at least 15 minutesafter failure to explode. Other persons shall not beallowed to return to the area of blast until an “ALLCLEAR” signal is given by the blasting foreman.

All wires shall be carefully traced and search madefor any exploded cartridge by the man-in-charge ofthe blasting operation. Sufficient time shall be givenfor the fumes to clear before permitting the labourto work for mucking operation.

(D) Misfires: Misfired holes shall be dealt by theblaster preferably by the same person who has donethe charging operations. If broken wires, faultyconnections or short circuits are determined as thecause of a misfire, the proper repairs shall be made,the firing line reconnected and the charge fired. Thisshall be done, however, only after a careful inspectionhas been made of burdens remaining in such holesand no hole shall be fired when the burden has beendangerously weakened by other shots. The chargeof explosives from a misfired hole shall not be drilled,bored or picked out. Misfired charges, tamped withsolid material, shall be detonated by a safe approvedmethod.

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The stemming shall be floated out by the use ofwater or air jet from hose until the hole has beenopened to within 60 cm off the charge, and the watershall be pumped out or siphoned off and the newcharge placed and detonated. Whenever this methodis not practicable, a new hole shall be drilled 30 cmdeep and spaced not nearer than 60 cm, shall beloaded and detonated. A careful search shall be madeof the unexploded material in the debris of thesecond charge.

(E) Scaling and Mucking: It is essential to carryout proper scaling operations after each blast toremove all the loose rock pieces and guard againstrock falls. Careful and frequent inspection of wallsand roofs as well as of tunnel supports shall becarried out. Thorough scaling of loose rocks at allweak spots is the best prevention against rock falls.Periodic inspection of unsupported sections of thetunnel from a travelling scaling platform shall becarried out for locating weak spots. Supportedsections shall also be inspected regularly to makesure that the weakness of the formation has notspread beyond the supports. Loosened rock shallbe supported/removed forthwith. All supports shallbe checked occasionally to make sure that there isno member under distress. All scaling platforms shallbe equipped with safe ladders.

(F) Explosives Disposal: No explosive shall beabandoned. They shall be disposed or destroyedstrictly in accordance with the approved methodsand in doing so the manufacturers or the appropriateauthority shall be consulted. The expired deliveriesshall be sent back to the manufacturer. Explosives,caps, boxes, or material used in packing of explosivesshall not be left lying around in places to whichchildren or unauthorised persons or livestock canhave access. Paper or fibrous material employed inpacking explosives shall not be put to any

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subsequent use. Such material shall be destroyedby burning in the presence of a responsible person.

(G) Explosive Accountal: A day-to-day accountof the explosives shall be maintained in a registerin an approved manner, which shall be open toinspection at all times by the concerned authorities.Explosives shall be issued only to competent personsupon written requisition signed by the blaster or byan official authorized for the purpose and onlyagainst the signature or thumb impression. Suchrequisitions shall be preserved by the person-in-charge of the magazine.

5.2 Installation of Supports: Design andinstallation of appropriate supports within thestand-up time for the particular type of rock is themost important step to ensure proper safety for allpersonnel inside the tunnels. Special watch shallbe maintained for uncontrolled collapse of the faceor adjacent areas, sliding in of muck and wateretc. The stability of temporary supports should bechecked regularly. The bolts should be tested atregular intervals determined by rock conditionsand the distance from vibration sources.

5.3 Structural Steel Erection: All employeesworking in places where they are exposed tofalling hazards should use safety belts. Workmenshould stand clear from lifted loads, when derrickis in operation. When guiding a beam, it should beso held that the hands do not get jammed againstother objects. When lifting an object in a group,one person should be designated to give the signalfor all to lift or set the object down in unison.There shall be no riding on steel that is beinghoisted, no riding on the overhauling weights,hooks, cables or slings, nor sliding down on ropesor cables.

5.4 Scaffolds: Safe means of access shall beprovided to every place in scaffolds at which

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workers are required to work. Construction anddismantling of every scaffold shall be under thesupervision of a competent person. Boards andplanks used for the floors shall be of uniformthickness, well jointed, closely laid, and securelyfastened in place. While dismantling, the entirescaffold shall be maintained stable and rigid so asto avoid the danger of collapse. Nails from theplanking and various members of the scaffold shallbe carefully removed and all material carefullypiled.

5.5 Working Platforms: All working platforms,from which workers are liable to fall more than2m, shall be of adequate width depending uponthe type of work to be done and the width shallnot be less than 60 cm. Suitable guardrails ofheight 1m above the working surface shall beprovided. Platforms shall be kept free fromunnecessary obstruction and when they becomeslippery, appropriate steps shall be taken toremedy this.

5.6 Concreting

(A) Mixing Plant: Precautions shall be taken toprotect workmen from falling objects. The operationsof the plant shall be coordinated by signals etc. toensure safety of all workmen. An air exhaust systemshall be installed to remove cement and other dustsfrom the inside of the plant. Duct masks should beworn when necessary.

(B) Pumped Concrete: The pipeline shall beanchored at all curves and near the end. Air releasevalves shall be installed at high points to releaseentrapped air. If and when necessary to open a pipeto clear it of an obstruction, the work must becarefully done in order that workmen are not injuredby concrete blown out by air pressure in the pipe.

(C) Grouting and Shotcreting: Only experiencedman should be employed for grouting and

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shotcreting, which are special type of concrete work.All hoses and mixers shall be inspected daily andmaintained in a safe working condition.

5.7 Welding and Cutting

(A) General: All welding and cutting shall be doneby workmen who are thoroughly trained in the work.Shields shall be placed around the work to protectperson from glare. Welding and cutting shall not bedone in the immediate proximity of flammablematerials. A helper shall always be at hand to shutoff the gas in case of an accident when the welder isworking in a space from where escape is difficult.All welding operations should be carried out in awell-ventilated space. Eye exposed to welding orflashes should be washed with rose water for betterrelief.

(B) Oxy-acetylene Cutting and Welding: Keephose and cylinder valves free from grease, oil, dustand dirt. Keep cylinders away from stoves, furnacesand other sources of heat. Only “Gas Lighter” shallbe used to light the torch. Valve protection capsshall be in place when cylinders are not in use.

(C) Gas Cylinders: Gas cylinders shall be keptupright in safe places where they cannot be knockedover and well separated from combustible materials.Loaded and empty cylinders should be kept inseparate places. Tampering with or attempting torepair safety devices or valves of gas cylinders shallbe prohibited. Cylinders found to have leaky valvesor fittings, shall be taken into the open away fromany source of ignition, and slowly drained of gas.

(D) Hoses and Torches: Special care shall be takento avoid interchange of oxygen and acetylene hoses,as the mixture of these gases is highly explosive.Compressed air shall never be used to clean hosesas it may contain oil from the compressor. Oxygenshall be used to clean oxygen hoses and acetyleneshall be used to clean acetylene hoses. Copper or

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brass wire shall be used to clean the tips. Hardwoodsticks may also be used.

(E) Electric Arc Welding and Cutting: Only heavyduty electric cable with unbroken insulation shallbe used. When it is necessary to couple severallengths of cables for use as a welding circuit andoccasional coupling or uncoupling is necessary,insulated cable connectors shall be used. Frames ofall electric welding machines operated from powercircuits shall be effectively grounded.

5.8 Paints: Most paint materials are highlycombustible, and every precaution should be takento eliminate danger from fire. Fire extinguishers ofappropriate capacity shall always be at hand whereflammable paint materials are being mixed, usedor stored. Sand buckets or extinguishers of thecarbon dioxide and carbon tetrachloride type aregenerally affective.

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SAFETY ASPECTS IN OPERATION IN TUNNELS

Three types of accidents can occur in tunnels;derailments, collision and fires. An integratedcomprehensive approach towards tunnel safety has tobe implemented at all the stages i.e. planning, design,construction and operations. The philosophy followedin design and implementation of safety aspects for trainoperation in tunnels vary from country to country. TheEuropean countries (who have probably longest andoldest network of railway tunnels) adopt UIC provisions,whereas Japan have their own standards which are quitedifferent from UIC standards. In USA, the provisions oftheir statutes (e.g. for Fire protection provision of NFPA-National Fire Protection Association - Code 130“Standard for Fixed Guideway transit and PassengerRail System - 2017”) are applicable. There are no laiddown standards for Indian Railway so far and, therefore,the standards being presented in this chapter are basedon UIC approach, as per UIC Code 779-9 R (1st Edition,Aug’2003).

1. General: Tunnels of length up to 1 km are termedas “Short Tunnels”, tunnels of length 1 to 15 km aretermed as “Long Tunnels” and tunnels of length morethan 15 km are termed as “Very Long Tunnels”.

The recommendations are for tunnels in mixedpassenger/ freight traffic and normal operatingconditions (up to 200 trains/day). It does not coverunderground platforms and surfaces in urban areas.

Safety in tunnels is the result of an optimumcombination of infrastructure, operations and rollingstock measures. A general principle shared by allrailways can be divided in following categories:

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(i) Prevent accidents(ii) Mitigate the impact of accidents(iii) Facilitate self-rescue(iv) Facilitate rescue

For each of the above categories, a set of safetymeasures can be defined for a combination ofInfrastructure Measures, Rolling Stock Measures andOperations Measures.

2. Important Safety Aspects: Some importantaspects related to safety are discussed in this para.

2.1 Tunnel Ventilation: Ventilation of tunnel isone of the important aspects related to passengerand crew comfort during passage of train insidethe tunnel. It is also important for workmenworking inside the tunnel from their health pointof view. Movement of trains inside tunneltransforms its environmental features. Some of thepollutant gases, emitted from locomotives, may bepotential hazards to the health, physiological andpsychological comfort of human being. For safeoperation, it is necessary that these hazardousfeatures, especially gases emitted fromlocomotives, should not cause discomfort to crew,passenger and workman inside the tunnel.Concentration of pollutant gases and rise intemperature of air inside tunnel depends uponeffectiveness of ventilation in tunnel. Thus, it isnecessary that tunnels are provided with adequateventilation, so that concentration of hazardousgases and rise in temperature of air inside tunnelremain within permissible limits.

2.2 Effect of Movement of Train inside Tunnel:Passage of a train in a tunnel creates followingenvironmental hazards:

(A) Air Quality Deterioration: Emission fromdiesel locomotive contains potentially hazardousgases such as oxides of nitrogen (NO, NO2), oxides

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of carbon (CO, CO2) Sulphur-dioxide (SO2) andhydrocarbons. These gases get mixed up with theair inside tunnel and pollute it. High concentrationof carbon monoxide gas causes headache anddiscomfort and may be fatal if stay is prolonged.Nitrogen Oxides (NO, NO2) have toxic effects.Sulphur-dioxide (SO2) is bronchial and nasal irritant.

(B) Thermal Environment Hazards: As a loco-motive/ generator cars traverses through a tunnel,heat from exhaust gases is emitted. The air insidethe tunnel gets heated up due to heat emitted fromexhaust gasses/ locomotive surface. For safeoperation of trains in the tunnel, the thermalenvironment is to be controlled within a safe rangefor efficient functioning of locomotive and comfortof passengers, crew and workman.

(C) Pressure Transient Hazards: When a trainpasses through a tunnel, aerodynamic effects comeinto play. Due to this, the drag and propulsion powerincreases and the pressure environment around thetrain gets changed. The change of pressureenvironment around the moving vehicle may causesevere discomfort to passengers.

2.3 Permissible Values of Pollutants: The per-missible values for the concentration of pollutantsin tunnels depend upon the time of exposure.Threshold levels for various pollutants insidetunnels are tabulated in Table 17.01.

* These values are from the consideration ofpassengers comfort and shall depend upon thelength of the tunnel and speed of the train.

As workers are required to remain in tunnel for 8hours, values for 8 hours exposure need to beconsidered for the design of ventilation system.

Maximum temperature of air inside tunnel needs tobe limited to 400C considering passengers andworkmen comfort.

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Table 17.01: Threshold Levels for Pollutants

Pollutant Gas 8 Hours 15 MinutesExposure Values Exposure Values*

CO 50 ppm 400 ppm

NO 25 ppm 5 ppm

NO2 5 ppm 5 ppm

CO2 5000 ppm 18000 ppm

SO2 5 ppm 5 ppm

2.4 Types of Ventilation Systems: The ven-tilation in a tunnel can be achieved by:

(A) Natural Ventilation: When a train travels insidetunnel at a relatively high speed and ratio of trainfrontal area to tunnel cross section is of the order0.5 to 0.6, it induces considerable air flow insidetunnel. This type of ventilation is called as naturalventilation. The amount of induced air flow willdepend upon orientation of tunnel and atmosphericpressure difference between inside and outside thetunnel. If length of tunnel is small, the induced airflow may be sufficient to keep the pollutantsconcentration and rise in temperature inside tunnel,within permissible limits. In such case there maynot be any necessity for provision of artificialventilation.

(B) Artificial Ventilation: In long tunnels, naturalventilation is not sufficient to keep concentration ofpollutant gases under permissible limit. In suchcases, artificial ventilation may have to be providedby means of ventilation shafts with or without ofventilation fans, with suction and deliveryarrangement. Where provision of shaft is notfeasible, longitudinal ventilation with the help of anaxial blower fan at the portal supplemented byauxiliary fan of smaller capacity, spaced at suitableintervals along the length of tunnel may beconsidered (Fig. 17.01).

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Fig. 17.01: Artificial Ventilation

2.5 Tunnel Illumination System: Tunnel lighting(Fig. 17.02) is required for the following purposes:(i) Passengers and staff to detrain and make

their way safely out of the tunnel.(ii) To assist train crews in their orientation and

improve their visibility of the track. Adequatetunnel lighting allows drivers to quickly adjustto the light within, identify possible obstaclesand negotiate their passage without reducingspeed.

(iii) To let the inspectors or workers clearly seethe track elements/condition or go throughtheir routine inspections without usingflashlights.

Fig. 17.02: Tunnel illumination System

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2.6 Design of Tunnel Lighting Systems: The designof the lighting system should also:

(i) Take full account of the possible conditions ofthe tunnel following an emergency (forexample, fire and smoke).

(ii) Consider when and how the lighting shouldfunction. Options include:• Permanently switched on• Switched on automatically following an

incident or condition• Manually controlled

(iii) Consider how maintenance illuminance will beprovided in the event of failure of the normalpower supply.

2.7 Normal Tunnel Lighting: Illumination systemfor General Lighting would be provided to achieveminimum 10 lux illumination level. Lightingfixtures for general lighting should be energyefficient type such as LED lighting fixtures. HPSV(High Pressure Sodium Vapour) and MH (MetalHalide) lighting fixtures which are powerconsumption intensive should be avoided. Whenthere is no train operation for more than half anhour in tunnel or no maintenance activity is goingon, the lighting should be reduced (about 30% oras considered appropriate) of full lighting toconserve energy in main and escape tunnels.

2.8 Emergency Tunnel Lighting: Emergency lig-hting having luminance of at least 5 lux atwalkway level shall be provided to guidepassengers and staff to a safe area in the event ofemergency. Emergency tunnel lighting shall beinstalled on one or both sides of the tunnel.Emergency lighting shall be reliable and operativeunder emergency conditions (like presence ofsmoke). Lighting fixtures shall be energy efficienttype such as LED type. Emergency lighting circuit

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shall be alternative supply system (such as on UPSand DG set supply).

2.9 Power Supply System: The Power Supplyarrangement shall be designed keeping in view thereliability, redundancy, voltage drop at the farthestend of feeding system. Generally, the PowerSupply should be availed at 33 kV on both thetunnel portals from two independent sources whichshould be stepped down to 11kV first and then to415 V at the sub-station near tunnel for feedingE&M system at 415V. The standby DG set of fullload capacity should be provided at both tunnelportals to provide the standby Power Supply withredundancy in the system. Electrical equipmentshould be protected against damage, mechanicalimpact, heat or fire. The design of distributionsystem should be able to tolerate unavoidabledamage by energizing alternate links. For PowerSupply of control system and emergency lighting,120 minute UPS backup with adequate SMFbattery bank should be provided at all the Sub-stations at Tunnel Portals and inside the tunnel.

Alternatively, on electrified routes, Power Supplyfor Tunnel Ventilation and illumination systems canbe tapped from OHE by providing 25kV/415V threephase Auxiliary Transformers in order to maintainreliability of the Power Supply. The step downtransformers of 25kV single phase to 415V threephase will need to be specifically designed.

2.10 Cables: The 33kV double circuit, 11kVdouble circuits and LT circuits with redundancyshall be provided for which suitable cable ducts/conduits/trays should be planned keeping in viewthe maintainability. Fire resistant, Halogen FreeLow Smoke (HFLS) cables on protected cable ductsshould be provided. Physical protection of cablesagainst impact from derailments or constructionwork should be provided.

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2.11 Leaky Feeder: In the main tunnel and inthe accesses, a leaky feeder cable (or otheralternative system) shall be used as antenna. Itconsists of a coaxial cable with gaps or slots in it,with a regular span in its outer conductor forradiating or receiving signals along its entire length.

2.12 Sensors inside Main and Escape Tunnels:For monitoring of environmental conditions/pollutants, CO Sensors, Visibility Sensors, TrainLocation Sensors, Temperature Sensors, WindVelocity Sensors and Heat detection cables shouldbe provided inside tunnels with preset thresholdvalues. The data of these sensors should betransmitted to the SCADA system provided inTunnel Control Room through Optical fiber cable/dupe line communication. The Tunnel ControlRooms would be manned round the clock wherethe tunnel operator can take the action as requiredas per the condition monitoring of the tunnelenvironment.

2.13 Safety walkway near tunnel portals:Safety pathways of adequate width (on both sidesof track) should be provided near portals for fulltunnel length for quick evacuation of passengers incase of emergency.

3. Infrastructure Measures

3.1 Speed Monitoring/Signaling System:Monitoring of speed can be affected on thelocomotives, on speed-checking sections throughautomatic train control at fixed points (ATCS), bymeans of radar or ahead of signals, using signalbased controls. It is specified based on Operatingcharacteristics; train density and speed (e.g. >160km/h).

(A) Specifications: The system should be able toprevent train from overrunning a stop signal andexceeding maximum speed, with a high reliabilitylevel.

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(B) Assessment: New Tunnels: Speed monitoringrecommended, if equipment on the specific route isplanned.

Existing Tunnels: If upgrade of an existing systemis possible, then tunnels should have high priorityfor upgrade.

3.2 Train Radio: A train radio system permitscommunication between the train crew andoperations centre and with passengers in coaches.It includes fixed installations in tunnels andequipment on board trains (including coaches).

(A) Specifications: System is installed dependingon the standard of the line on which tunnels arelocated. Reliability is highly important.

(B) Assessment: New Tunnels: Recommended asa standard measure (including the possibility tocommunicating messages to all coaches in a train).

Existing Tunnels: Recommended (a) if tunnel is ona line equipped with train radio system, then tunnelshall be also equipped. (b) if line is not yet equipped,tunnels shall be a relevant argument when settingpriorities (c). A tunnel may not be equipped if it ison a line with a low volume of traffic or of secondarystandard.

3.3 Train Detection (axle counter, track circ-uit): Checking that a track section has beencompletely cleared and trains are complete.

(A) Specifications: installation depending onoperations conditions also e.g. very low-densitytraffic. It is combined with an adequate trainprotection/signaling system. This measure is notspecific to tunnels only: if decided to equip, it meansequipping the whole line.

(B) Assessment: New Tunnels: Recommended asstandard measures. Both measures are equallyeffective.

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Existing Tunnels: Recommended as standard(exceptions: for lines with very low-density trafficand simple operating conditions).

3.4 Train Control: Lineside fixed temperaturesensors for the detection of hot axles and wheels,so that trains can be stopped in a safe placebefore entering the tunnel.

(A) Specifications: Appropriate distance betweentwo installations: depending on the network-wideconcept adopted for installations (typical range ofbetween 25 and 100 km). Depending on theoperation mode for double-track lines, one or bothtracks are equipped. Rules and procedures to checka train.

(B) Assessment: New and Existing Tunnels:Recommended at the approach to sections withmany tunnels. Isolated tunnels will be covered bythe ordinary network of installations.

3.5 Arrangement of Switches: In tunnels and atthe approach to tunnel entrances, the installationof switches or other track discontinuities should beavoided (completely remove or shift the location).Accidents caused or influenced adversely byswitches will then not occur in tunnels.

(A) Specifications: Minimal distance betweenswitches and tunnel entrance optimized to take intoaccount line speed.

(B) Assessment: New and Existing Tunnels:Switches or other track discontinuities should bereduced to the operating minimum in tunnels. Ifnot possible, movable-point-frog switches should beconsidered (depending on speed, axle load andoperating requirements).

3.6 Access Control: Measures to prevent unau-thorized access to the tunnel portals or exits:signs, fencing, secure locks, remote or localsurveillance.

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(A) Specifications: Signs: warning and entryprohibited at tunnel entrances. Emergency exits:locked doors, possibility of opening doors from insideby anyone and from outside by railway/rescueservices (remote or on the spot). Fences for theportal area and emergency exits depending on theexposure and possible security hazard scenarios.Large doors for emergency access. Closed-circuit TV-monitoring of sensitive areas such as tunnelentrances depending on the exposure and possiblesecurity hazard scenarios. Remote monitoring by acontrol/operations centre for the tunnel.

(B) Assessment: New and Existing Tunnels:Security measures must be taken on the basis of arisk assessment including the location/exposure,accessibility of tunnel objects, their attraction astargets for vandalism or sabotage and localexperience/tendency to be subjected to vandalismand sabotage. For new tunnels, it is recommendedas a standard measure to put signs and fences attunnel entrances and to lock all exits (seespecifications). Further measures are recommendedonly if an assessment of security risks shows highrisks. It is recommended that existing tunnels beupgraded (optimization) if reasonable because ofthe local situation (in general in urban areas).

3.7 Double-bore Single-track Tunnels: Double-bore single-track tunnels instead of single-boredouble-track tunnels to avoid accidents caused bya derailed train obstructing the adjacent track andallow for better rescue conditions in the event ofan accident or fire.

Assessment: New Tunnels: The optimal systemshould be result of evaluation of all relevantparameters. The more cost-effective system shouldbe chosen provided that required escape distancesand operating restrictions (e.g. mixed traffic) canbe observed.

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Existing Tunnels: Not applicable if the tunnel is adouble-track tube.

3.8 Fire, Smoke and Gas detection in Tunnels:Installation of fire, smoke and gas detectors intunnels, enabling rapid location of a fire in ignitionphase in main tunnel and in technical rooms.

(A) Specifications: A distinction should be madebetween the main tunnel and technical rooms.

(B) Assessment: New Tunnels: Main tunnel - Notrecommended as standard. Gas detectorsrecommended for tunnels with a low point in thetunnel (u-shaped) and if gas could enter the tunnelfrom the surroundings.

Technical rooms - Fire and/or smoke detectors arerecommended for technical installationsconcentrated in separate rooms in a tunnel.

Existing Tunnels: In the course of a renewal/generalupgrade, the recommendations for new tunnelshould be followed as far as reasonable.

3.9 Fire Extinguishing System: Automatic ormanually-triggered fire extinguishing systems inorder to fight the fire in an early stage, in themain tunnel and in technical rooms.

(A) Specifications: To define the efficiency is partof a specific project.

(B) Assessment: New and Existing Tunnels: Maintunnel- no extinguishing systems are recom-mended. Automatic extinguishing systems arerecommended only for rooms with highly sensitivetechnical installations.

3.10 Ventilation System: A distinction must bemade between three main situations:

(a) Main tunnel: mechanical smoke extractionsystem in the main tunnel to draw out smoke or tocreate a defined air stream in order to obtain asmoke-free site for rescue.

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(b) Smoke extraction if a tunnel on a double-trackline consists of double-bore single-track tubes or atpassages between double-bore single-track tubes(to keep the parallel tubes free of smoke, to preventair streams)

(c) Safe places: ventilation systems to keepemergency exits, cross passages or a parallel safetytunnel free of smoke (produce overp-ressure).

(A) Specifications:

(a) No specification for the main tunnel.

(b) Combination of double-track/single-track tunnelor at passages between double-bore single-tracktubes: the ventilation/smoke extraction system hasto be designed so that smoke transfer from one tubeinto the other through the passage between the twotubes is reduced to a minimum. A detailed conceptand sufficient dimensioning of the system arenecessary.

(c) Safe places: The ventilation system has to bedesigned so that smoke transfer into the safe placeis reduced to the minimum when opening doors tothe main tunnel. If there are alternatives to an activeventilation system meeting that requirement, theyare acceptable as well.

(B) Assessment: New Tunnels:

(a) Smoke extraction in the main tunnel: notrecommended as standard measure to control smokespread.

(b) & (c) Recommended for specific situations, wheresafe areas should be kept free of smoke (e.g. paralleltubes, emergency exits). In order to achieve thisgoal, alternative measures such as doors or locksmay also be adequate.

Existing Tunnels: Not reasonably feasible for existingtunnels.

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3.11 Escape Routes (Routes, Handrail, Mar-king): Provision of walkways in tunnels tofacilitate escape (normally beside tunnel wall, alsoin or between tracks if there is not enough space).Handrail along the tunnel wall and especiallyaround obstacles. The escape route and directionsare marked by pictograms.

(A) Specifications: Minimum width of walkway fornew tunnels: >70cm, optimally 1.20m. In double-track tunnels on both sides of tunnel. Existingtunnels: optimisation of surface (e.g. compressedgravel, cable duct with larger slab). Hard and smoothsurface, free of obstacles as far as possible. Handrailleads around obstacles. Signs are to be located atlighting points: indication of escape direction anddistance to nearest exit.

(B) Assessment: New Tunnels: Recommended asa standard measure as specified.

Existing Tunnels: Improvements to enable adequatemovement are recommended for existing tunnelsas basic equipment, solutions should be optimisedand consider the specific risk situation (tunnellength, traffic, rescue concept).

3.12 Emergency Tunnel Lighting: Lights alongone or both tunnel walls for lighting the escaperoutes in the event of a train evacuation. Thelighting shall ensure uniform illumination of theescape route in order to enable evacuees to walksafely.

(A) Specifications: Following specifications arebased on the assumption of electric lighting.Alternative technical solutions are possible as wellif they fulfil the intended functions.

• On one or both sidesSingle-track tube: one side (same as walkway)Double-track tube: both sides

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• Luminosity

Enable a safe walking as far as possible alsounder smoke conditions and poor visibility

• Height of lights

Above walkway, as low as possible, dependingon free space

• Autonomy and reliability:

(a) Guaranteed power supply for emergency oralternative concepts to ensure high reliability.

(b) Supply cables protected against mechanicalimpact and fire

(c) It is recommended to build sections for powersupply/lighting

• Other specifications:

Possibility to switch on from operating centre,portals and inside the tunnel.

Minimum distance between portal and firstswitch is 250 m if security aspects are relevant.

Under normal operation, lighting is switched off.

(B) Assessment: New Tunnels: Reco-mmended fornew tunnels as specified in the specifications.

Existing Tunnels: Tunnel lighting is generallyrecommended for tunnels of about >1 km. Decisionsshould be based on a risk assessment consideringat least operations data and tunnel length. In orderto keep a good cost-effectiveness ratio, specificationsmay be less stringent: lighting on one side only,lower requirements for luminosity and reliability.

3.13 Emergency Telephones/CommunicationMeans: Emergency telephones or similar means ofcommunication so that passengers, too, can usethem in emergencies, connected with operationscentre (independent of train radio or mobilephone). Emergency telephones shall permit

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adequate and reliable communication during anyemergency.

(A) Specifications: Clearly visible and easy to use(indications necessary). Reliability. Direct and easy-to-use connection to the operating centre. Distancebetween phones: 500–1000 m as guideline(depending on distance between exits or crosspassages). Additional/alternative locations: Portalsand exits.

(B) Assessment: New Tunnels: Recommended asstandard.

Existing Tunnels: Recommended as standard,optimisation of existing telephones as far asreasonably possible.

3.14 Escape Distances: A maximum distancebetween two safe places (portal, emergency exit,cross passage) in the tunnel is defined in order toenable self-rescue.

(A) Specifications: Distance between safe places:1000m (mean escape distance for self-rescue of500m) as general guideline. For double-bore single-track tubes and parallel safety tunnel: reduceddistance of 500m (cost-effective). This distance canvary depending on the local situation, operatingparameters and the total safety concept.

(B) Assessment: New Tunnels: The costeffectiveness ratio depends on the local situation(costs). Under favourable conditions, a good cost-effectiveness ratio can be assumed.

Existing Tunnels: For existing tunnels additionalconstruction work is very expensive and if for safetyreasons only, the cost-effectiveness mostly will bevery unfavourable.

3.15 Vertical Exits/Access: Construction of ver-tical exits from the tunnel which are used forescape as well as for access by rescue services.

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(A) Specifications: Maximum height should be lessthan 30 m, width of stairs of about 1.2m as aguideline. Design or installation necessary thatprevents smoke spreading into the safe place(possible solution: locks or ventilation system).Equipped with lighting and communication means(e.g. telephone). Design or installation that preventsunauthorised access from outside.

(B) Assessment: New Tunnels: If vertical exits areplanned, a distance of about 1000m between theexits.

Existing Tunnels: For high risk tunnel: possibility toimprove tunnels in the course of a total renewal, ifopportunities (nearness to surface) exist. Decisionshould be made based on a sound evaluation.

3.16 Lateral Exits/Access: Construction oflateral exits from the tunnel which are used forescape as well as for access of rescue services.

(A) Specifications: Cross section: 2.25m x 2.25mas a guideline. Maximum length of about 150m as aguideline, but if longer, it should be accessible withroad vehicles. Design or installation that preventssmoke from spreading into the safe place (possiblesolution: locks). Equipped with lighting andcommunication means (e.g. telephone). Design orinstallation that prevents an unauthorised accessfrom outside.

(B) Assessment: New Tunnels: If lateral exits areplanned, a distance of about 1000m between theexits.

Existing Tunnels: For high-risk tunnels: possibilityto improve tunnels in the course of a total renewal.Decision should be made based on a soundevaluation.

3.17 Cross Passages: Cross passages betweendouble-bore single-track tunnels or between adouble-track tunnel and a safety tunnel.

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(A) Specifications: Cross section: 2.25m x 2.25mas a guideline. Design or installation that preventssmoke from spreading into the safe place. Equippedwith lighting and communication means (e.g.telephone). Design or installation that preventsunauthorised access to the neighbouring tube if trainoperation has not yet been stopped.

(B) Assessment: New Tunnels: If cross passagesare planned, a distance of about 500m between thepassages is recommended.

Existing Tunnels: For high risk tunnels: possibilityto improve tunnels in the course of a total renewal.Decision should be made based on a soundevaluation

3.18 Parallel Service and Safety Tunnel:Provision of a service and safety tunnel parallel tothe main tunnel (double-track). The tunnel is keptfree of smoke and provides a safe place in theevent of fire and other accidents. The safety tunnelcan also be used by emergency services.

(A) Specifications: Cross passages to the maintunnel. Cross section: 3.5m x 3.5m as a guideline,accessible by road vehicles, possibilities to reverseand pass. Independent ventilation system (or similarinstallation) in order to keep the safety tunnel freeof smoke (produce overpressure in relation to thecross passages and the main tunnel).

(B) Assessment: New Tunnels: Should be the resultof an evaluation of the optimal system. Notrecommended as general solution.

Existing Tunnels: For high-risk tunnels: possibilityto improve tunnels in the course of a total renewal.Decision should be made based on a soundevaluation.

3.19 Access to Tunnel Entrance and Exits:Access road to portals and emergency exits forrescue services.

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(A) Specifications: Access roads shall be accessiblefor normal fire brigades vehicles. Solid surface(damage after a large intervention is acceptable).Minimal width: 3m. The road ends at the rescuearea or at a solid turning place. As close asreasonable to the entrance, depending on localtopography.

(B) Assessment: New Tunnels: Recommended incombination with rescue area.

Existing Tunnels: Recommended to improvesituations as far as reasonably practicable. If notpossible, helicopter landing areas in the vicinityshould be defined and prepared as far as reasonablypracticable.

3.20 Rescue Areas at Tunnel Entrance orExits: Rescue areas are situated in the vicinity oftunnel entrances and emergency exits as the basefor rescue operations.

(A) Specifications: The area at the entrances ofnew tunnels should include:

• Access road to the area, accessible forfire-fighting lorries, solid road surface,possibility for two vehicles to cross onthe way.

• Power supply, lighting, fixed provisions/installations for communication.

• Possibility for water supply (on site or inthe vicinity).

• Defined helicopter landing area(20mx20m) with road connection to therescue area.

• Access to the portal.The area at exits should include:

• Access road to the area, accessible forfire-fighting lorries, solid road surface,turning area and if this is not possible,helicopter landing area.

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• Power supply, lighting, possibility forwater supply (on site or in the vicinity).The area for existing tunnels shouldinclude

• Access road to the area, accessible withheavy fire-fighting lorries, turning areaand/or defined helicopter landing area(20mx20m) as far as reasonablypracticable.

• Power supply, lighting, possibility forwater supply (as far as reasonablepracticable).

Local possibilities have to be taken into accountoptimisation.

Existing roads, places and land area should beincluded in the considerations.

(B) Assessment: New Tunnels: Recommended asstandard safety measure, in the light of localpossibilities.

Existing Tunnels: Generally recommended with therestriction that local topography and possibilitiesshould be taken into account for optimising.

3.21 Water Supply (as access, in tunnel):Continuous water main through the tunnel:permanently filled or dry pipe. Branch lines totunnel entrances: portals, emergency exits:permanently filled or dry pipe.

(A) Specifications: Supply of water pipe: pool,hydrant in the vicinity, connected to water supplysystem, other sources (e.g. river). Reserve of 100m³ at tunnel entrances if water supply is based onpools. Hydrants in the tunnel: every 250m if thereis a continuous pipe; at emergency exits, if supplyis only through these exits. Installed on one side ofthe track. Design should especially considermaintenance aspects.

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(B) Assessment: New Tunnels: Water supply ascontinuous pipe through the tunnel or branch linesto portals and exits are recommended as standard.Alternative solutions with mobile railway means areadequate too, if they are based on “professional”rescue organisation (e.g. fire-fighting and rescuetrain).

Existing Tunnels: If the intervention concept is basedon railway resources: mobile water supply isrecommended (e.g. rescue train). If the concept isbased on fire brigades: water supply to the tunnelportals is recommended, e.g. mobile means by road,water reserves in the vicinity. Additional equipmentof an existing tunnel only in the course of a renewal.

3.22 Electrical Supply for Rescue Services:Power supply suitable for the equipment needed byemergency services in tunnels.

(A) Specifications: Distance between outlets: 125–250m. Ensure compatibility for rescue service andmaintenance. Location in niches, concentrated withother electrical installations and communicationmeans. On one or both sides of the track. For shorttunnels and/or existing tunnels: mobile means asalternative

(B) Assessment: New Tunnels: Recommended tointegrate in a comprehensive concept for powersupply and installations.

Existing Tunnels: Recommended to upgrade existingtunnels in the course of a renewal of a tunnel orelse to provide mobile means.

3.23 Radio Installation for Rescue Services:Ensure radio communication for emergencyservices in a tunnel between emergency services,operations centre, railway personnel (in general:own frequencies for rescue services).

(A) Specifications: Channel with commonfrequency necessary.

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(B) Assessment: New Tunnels: Recommended asa standard measure.

Existing Tunnels: Generally recommended, butdepends on the specific situation, alternativespossible.

3.24 Control Systems: Tunnels with largeelectromechanical installations shall be equippedwith a centralised control system (tunnel controlcentre).

(A) Specifications: Control of: ventilation/smokeextraction system, lighting, communication means,power supply and all other safety systems, etc.Security measures such as closed circuit TVmonitoring system. Eventually also operating tasks.Professional staff/24h-operation.

(B) Assessment: New Tunnels: Not recommendedfor new tunnels less than 15 km long. It is reasonableto integrate these functions into ordinary operationscentres which are also responsible for the stretcheson the approach to a tunnel.

Existing Tunnels: Only for new tunnels.

3.25 Tunnel Rescue Train: Railway vehicles forrescue purposes can be defined on different levels:(a) Provision of carrier wagons to carry rescue

vehicles and of tank wagons for watersupply. Fire brigades load their vehiclesonto the carrier wagon hauled by alocomotive or tractor.

(b) Special rescue unit/train: rescue train forrapid transport of staff and equipment. Thetrain is specially built for intervention andserves as a means of transport, a base forfire-fighting, first aid, transport of injuredpeople and for communication. The staff iscomposed of railway staff and local firebrigades.

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Assessment: New and Existing Tunnels:(a) Recommended if it is part of a

comprehensive rescue concept whichincludes alternative ways to reach the siteof an accident in a tunnel (e.g. throughexits) or for an individual tunnel in a lowerrisk class.

(b) Recommended if it is part of acomprehensive rescue concept basedprimarily on railway resources for rescue.

3.26 Road/Rail Vehicles for Rescue: The rele-vant fire brigades are provided with road/railvehicles which are able to run on track to conveystaff and equipment rapidly to the accident site.The main goals are interception, support of self-rescue, first aid and initial fire-fighting action.

(A) Assessment: New and Existing Tunnels:Recommended only if road/rail vehicles are part ofa comprehensive rescue concept based on firebrigades.

4. Prevention of Fire on Rolling Stock

4.1 Fire Load and Prevent Fire Spreading:(a) Constructive measures/vehicle design to

prevent outbreak and spread of fire.(b) Avoiding the use of materials producing toxic

substances/a large amount of smoke in theevent of fire.

(A) Specifications: Reduction of the fire load;separation (compartment-type construction withinterconnecting doors constructed as fire doors); useof fire-resistant materials; replacing flammable byhardly-flammable material; introducing fire resistantlayers inside seats although these increase the fireload.

(B) Assessment: New and Existing Tunnels: It isrecommended that fire safety aspects be emphasisedand integrated in the specifications for new rolling

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stock and also that it be ensured that they are takeninto account for coach renewals.

4.2 Onboard Fire Detection:(a) Automatic fire detection on traction units to

detect fire at an early stage (with notificationto the driver).

(b) Automatic fire detection on coaches to detect fireat an early stage (with notification to the driver).

Assessment: New and Existing Tunnels:(a) Recommended for traction units(b) Not recommended for passenger coaches in

general. To be considered for technicalinstallations in separate compartments.

4.3 Onboard Fire Extinguishing Equipment:(a) Portable fire extinguishers on traction units

and in coaches. The use of more effectiveextinguishing agents would improveextinguishing performance, reliability andease of use.

(b) Automatic or manually-operated extinguishingsystems on traction units (e.g. sprinklers fordefined compartments).

(c) Automatic fire-extinguishing systems incoaches (technical compartments, passengercompartments).

Assessment: New and Existing Tunnels:(a) Portable fire extinguishers on traction units

and coaches: Recommended as a standardmeasure, ensure proper functioning andimprove the effectiveness.

(b) Automatic or manually-operated extinguishingsystems on traction units: Recommended fornew traction units and for specifiedmechanical or electrical components onnetworks with large number of tunnels(especially very long tunnels).

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(c) Automatic fire-extinguishing systems incoaches: Not recommended for all coaches.May be a reasonable solution under definedconditions such as a closed network,operation with fixed consists (typicallycommuter trains).

4.4 First-aid Equipment Onboard: Each train isequipped with at least one first aid box.

Assessment: New and Existing Tunnels:Recommended as a general safety measure (notonly tunnel safety).

4.5 Escape Equipment and Design of Coaches:(a) Escape equipment: the train crew is equipped

with megaphones for communication andlamps to be able to inform passengers in theevent of evacuation, also for use outside thetrain.

(b) Escape design: coaches (doors, windows,body shell) are designed with definedemergency exits/accesses. The respectiveplaces are visible/indicated for passengersand rescue services.

Assessment: New and Existing Tunnels:(a) Recommended as suitable(b) It is recommended to integrate the aspect of

emergency exits/accesses in furtherspecifications for coaches (but this is notsomething specific solely to tunnels)

5. Operations Measures

5.1 Regulations for Carriage of DangerousGoods: Restrictions on transit through tunnels ofpassenger trains and freight trains carryingdangerous goods:(a) Dangerous goods in general (including single

loads or wagons in a freight train).(b) Block trains carrying dangerous goods only.

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Assessment: New and Existing Tunnels:Recommended for high risk tunnels, if operatingconditions permit (optimise operations from safetystandpoint).

5.2 Emergency Information for Passengers:Passengers are informed about what to do in theevent of an emergency with special emphasis onincidents in tunnels.

(A) Specifications: Means are: Posters, leaflets,spot publicity, on-board TV.

(B) Assessment: New and Existing Tunnels:Implement.

5.3 Emergency and Rescue Plans: Preparationof emergency plans consisting of:• Strategy for dealing with critical events• Emergency call-out plans• Tunnel-specific plans for rescue services

Assessment: New and Existing Tunnels:Recommended as a standard measure.

5.4 Information on carriage of DangerousGoods:(a) Notification of movements of exceptionally

dangerous goods (to be defined, e.g.chlorine, propane, vinyl chloride) to informrescue services concerned along the route tobe prepared in the event of emergency andto be able to take the right action in time(e.g. evacuation).

(b) Information system to identify rapidly theloads involved in the event of an accident inorder to take the right precautions and actionfor intervention (precise and rapidlyaccessible database).

Assessment: New and Existing Tunnels:Reasonable as a general safety measure ifinformation concerning carriage of dangerous

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goods is improved, but not recommend as safetymeasure specific to tunnels.

5.5 Provision of Rescue Equipment: Provisionof rescue equipment for fire-fighting in a tunnel.

(A) Specifications: All rescue services persons areequipped with breathing apparatus for use whenfighting a fire in tunnel. Rolling pallets are locatedat tunnel entrances and exits. Depending on therescue concept, further minimum equipment islocated at entrances.

(B) Assessment: New and Existing Tunnels:Provision of adequate breathing apparatus is astandard measure (prerequisite for an intervention).

6. Additional Measures for Very Long Tunnels

6.1 Segmentation of Overhead Lines: Dis-connection of the overhead line into segments invery long tunnel. Earthing devices includingvoltage measuring instruments should bepositioned at the entrances, portals and emergencyexits for the nearby segments of the overheadline.

(A) Specifications: The different segmented partsof the OHL must take into account the escape routespossible for trains in the tunnel in the event of anemergency so that rescue services can enter thetunnel safely and “captured” electric trains can exitfrom the tunnel. Communication means (e.g.telephone) and lighting of the place are ensured.

Procedures and responsibilities are defined (includingcommunication between rescue services and therelevant centre).

(B) Assessment: New and Existing Tunnels:Recommended as a safety measure for very longtunnels as specified.

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7. Safety measures for existing tunnels: For existingtunnel up to 2 km long, suitable safety measures maybe adopted, on case to case basis and depending onthe site conditions, keeping in view the principleselaborated above for the long tunnels.

8. Mock Drills: Mock drills should be conductedperiodically (recommended at least once every twoyears) on at least one tunnel in section of every ADENto assess the level of safety preparedness/awareness.Concerned State Government/District administrationauthorities should also be briefed about and involvedin conducting mock drills. Mock drill can includescenarios for evacuation and rescue/relief in case of“fire accidents” serious injuries to passengers etc.

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TUNNEL PORTALS

There are generally two modes of access to tunnelconstruction face: through a portal providing directaccess at the surface or through a shaft providingvertical access to the level of tunnel operations. Whetherto use portal, shaft or a combination of the two isdetermined by the elevation of the tunnel, utility ofthis access during construction and whether the ingressis temporary or permanent. Generally speaking,transportation tunnels rely on portals because of therelative elevation. Usually, the tunnelling throughmountain will use portals and sometimes congestionmay require that the portal to the tunnel be below thesurface, as is often the case with urban rail tunnels.When this is the situation, a ramp is constructed bycut-and-cover method.

1. Need for Portal: The portals are constructed toprovide well defined access to tunnel and protect theentrance of tunnel from rock or other objects falling onit. Structurally, the portal protects and supports thetunnel entrance and approach from the earth and rockabove it. It prevents surface water from entering thetunnel and provides a means to drain the water runningdown from above the portal.

Often acting as a retaining wall, the portal is more criticalthan often realized and it should be designed andconstructed based on that criticality. The loads that theportal has to support will sometimes require theutilization of geometric shapes for strength (e.g. theportal being curved in shape to use the arch support).

2. Design of Portal: Design of tunnel portal is quitesimple and following factors are taken into account:

(i) Load on beam is taken as self-load plus auniformly distributed live load calculated by

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assuming 450 dispersion above the beam.

(ii) Design the portal frame by calculating Bendingmoments, Shear forces and Displacements.

(iii) Provide section of beam and columns andreinforcement accordingly.

(iv)Stabilize the slope above the portal.

3. Construction of Portal: The steps normallyfollowed in construction of tunnel portals are as following(Fig. 18.01):

(i) Construct approach to reach working face.

(ii) Remove the loose overburden consisting ofweathered rock in first few meters.

(iii) Make a working platform, at the invert level,and mark outline of the tunnel face on theexposed rock face.

(iv)Construct a RCC or Steel frame portal aroundthe periphery of the proposed tunnel. Steelportal is embedded in concrete (Fig. 18.02).

Fig. 18.01: Portal Development

If the tunnel begins at a soil slope, the slope should becut back so that the ground above is minimum one totwo diameters distance above the portal. The groundmay consist of competent rock or talus on a mountainside and everything in between.

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Fig. 18.02: Tunnel Face Opening

If the ground is stable rock, a portal structure may notbe necessary. Using rock bolts or rock bolts with wiremesh or adding shotcrete to the bolts and mesh maysuffice. If bolt holes can be drilled vertically from thesurface, it is recommended to blast the slope so thatthere is vertical wall of rock and where appropriate toclean the surface using controlled blasting, to providea wall with limited blasting damage. If required, theslope can be benched and supported with wire mesh,shotcrete and tie backs. On benches, ditches (catchwater drains) are provided to collect water and drain itaway from the portal.

Portal construction involves excavating and supportingthe tunnel entrance. If support is required to stabilizeor hold the crown, Spiling can be installed. Spiling canbe pipes (known as Pipe Roofing) that are driven and/or drilled above the tunnel crown (Fig. 18.03). Thesepipes support the roof of the excavation. The pipes inthe soft ground have angular cuts at end, making themself-drilling. The pipes are then grouted to provide asolid shell above the portal.

The goal is to support the crown. Steel beams can alsobe used for stabilization of the roof, for the permanentportals, or they can be used in combination with pipes(Fig. 18.04). The criticality of the portal requires that itbe totally supported. Rock should be completely boltedin the portal area, with spacing of the rock bolts varyingfrom 1.20m to 3m depending upon the quality of the

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rock. In soft ground or soil, it may be necessary to useforepoling with pipe or one of the method discussed inChapter-14.

Fig. 18.03: Spiling above Portal

Fig. 18.04: Using Steel Beams for Portals

The portal can be constructed and supported using ribsand lagging, shotcreted ribs and lagging, and liner plate,to name a few. The portal must be constructed tofacilitate water handling on a slope and requisite bermsand/or catch water drains may have to be built for this.Irrespective of the portal’s surroundings, there mustbe a means to handle water, whether it is from thetunnel, groundwater, or runoff. The portal should begraded to create a low spot with a sump and a pump tohandle water that enters the portal area. When planningthis, one should check local regulations regardingdisposal of the water.

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The portal should extend from the tunnel entrance farenough to prevent falling or rolling material from landingon the approach invert. To facilitate this, a facade (alsoknown as False Portal) may be constructed extendingto the requisite distance from face of the tunnel (Fig.18.05). Normally, a suitable thickness of earth pad (alayer of earth) is provided on the roof of false portals,with parapets on both sides. Any boulder or rollingmaterial is retained on this earth pad with very lessimpact load on the roof and these materials are clearedfrom there as part of maintenance operations.

Fig. 18.05: False Portal

Fig. 18.06: Portal Construction using Ribs and LinerPlate

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Portal, including false portal, can be constructed usingliner plate and steel beams (Fig. 18.06). At the base ofliner plates, footings are constructed for long termstability. If portal is on a slope, the slope may be cleanedand shotcreted, once the liner plate is tied into the slope,to protect from the rocks rolling down the slope. Theshotcrete prevents erosion of the slope into the portalroof and strengthens the portal.

In addition to considering ground and water controlrequirements, it must be kept in mind that the portalarea serves as the focal center for the tunnellingoperations. Since everything goes through it, it mustbe designed and equipped to facilitate movementthrough it. Generally, electrical panels are either in theportal area or above the portal elevation on the top ofthe bank. If that is the case, there should be easyaccess from the portal to the panel, for example, usingstairs.

There should be a ventilation fan in the portal area, atleast during tunnel construction phase. It should bemounted on a solid frame, elevated out of the way, andshould be as quiet as possible. It is dangerous forworkers in the portal area to not be able to hear. Storagefor general-use small tools and materials should bereadily available in the portal area.

The workers, materials and muck will be travelling intoand out of the portal area. If trains are used for muckhaulage during construction of tunnel, the train will haveto pass through the portal area to dump the muck cars.All supplies, utilities, equipment, workers and possiblyliner segments will be loaded in the portal area. It willbe a very active area, which means adequate safetyconsiderations must be taken. If trains are moving inthe area, walkways should be easily be seen by thelocomotive operator. The portal location, design andconstruction should take into account all these factors.

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Shafts are vertical or nearly vertical openings connectingthe surface and the underground structure and whenused during construction serve the same purpose asportals. Many times, the words “Shaft” and “Adits” areused interchangeably but technically speaking “adit” isan entrance to an underground structure which ishorizontal or nearly horizontal in contrast to “shaft”which is vertical or near vertical entry to theunderground excavation.

1. Need for Shafts: During construction phase, theshafts and adits provide extra working faces for tunnelexcavation, thereby increasing the progress of tunnelconstruction work. Tunnel shafts can be temporary orpermanent. Temporary shafts are for use duringconstruction only. Permanent shafts may be used duringconstruction, but will become an integral part of thetunnel structure. Permanent shafts can be used foraccess to tunnels, elevators/stairways or both,ventilation, pumping, utility lines or manholes, or theymay be enlarged to house stations. Temporary shaftsare normally backfilled at the end of construction.

2. Location of Shafts: The location of shafts is criticalin planning efficient construction. Locating a shaft atthe midpoint of a tunnel will permit tunnel driving intwo directions; also a single set of excavation machineryand office accommodation can serve both excavationfaces. Locating a shaft near vacant land will facilitatethe erection of temporary buildings. The proximity ofmuck disposal locations and routes should also beconsidered.

While selecting the location of shaft, the purpose of theshaft during construction, end use, proximity to utilitiesand the underground needs have to be considered. If

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the shaft has to serve as ventilation shaft in the transporttunnel, then the shaft location alignment is fixed.

Another major consideration is the effects on theenvironment. If the shaft is located on a street, trafficand local businesses will be affected: resulting in bothtraffic congestion and loss of business income. Inaddition, it may be prohibited to locate a shaft near anenvironmentally sensitive area such as a stream or itmay create environmental problems and requirefacilities to treat the water, for example.

Once the shaft has been located in a grade, a pumpchamber and a sump may be excavated if required.The pump should provide sufficient capacity to handlethe maximum anticipated flow. Information regardingthe amount of water that will enter the shaft is obtainedduring the shaft excavation process, and an estimateof groundwater seepage can be based on previousexperience in the same soil or rock medium.Unexpectedly large inflows may occur if water-bearingstrata or seams are encountered during excavation.

3. Shapes of the Shaft: Theoretically, shafts can bein any geometric shape. However, the most commonshapes are the circle, rectangle, square and ellipse. Theshape of the shaft is dependent on the use and groundconditions. The circular shaft is generally preferred, asit is structurally much stronger and can be moreefficiently supported. Therefore, in ground with highlateral stresses or heavy loading on the shaft groundsupport, a circular shaft is preferred.

4. Design of Shaft: The shaft construction method isdetermined by:(i) Depth of groundwater table,(ii) Type of ground to be excavated,(iii) Extent of working space needed, and(iv) Depth of shaft.

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While determining shaft construction method, minimumsize of shaft has to be decided. During design, theminimum dimensions are typically determined by thephysical layout of the final structure to be constructedor space needed for launching a Tunnel Boring Machine(TBM). In transport tunnels, shafts can be used foraccess, elevators, ventilation, transit stations and utilitydrops. It is difficult to determine exactly what sizeshaft the contractor will need because proposed meansand methods of excavation and exact type of equipmentthat will be used, are not known at this stage.

5. Collar: As with any construction, if the activity isnot started correctly, there will be problems for the entireactivity that could have been avoided. The same istrue with sinking a shaft. The starting point in theconstruction of a shaft is the collar. The collar isuppermost section of the shaft. A concrete collar, a ringof concrete, should be placed around the top of theshaft. The collar prevents distortion of the shaft’sprimary lining and prevents surface water and debrisfrom falling into the shaft. The depth of the collardepends on the depth to the rock, the sinking methodand the groundwater. It is generally considered thatthe collar extends to bedrock and in some shallow shaftsused for the civil tunneling applications that can be theentire depth. The collar structure is constructed priorto shaft sinking. The shaft excavation area is excavated,generally as a trench. For small shafts, the collar canbe made simple but should be no smaller than 1m wideand 1.5m deep and should be constructed of reinforcedconcrete.

The collar beam should be at least 0.3m above theground surface to prevent things being inadvertentlykicked into the shaft and to prevent the inflow of water.

The collar is where the survey is established andtransferred to the tunnel. It should be rememberedthat a shaft collar is load-bearing structure. Therefore,a complete analysis of the loads must be conducted.The size of the collar area may be increased or modified

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to suit operations. For example, if a gantry crane is tobe used for shaft excavation, a concrete slab will needto be appended to the collar for the crane’s rail andother equipment.

The shaft collar should be constructed based on anengineered solution. That is, it should be designedbased on the engineering properties of the soils andloading of the collar area.

The material used for the collar should be the same asfor the shaft lining. The lining immediately follows belowthe collar. The transition from collar to shaft should notbe apparent.

6. Area around Shaft: The shaft area should be gradedto drain all surface water away from the shaft. It is agood idea to place gravel or stone (concrete is evenbetter) on the surface area to maintain a neat andorganized working area. If, to save money, the shaftarea is not organized well, in the long run this decisionmay cost much more than proper preparation wouldhave.

The top of the shaft should be reviewed for preventionof falls and dropping things down the shaft. Handrailsor another type of safety protection around the top ofthe shaft must also be provided. The federal, state orlocal safety regulations must be complied. The safetyregulations may require only a standard railing of 1.1mheight but that may be inadequate for great depthsand wherever possible 1.2m high chain link fencingshould be provided on the perimeter of the shaft, whicheliminates considerable risk.

7. Shaft excavation in Soft Ground: Shafts in softground are normally excavated with a crane using aclamshell bucket to host the muck from the shaft anddrop it into a hopper or a stockpile or directly into atruck on the surface.

Generally, the most difficult shaft sinking is throughthe overburden, because the overburden is generally

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not as stable as rock. It can be practically difficult atthe interface of the overburden and the rock. This isoften an area of water ingress.

Primary shaft linings are normally installed at every 4or 5 feet of advance. However, shafts up to 30 feethave been sunk without supports. The rate of installationdepends on the type of lining and the nature of the soilmedium. A permanent shaft usually will have a finallining of concrete and the concrete for lining may becast either with forms on both sides or forms on theinside only with the ground support system on theoutside.

Soft ground shaft sinking may disturb or damageneighbouring buildings, utilities, pipelines or stresses.The problem is especially acute with soft plastic soils;when plastic soil is excavated, the load over theexcavated area is reduced, and plastic yielding mayresult, causing ground yielding at the surface. A properlydesigned shaft support system can prevent plasticyielding. Soil characteristics, shaft depth, diameter, andeconomic factors will dictate the choice among the manyavailable sheeting and bracing system.

8. Support Systems used for Shaft Excavation:There are many methods for supporting the ground toexcavate through the overburden. Sinking a shaft hasan advantage over tunnelling when it comes to handlingbad ground. With a shaft, one is on top of the groundbeing excavated rather than having to go under it. Thatmeans, when approaching bad ground, one is in a betterposition to implement ground support measures.

A shaft can be sunk through soil using various types ofsupport depending on depth and material to beexcavated. Some of these support types are as under:

8.1 Timber Sheet Piling: This is normally usedonly in shallow shafts, since driving the thicktimbers is difficult. The method can be economicalto start excavating in soft material, not deeperthan about 20 feet of soil overlying rock. Although

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easy to install, wood sheet piles are poor forground water control. Moreover, availability oftimber is an issue in present times.

Timber or wood sheet piles (3 to 4 inch thick) aredriven vertically on the shaft perimeter prior toexcavation (Fig. 19.01). Horizontal steel ribs areinstalled against the interior of the sheeting, tocounter the lateral earth pressure acting on thesheeting, by compression or bending action of thering depending upon whether the shaft is circularor rectangular. It is important that a template isused to maintain the line and plumb of sheet pilewall.

Fig. 19.01: Timber Sheet Piling

8.2 Steel Sheet Piling: Steel sheet piles consistof steel that is forged to different shapes providingadditional flexural strength and an interlockingcapability. They are constructed by drivingprefabricated sections into the ground. Interlockingsteel sheet piles are commonly used to brace soft,water bearing ground if the excavation depthexceeds about 20 feet. Steel sheet piles aregenerally limited to depths of 25m (80 feet). Forgreater depths, the shaft can be stepped in andanother level of sheet pile can be driven. However,the stepped-in area must be wide enough to allowthe driving equipment; thus it has limited uses.Steel sheet piles can be driven into harder ground.Because of its lock ends, it is very good for watercontrol as compared to wood sheet piles.

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Steel sheeting is desired when depths are so greatthat wood sheet pile cannot work and where highloads are present. Also, if it is required to leavethe sheeting in the ground for a long period oftime, steel sheeting can resist corrosion. Moreforce is required to penetrate the ground, and thusheavier equipment is necessary. They are usuallydriven by a pile hammer or vibratory hammer,depending upon the type and condition of theground being penetrated.

The steel sheet pile wall is formed by sequentiallyconnecting the joints of adjacent sheet pilesection. Steel piles must be driven carefully, toensure proper interlocking of the joints to cut-offwater seepage. Each pile is driven to the planneddepth, then the next pile is driven and the two arelocked together. This process is continued till theshaft wall is complete. Excavation usually beginsafter the pile diving operation is completed, unlessthe shaft is unusually deep. Horizontal steel ribsets (wales) can then be installed progressively atappropriate vertical depths as the excavationprogresses downwards (Fig. 19.02). The lateralearth pressure is transferred to the wales.

Fig. 19.02: Steel Sheet Piling

Steel sheet piling is most commonly used becauseit provides high resistance to driving stresses, is

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light weight and the pile can be easily changedusing welding or bolting. Because of its reusefeature, its cost can be amortized over severalprojects. During driving, the joints are less likelyto deform and, with some protection, can have along service life below water.

Problems can develop when trying to drive sheetpiles through difficult soils or boulders, becausethen driving them to desired depth may not bepossible. Sheet piles can rarely be used as apermanent structure. Because of noise andvibrations, there may be complaints from habitantsstaying near site of work.

8.3 Soldier Piles and Lagging: Soldier piles andlagging used to retain earth are among oldestforms of retaining systems in deep excavations.The system consists of H-piles called soldier pilesdriven or placed in drilled holes, usually spacedfrom 6 to 10 feet apart. Once the soldier beamshave been installed, the soil is excavated alongone side of the beams to partially expose the frontfaces of the beams. Then, the wood lagging isinstalled to temporarily hold back the soil (Fig.19.03). In some case, reinforced concrete panelscan also be utilized for permanent conditionsinstead of timber. In moist to wet soils, the waterusually drains between the lagging boards.

In deeper excavations, where large earth pressuresare encountered, horizontal steel ribs sets (wales)are installed. When the shaft is greater than 6m orthe deflection near existing structures (such asbuildings and utilities) is very strict, wales areoften used. The steel rib sets must be designedfor either ring compression or bending, dependingon whether the shaft is circular or rectangular. Inlarge rectangular shafts, steel struts can beinstalled to span between wales. In this system,the moment is resisted by the soldier pile, and thelagging does not provide resistance to the

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moment. Embedding the soldier piles below theshaft bottom provides passive soil resistance,whereas the lagging between the piles, retainingthe soil, transfers the lateral load to the soldierpiles.

Fig. 19.03: Soldier Piles and Lagging

With the soldier pile being the only part of the wallembedded beneath the subgrade, it is difficult tocontrol basal movement and the system is not asstiff as other support systems. Hence soldier pilesand lagging walls are generally used in temporaryconstructions, and if not properly backfilled behindthe lagging, there can be a problem with groundlosses and surface settlement.

8.4 Liner Plates: Liner plates are corrugatedpressed steel pans with bolt holes in the flangeson the sides and ends to permit bolted erection ofthe ring. They are curved longitudinally accordingto the curvature of the tunnel (Fig. 19.04). Linerplates come in different thicknesses (gages) to suitthe given loading conditions.

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Fig. 19.04: Liner Plates

The main advantage of liner plates is that theirsmall size permits ease of operation in the limitedworking spaces, and it is not necessary to havespecial equipment to lift or place the liner plates.

Shaft excavation begins by precisely erecting thefirst ring of liner plates on the ground surface andplacing a concrete or earth collar around it. Thesoil is then excavated within the ring, and asspace becomes available a liner plate is bolted tothe bottom flange of the first liner plate ring;assembly of the succeeding rings proceed in thesame manner. The joints of individual liner platesare staggered from the joint immediately above toincrease strength. If voids are observed whilesinking, material should be stuffed into them toreduce the chances of localized caving because ifleft unfilled such voids can continue to expand andeventually cause major ground failure. The fillbehind the liner plate does not have to bestructural. Hay is often used to fill such voids butbeing inflammable, the risk of fire must be kept inmind.

The soil pressure is carried by the liner plates inring compression. In larger diameter shafts or inshafts where the lateral earth pressure is large,circular horizontal steel rib sets (ring wales) canbe set at a predetermined vertical spacing insidethe liner plate ring to increase strength.

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Grout holes are fabricated in the liner plate topermit the grouting voids behind the liner plate orto stop/reduce the inflow of water.

8.5 Horizontal Ribs and Vertical Lagging: Thehorizontal ribs and vertical lagging method, alsocalled as ribs and lagging method, is somewhatsimilar to liner plate construction. Rings are madeof structural steel members, cold-formed torequired curvature, the sections butted at eachend. Butt plates welded to the ends of thesegments are provided with bolt holes. Six to eightfeet length of timbers are usually used for lagging(Fig. 19.05).

This method requires excavation of the soil to adistance equal to the length of the lagging. Curvedring segments are bolted together and held inplace by tie rods and spacers that are placedbetween the webs of the rings. Placement of thevertical lagging follows. The steel rings can beplaced at varying vertical spacing to resist thevarying lateral earth pressure.

Fig. 19.05: Ribs and Lagging

Since the soil must be initially somewhat self-supporting for the height of lagging, this method is

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usually employed in cohesive soils, although it canbe used when the ground has moderate stand-uptime. During construction, the interval betweenexcavation and lagging placement should beminimal to prevent ground loss.

8.6 Slurry Walls: Bentonite, naturally occurringclay, has a large capacity for absorbing water. Insuspension, it is a liquid when agitated, and a gelif left to stand.

A trench is dug in to build a wall using slurry,consisting of water and bentonite. Prior to startingthe excavation, guide walls are constructed tomaintain the slurry wall alignment. The guide wallsare constructed on the surface and areapproximately 50cm wide by 1m deep. The trenchis excavated and kept full of slurry at all thetimes. The slurry prevents the trench fromcollapsing by providing outward pressure whichbalances the inward hydraulic forces and preventswater flow into the trench. Excavation is generallydone upto the design depth using a specialclamshell shaped digger. The excavator is thenlifted and moved along the trench guide walls tocontinue the trench with successive cuts asneeded. While maintaining the slurry level toprevent the cave-in, the excavator is moved tocomplete the entire length of wall once it reachesrock or designed depth. Then pre-fabricated steelreinforcement cage is lowered into the slurry filledtrench, using spacers to ensure correct positioningof cage. During excavation and installation ofrebar, the viscosity and density of slurry aremonitored. Tremie pipes are used to pour concreteinto the slurry trench. The concrete displaces theslurry and the slurry is captured and saved forother excavations (Fig. 19.06). A cofferdam canthen be created by bracing the concrete with steelor reinforced concrete wales and struts.

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Fig. 19.06: Slurry Wall Cycle

Slurry walls are useful in soil that is unstable orhas a high water table. Slurry walls are used asboth temporary and permanent earth retentionsystems to support the sides of deep excavation.

Another method, known as “interlocked elementtype”, is used when the soil is very hard andbouldery, or when the excavation must reachconsiderable depths. Primary holes upto desireddepth are first drilled using percussion rigs withthe assistance of bentonite slurry. Generally thespacing of primary holes is kept as twice theirdiameter. This also ensures that penetration ofthe soil between primary holes by the bentoniteslurry is almost complete, and therefore theexcavation of the secondary elements involves azone of soil stabilized through gelation in its pores.Concrete is placed in primary holes with the aid ofTremie pipes. A hydraulically expandable chisel isthen used to excavate the panels between theprimary holes. The secondary elements are thenput into these spaces between the primaryelements, interlocking with the primary elements.

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The structure built can be reinforced by placing thesteel in the slurry filled primary holes beforeplacing the concrete.

8.7 NATM System: A recent development in thedesign of support for shafts in soft ground hasbeen the adaption of the New Austrian TunnellingMethodology (NATM), in the same way as it isdone for tunnels.

In general, the NATM support system followssomewhat the same sequence as the installation ofliner plate. A lattice-type rib ring is a key elementof this system. After excavation of a section of theshaft, lattice ring is installed. After this, shotcretewith wire mesh is applied to the excavatedexposed surface. Usually about 6” of shotcretethickness is adequate, but if the instrumentationindicates movement of the earth or otherundesirable characteristics, more shotcrete can beapplied to obtain a thickness of 12” or more.

9. Shaft excavation in Soft and Wet Ground:Excavation in soft, wet ground can be accomplished ina number of ways. The most common method is tolower, by any of several means, the groundwater tablein the working area. Other methods include freezing ofthe soil, the use of slurry, grouting, sinking a pneumaticcaisson and sinking a dredged drop caisson with a Tremieconcrete seal.

9.1 Lowering of Groundwater: Although thismethod is time consuming, it ensures dry, safeand firm working conditions. To determine theproper type of groundwater control system,geological and soil information should be evaluateda pumping test should be performed on the soil.The pumping test should yield results such aswater volume pumped, well yield and timerequired to reach equilibrium.

Dewatering can result in lowering of the watertable under adjacent areas. Therefore, extreme

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caution must be observed if large structures arelocated in the vicinity of the dewateringoperations. Dewatering increases effective stress inthe soil, which in turn causes settlements. Also, incases where adjacent structures are supported onpiles, enough drawdrag can be developed on thepile foundation to cause settlement.

9.2 Open Pumping: This method consists ofdriving steel sheet piling, excavating and pumpingwater from the bottom of the excavation.

However, the pumping operation may causeseepage of water (and loss of fines) around thetoe of the sheeting. Furthermore, if the pressuredue to the upward seepage of water becomesgreater than the soil pressure at the bottom of theexcavation, it can result into a quick or “boiling”condition in the soil. Also, if the seepage of wateraround the toe of the sheeting significantlydislodges soil particles, the sheeting can beundermined.

9.3 Wellpoint System: Wellpoints are wellscreens, which require suitable filter materialaround the screen to prevent the collection of soilparticles with the water. This system is generallyused for dewatering to a depth of about 15 feetand this technique is best suited for medium tofine sand, for work of short duration.

Wellpoints are placed at 3 to 12 feet spacing,around the area to be excavated, and connected toa common header pipe, which is connected to apump. The wellpoint system has to be kept inoperation during shaft excavation also otherwisewater table can return to its original level.

9.4 Deep Wells: Deep wells can be used todewater pervious materials to whatever depth theexcavation requires, and they can be installedoutside the zone of excavation.

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The deep well system consists of spacing 6 to 18inches diameter wells at 20 to 200 feet spacing,depending on perviousness of ground and depth ofdewatering required. These wells have acommercial type of water well screens surroundedwith a properly graded sand-gravel filter. Each wellis equipped with its own submersible pump. Theexcavation for the shaft can begin after waterdrawdown to the required elevation has beenaccomplished.

9.5 Freezing: In water-bearing ground whereeven minimal surface subsidence cannot betolerated, such as adjacent to large buildings, themost reliable method of handling the excavation isto freeze the soil and then excavate it. There is nolimitation on the depth upto which freezing may beused.

The procedure consists of sinking pipes around thearea to be excavated and circulating a cold brinesolution through the pipes, thereby freezing a wallof soil. As an alternative to brine refrigerant, liquidnitrogen is sometimes used to accelerate thefreezing process. Excavation can then begin. Ifconcrete of shaft lining is to be installed, theconcrete can be placed against the frozen soil.

The main disadvantage of this technique is thetime required to freeze the soil and the cost of theequipment.

10. Shaft excavation in Rock: Shaft excavation inrock is usually performed by the Drill and Blast method,like in tunnels. Shaft excavations for tunnels arenormally less than 120 feet deep and, therefore, use ofmore sophisticated excavating equipment is not ofteneconomical. However, for shaft excavation for more than120 feet depth, other methods may have to be explored.Prior to the start of rock excavation, it is advisable togrout and seal the overburden if ground water infiltrationfrom the overburden can become a problem.

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10.1 Raises: A raise is a vertical excavationproceeding from a lower elevation to a higherelevation, perhaps from one tunnel to another, orfrom a tunnel to the ground surface. The raise canbe used to intersect another tunnel above anexisting one. Shaft raising is sometimes used inurban tunnel construction to minimize surfacedisruption.

Fig. 19.07: Raise Boring

Raises are usually excavated by drilling pilot holes,usually of diameter about 9 to 12 inches, thenreaming the hole to the proper diameter. The mostcommon and successful system is to drill the pilothole down and reaming up the required raise (Fig.19.07). Only a few types of raise drills are widelyused, and the nature of rock should be studiedcarefully to assure the use of proper cutters forefficiency and economy.

Raise excavation by drilling and blasting is oftenquite hazardous, especially the scaling down ofloose rock after the blast.

10.2 Temporary Supports: In sedimentary,fractured, or blocky rock, the walls can be quitetreacherous. The support can consist of steel ribsand liner plates, steel ribs with lagging, rock boltswith or without wire mesh, or shotcrete. The NATMsupport system is particularly adaptable for rockshafts that require temporary supports.

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If rock is of reasonable good quality, it may beadvantageous to merely install rock bolts into thewall and fasten wire mesh to the bolts to keeprock spalls from dropping on the workers. It issometimes good practice to apply shotcrete to thewalls. Shotcrete linings are also becoming popularas permanent linings.

11. Other Mechanized Methods: Methods are beingdeveloped to sink shafts by mechanical methods.Herrenknecht and Robbins have developed a shaftsinking machine that can sink all type of shafts. Theyare designed to be used in both stable and unstablegrounds. They mainly consist of a sinking unit and ashaft boring machine (Fig. 19.08). Functioning like agripper TBM, the machine is braced with gripper armsagainst the solid rock. The machine receives all suppliesand is controlled from the surface and, with informationbeing monitored, the operator can be constantly awareof the machine’s technical parameters.

12. Lining of Shafts: Permanent shafts are usuallylined. The planned permanent usage of the shaftdetermines the type of final lining. Shafts sunk throughsoft ground require an initial or “primary” lining forconstruction support. The secondary lining, if required,can either be poured against the primary lining, or itcan be formed from both the outside and inside. If thelining is formed on the outside, the annular spacebetween the primary and secondary lining should betightly backfilled or packed with pea gravel, well gradedsand or other suitable material.

Precast concrete segments can be used for secondarylining, where it can be placed from the bottom up, butthe annular space created should be filled up.

If the shaft has been excavated through water-bearingground, placing an impervious sheeting material on theface of primary lining, prior to the placement of thefinal concrete lining, has become a common practice.Additionally, grouting may be required to prevent waterseepage into the finished shaft.

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Fig. 19.08: Vertical Shaft Sinking Machine

Calculation of lining thickness is done according tostructural loads and stresses to which the collar will besubjected.

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The philosophy of Inspections and Maintenance oftunnels remains more or less same in the tunnels,irrespective of the owning organization of the tunnel.But the frequency of the inspection, roles/responsibilitiesof officials involved and reporting system etc. vary fromone organization to another. Therefore, it is difficult tolay down a common inspection and maintenancemethodology in any text book. In this chapter, themethodology to be followed in Indian Railway, as per“Guidelines for Civil Engineering Inspection, Maintenanceand Safety in Existing Tunnels” (No. RDSO/GE/G-0015,August’2012), have been covered.

1. Pre-requisites for Tunnel Inspection

1.1 Qualifications of Inspecting Officials: Allofficials performing tunnel inspection work shouldpossess basic knowledge of tunnel components andunderstand how they function. The inspectingofficial(s) should have the ability to identify andevaluate defects that pose a threat to the integrityof a structural member and should be able toassess the degree of deterioration of structuralcomponents of tunnel. Training courses for buildingcompetency in tunnel inspections should beorganized periodically.

1.2 Equipment/Tools for Inspection: Reco-mmended list of equipment and tools to be usedfor tunnel inspections is as under:• Tunnel Inspection Unit/Requisite staging

mounted on the mobile units, such as openwagon, dip lorry or rail motor (Fig. 20.01):To carry out thorough inspection of the sides

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and roof of the tunnels. These should be keptat suitable points for urgent use as required.

Fig. 20.01: Tunnel Inspection Unit

• Calipers: To measure steel plate thicknesses.• Feeler gauge/Crack width ruler/Optical Crack

width Microscope: For measuring crack width.• Digital Camera (with Flash): To take

photographs for documentation of theinspection.

• Markers: For making reference marks ontunnel surfaces.

• Chipping Hammer: To sound concrete• Extension Cord: To get electricity to

inspection area.• Flashlights: Used in dark areas to improve

visibility during inspection.• Plumb Bob: To check plumbness of columns

and wall faces.• Pocket Knife: To examine loose material and

other items.• Scraper: To determine extent of corrosion

and concrete deterioration.• Screw Driver: To probe weep holes to check

for clogs.• Wire Brush or Brooms: To clean debris from

surfaces to be inspected.

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• Pocket Tapes: To measure dimensions ofdefects.

• 30m Tape (Non Metallic): To measureanything beyond the reach of pocket tapesand folding rules.

• Appropriate Safety gear.

Various non–destructive inspection methods such asultrasonic, geo-radar and seismic testing may be usedto supplement the regular inspection methods, ifrequired.

1.3 Advance Preparation

(A) Study of Tunnel Records: To perform theinspection efficiently, it is important to plan andorganize for inspection in advance. This involvesstudy of available tunnel drawings, previousinspection reports, construction phase geologicalrecords and tunnel instrumentation data records (ifavailable).

(B) Marking of Reference System: It is neces-sary to establish a system by which the location ofa defect can be recorded and understood. This willallow the inspections to be referenced historicallyfor future monitoring of condition of any defect andwill increase the efficiency of the overall inspectionprocess. In addition to locating a defect by itslongitudinal position, it is necessary to note thedefect’s position within the tunnel cross-section.Horseshoe, rectangular and other circular tunnelscan be broken down into consistently named cross-sectional elements. Nomenclature used in inspectionof tunnel should be illustrated in initial page(s) ofTunnel Inspection Register.

(C) Tunnel Inspection Record: To properly gatherand record tunnel inspection data, it is recommendedto have following two inspection/ informationregisters for each tunnel:

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(1) Tunnel Inspection Register: For recordingof Tunnel Condition Codes & comments.Recommended Proforma is placed at Appendix-20.01 and Appendix-20.02

(2) Supplementary Tunnel Information Reg-ister: For detailed sketches and/or photographsof defects found in areas of the tunnel, it wouldbe helpful to make sketches/take photographsof the same conditions or defects as previousinspections, so that the rate of deterioration canbe ascertained. As far as possible,supplementary information should be recordedduring the inspection itself. In cases (likephotographs) where this is not possible, the dateof recording Supplementary information shouldnot be two weeks later than date of inspection).Recommended Proforma is placed at Appendix-20.03

(D) Inspection Methodology: Identification ofstructural defects during the inspection can beaccomplished through visual inspection or througha combination of visual inspection and non-destructive techniques. In case any special non-destructive testing is required to be used forinspection, advance planning & preparation shouldbe made for the same.

(E) Ensuring Safety: Inspection team shouldensure that safety practices are followed at all times.Along with the safety of inspection personnel, theinspection team should take appropriate measuresduring inspection to prevent danger to the traffic,to staff and to members of the inspection team.

2. Common Structural Defects: Identification ofstructural defects during the inspection can beaccomplished through visual inspection or through acombination of visual inspection and non-destructivetechniques.

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The visual inspection must be made on all exposedsurfaces of the structural elements. All noted defectsshould be measured and documented for location.Severe spalls in the concrete surface should bemeasured in length, width and depth. Severe cracksshould be measured in length and width. Corrosion onsteel members should be measured for the length,width, and depth of the corrosion. The inspectors shouldclear away debris, efflorescence, corrosion or otherforeign substances from the surfaces of the structuralelement prior to performing the inspection. Once thedefect is noted, it should be classified as minor,moderate, or severe as explained in the followingsections.

Particular attention should be paid to determining ifdifferential settlement has occurred in transition areasof the tunnel. Transitions are those areas in which thetunnel support conditions change, such as betweensections of rock and soil tunneling.

In addition to visual inspection, structural elementsshould be periodically sounded with hammers to identifydefects hidden from the naked eye. As a result of ahammer strike on the surface, the structural elementwill produce a sound that indicates if a hidden defectexists. A high-pitched sound or a ringing sound fromthe blow indicates good material below the surface.Conversely, a dull thud or hollow sound indicates a defectexists below the surface. Such a defect in concrete maysignify a delamination is present or that the concrete isloose and could spall off. Once the defect is found, thesurface in the vicinity of the defect should be tappeduntil the extent of the affected area is determined.

For concrete or masonry surfaces that are accessible,non-destructive, ultrasonic test method such as“Impact-Echo” may be utilized. Impact-Echo is anacoustic method that can determine locations and extentof f laws/deteriorations, voids, debonding ofreinforcement bars and thickness of concrete. The useof this method helps to mitigate the need for major

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rehabilitation since the deterioration can be detectedat an early stage and repairs performed. Commonstructural defects are briefly described below:

2.1 Concrete Structures

(1) Scaling: Scaling is the gradual and continuingloss of surface mortar and aggregate over an area.This is classified as follows:

• Minor Scale: Loss of surface mortar upto 6mm deep, with surface exposure ofcoarse aggregates.

• Moderate Scale: Loss of surface mortarfrom 6mm to 25 mm deep, with someadded mortar loss between the coarseaggregates.

• Severe Scale: Loss of coarse aggregateparticles as well as surface mortar andthe mortar surrounding the aggregates.Depth of loss exceeds 25mm.

(2) Cracking: A crack is a linear fracture in theconcrete caused by tensile forces exceeding thetensile strength of the concrete. Cracks can occurduring curing (non-structural shrinkage cracks) orthereafter from external load (structural cracks).They may extend partially or completely throughthe concrete member. Cracks are categorized asTransverse Cracks, Longitudinal Cracks, HorizontalCracks, Vertical cracks, Diagonal Cracks, Map Cracks,and Random Cracks etc. All cracks may be classifiedas follows:

• Minor: Up to 0.80mm.• Moderate: Between 0.80mm and 3.20mm.• Severe: Over 3.20 mm.

(3) Spalling: Spalling is a roughly circular or ovaldepression in the concrete. It is caused by theSeparation and removal of a portion of the surfaceconcrete revealing a fracture roughly parallel or

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slightly inclined, to the surface. Spalling may beclassified as follows:

• Minor: Less than 12mm deep or 75mmto 150mm in diameter.

• Moderate: 12mm to 25mm deep orapproximately 150mm in diameter.

• Severe: More than 25mm deep andgreater than 150mm in diameter andany spall in which reinforcing steel isexposed.

(4) Pop-outs: These are conical fragments thatbreak out of the surface of the concrete leaving smallholes. Generally, a shattered aggregate particle willbe found at the bottom of the hole, with a part ofthe fragment still adhering to the small end of thepop-out cone. These are classified as under:

• Minor: Leaving holes upto 10mm indiameter or equivalent.

• Moderate: Leaving holes between 10mmand 50mm in diameter or equivalent.

• Severe: Leaving holes 50mm to 75mmin diameter or equivalent. Pop-outslarger than 75mm in diameter arespalls.

(5) Efflorescence: This is a combination of calciumcarbonate leached out of the cement paste and otherrecrystallized carbonate and chloride compounds,which form on the concrete surface.

(6) Staining: Staining is a discoloration of theconcrete surface caused by the passing of dissolvedmaterials through cracks and deposited on thesurface when the water emerges and evaporates.Staining can be of any colour although brownstaining may signify the corrosion of underlyingreinforcement steel.

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(7) Hollow Area: This is an area of a concretesurface that produces a hollow sound when struckby a hammer. It is often referred to as delaminatedconcrete.

(8) Honeycomb: This is an area of a concretesurface that was not completely filled with concreteduring the initial construction. The shape of theaggregate is visible giving the defect a honeycombappearance.

(9) Leakage: This occurs on a region on theconcrete surface where water is penetrating throughthe concrete.

• Minor: The concrete surface is wetalthough there are no drips.

• Moderate: Active flows at a volume lessthan 30 drips/minute.

• Severe: Active flows at a volumegreater than 30 drips/minute.

2.2 Steel Structures

(1) Corrosion: Corroded steel varies in color fromdark red to dark brown. Initially, corrosion is finegrained, but as it progresses, it becomes flaky orscaly in character. Eventually, corrosion causespitting in the member. All locations, characteristicsand extent of the corroded areas should be noted.The depth of severe pitting should be measured andthe size of any perforation caused by corrosion shouldbe recorded. Corrosion may be classified as follows:

• Minor: A light, loose corrosion formationpitting the paint surface.

• Moderate: A looser corrosion formationwith scales or flakes forming. Definiteareas of corrosion are discernible.

• Severe: A heavy, stratified corrosion orcorrosion scale with pitting of the metalsurface. This corrosion condition

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eventually culminates in loss of steelsection and generally occurs wherethere is water infiltration.

(2) Cracks: Cracks in the steel may vary from hairline thickness to sufficient width to transmit lightthrough the member. Any type of crack is seriousand should be reported at once. Look for cracksradiating from cuts, notches, and welds. All cracksin the steel will be classified as severe.

(3) Buckles and Kinks: Buckles and kinks developmostly because of damage arising from thermalstrain, overload, or added load conditions. Erectiondamage may also cause buckles and kinks.

(4) Leakage: This occurs on a region of the steelsurface where water is penetrating through a jointor crack.

• Minor: The steel surface is wet althoughthere are no drips.


• Severe: Active flows at a volumegreater than 30 drips/ minute.

(5) Protection System: Steel is generally protectedby a paint system or by galvanizing. Most existingstructures use either paint or galvanized steel. Paintsystems fail through peeling, cracking, corrosionpimples and excessive chalking. The classificationof the degree of paint system deterioration is tiedto both the physical condition of the paint and theamount of corrosion of the member as follows:

• Minor: General signs of deterioration ofthe paint system but no corrosion yetpresent.

• Moderate: Paint generally in poorcondition and corrosion is present butnot serious. No section loss.

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• Severe: Paint system has failed and there isextensive corrosion and/or section loss.

2.3 Masonry Structures

(1) Masonry Units: The individual stones, bricksor blocks should be checked for displaced, cracked,broken, crushed or missing units. For some types ofmasonry, surface deterioration or weathering canalso be a problem.

• Minor: Surface deterioration at isolatedlocations. Minor cracking.

• Moderate: Slight dislocation of masonryunits; large areas of surface scaling.

• Severe: Individual masonry unitssignificantly displaced or missing.

(2) Mortar: The condition of the mortar should bechecked to ensure that it is still holding strongly. Itis particularly important to note cracked,deteriorated or missing mortar if other deteriorationis present such as missing or displaced masonryunits.

• Minor: Shallow mortar deterioration atisolated locations.

• Moderate: Mortar generally deteriorated,loose or missing mortar at isolatedlocations; in?ltration staining apparent.

• Severe: Extensive areas of missingmortar; infiltration causing misalignmentof tunnel.

(3) Shape: Masonry arches act primarily incompression. Flattened curvature, bulges in wallsor other shape deformations may indicate unstablesoil conditions.

(4) Leakage: A region on the masonry surfacewhere water is penetrating through a joint or crack.

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• Minor: The masonry surface is wetalthough there are no drips.


• Severe: Active flows at a volumegreater than 30 drips/minute.

2.4 Connection Bolts: The connection bolts onfabricated components may be discolored due tomoisture and humidity conditions in the tunnel.This condition does not down grade the structuralcapacity of the bolt. Particular attention should begiven to bolts in regions of leakage to ensure thatno detrimental loss of section has occurred. Iflosses in section are observed, such bolts shouldbe noted for replacement. Also, the location of allmissing or loose bolts should be noted.• Minor: Bolts are discolored, but have no

section loss.• Moderate: Bolts are deteriorated with up to

15% loss.• Severe: Bolts are deteriorated with greater

than 15 % section loss. However, bolts withdeterioration approaching 50% or moreshould be replaced.

2.5 Shotcrete/SFRS: Inspector should part-icularly look for any cracking, spalling, scaling andwater seepage and rate it as mentioned earlier forconcrete structures.

2.6 Rockbolts: Generally shotcreting is done afterthe work of rock bolting is complete and thereforethe rock bolt ends are generally embedded inshotcrete. The protrusion of bolt end may only bevisible. The surroundings of bolts should be closelywatched for any signs of cracks, loosening ofshotcrete, dislocation etc.

If any sign of looseness/cracks is observed,sounding of the location to be done for about 10m

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on both side and suitable action to be taken. Anydampness observed around the rock bolt to berecorded.

A sample rock bolt selected in random in a tunnelat the rate of one rock bolt per 500 bolts or partthereof should be subjected to pull out test as aconfirmatory test once in 10 years after provisionof rock bolt.

3. Major Items to be inspected

3.1 Portal (including cuttings at approaches)(a) Signs of instability of slopes.(b) Possibility of boulder/loose mass fall.(c) Possibility of tree fall.(d) Drainage arrangements: Catch water drains,

side drains and sumps. Catch water drainsabove the portals should drain away and notbe allowed to percolate into the tunnel orbehind the portal masonry.

(e) Weep holes: In retaining walls/breast wallsand portal structure.

(f) Adequacy & condition of measures (likeshotcreting, rock bolting etc.) taken forstrengthening/ stabilization/erosion control ofslopes/tunnel face.

(g) Structural condition of portals.(h) Emergency access & communication.

3.2 Section of tunnel in relation to movingdimensions(a) Visible signs of any convergence/deformation

in tunnel supports.(b) In case the conventional measurements

indicate some movement/convergence,systematic convergence measurement usingtape extensometer/optical methods should bedone.

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(c) Scrutiny of past convergence/deformationhistory and instrumentation data.

3.3 Tunnel Roof, Walls and Invert(a) Signs of any structural distress in tunnel

supports (concrete/RCC/Steel/Shotcrete/SFRS/Rock bolts etc.).

(b) Signs of problem in formation (likedisturbance in track alignment, levels).

(c) Seepage/dampness.

3.4 Tunnel Refuges (Man/Trolley) and Wal-kway(a) No unwanted material, free of vegetation.(b) Firm and level.

3.5 Drainage(a) Side drains upto outfall: Cleanliness, No

unwanted material& whether functioningsatisfactorily.

(b) Weep holes: Check for clogging/choking.(c) Functioning of dewatering pumps.(d) Desilting of sumps.

3.6 Track: As per provisions of IRPWM. Detailedinspection needs to be carried out in locationswhere tunnel convergence/ deformation has beenobserved.

3.7 Ventilation, Lighting, Telephone Com-munication, Firefighting preparedness andElectrical/ Mechanical Systems(a) Functioning of designed systems.(b) Comments on adequacy of available systems.

General comments on functioning of mechanical,electrical and signaling systems should be recordedduring inspection. Any issue requiring urgentattention should immediately be brought to noticeof concerned department.

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It should be ascertained whether Ventilationshafts/Adits are adequate and maintained free ofvegetation and other growth. For tunnels morethan 200m long, level of pollution and temperaturecondition should be enquired from the gang andKeyman working in that location of tunnel keepingin view passenger comfort and working conditionsfor working inside the tunnel.

4. Inspection Documentation

4.1 Recording of Defects: The inspectionshould be thoroughly and accurately documented.For the tunnel structure, the documentation ofsevere defects should include a sketch showing thelocation and size of the defect and a verbaldescription of the defect. All severe defects shouldbe photographed. However, a representative photoof minor or moderate defects will be sufficient. Alldefects should be described but sketches need onlyto be made for severe defects.

The sketches should show the necessary plan andelevation views of the defects in structural elementto which they pertain. All defects should be locatedon sketches by dimensioning their location inreference to the beginning or end of the element.Each defect should be dimensioned showing itslength, width, and depth (if applicable).

In documenting the inspection, consistentabbreviation system (such as that given below)should be used in description, sketches andphotographs to describe the defect and to classifythem as minor, moderate or severe:

Description of Defect Classification

Crack- CR 1 - MinorScaling - SC 2 - ModerateSpall - SP 3 - SevereStaining - ST

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Exposed Reinforcement - ECorrosion - CHoneycomb- HHollow Area - HADebris - DBuckle- BEfflorescence - EFLeakage - LK

For example, a moderate crack should be labeledas CR2, a severe leakage as LK3 etc. Thisdesignation should be placed on the sketch/photograph.

Before placing any information on SupplementaryTunnel Information Register, always ensure thatthe structural element (that the defects pertain to)is correctly identified.

4.2 Condition Rating System: The tunnelcomponents mentioned in Proforma of TunnelInspection Register should be rated as below:

Excellent condition: No defects found.

Good condition: No repairs necessary. Isolateddefects found

Fair condition: Minor repairs required but elementis functioning as originally designed. Minor,moderate, and isolated severe defects are presentbut with no signi?cant section loss.

Poor condition: Major repairs are required andelement is not functioning as originally designed.Severe defects are present.

Serious condition: Major repairs requiredimmediately to keep structure open to traffic.

The rating is dependent upon the amount, type, sizeand location of defects as well as the extent to which

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the element retains its original structural capacity.To judge the extent to which the structural elementretains its original structural capacity, the inspectormust be able to appreciate how the element isdesigned and how the defect affects this design.

Defects should be described as “Minor”, “Moderate”or “Severe”. “Moderate” or “Severe” defects shouldbe sketched and/or their photographs should beplaced in Supplementary Tunnel InformationRegister.

4.3 Repair Priority Classifications: When sum-marizing inspection data and makingrecommendations for future repairs (in “TunnelInspection Register” and “Tunnel InspectionReport”) it is recommended to mention priority ofthese repairs to be performed under followingclassification:

(A) Critical: The inspection may reveal severedefects that could pose danger to the traffic or totunnel personnel. When this occurs, this particularsevere defect should be categorized for a “criticalrepair”. A defect requires this designation if itrequires “immediate” action including one of thefollowing critical actions be taken:

• Close the tunnel/keep the defect undercontinuous watch (with appropriatespeed restrictions, if necessary) untilthe severe defect is removed orrepaired.

• Shore up the structural member if thisis appropriate.

(B) Priority: Refers to conditions for which furtherinvestigations, design and implementation of interimor long-term repairs should be undertaken on apriority basis, i.e. taking precedence over all otherscheduled work.

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(C) Routine: Refers to conditions requiring furtherinvestigation or remedial work that can beundertaken as part of a scheduled maintenanceprogram, other scheduled project or routine facilitymaintenance depending on the action required.

4.4 Specialized Testing Reports: If the ins-pection utilizes any specialized testing agenciesand equipment, all such reports derived from thesespecial testing shall become a part of thedocumentation of the particular inspection period.

4.5 Tunnel Inspection Report: Upon completionof the inspection, Tunnel Inspection Report shouldbe developed that summarizes the ?ndings ofinspection. This report should be submitted alongwith Tunnel Inspection Registers & SupplementaryTunnel Information Registers to higher official(s).The report will facilitate appreciation of items inthe tunnel requiring urgent attention and will helpthe higher authorities in their decision making.

Suggested outline for the report along with adescription of the contents to be included in eachsection is as under:• Report Number/Letter number: Identi?

cation number of report• Table of Contents: Self-explanatory.• Executive Summary: Provide a concise

summary of the inspection, findings, andrecommended repairs.

• Major Inspection Findings: SummarizeMinor, Moderate & serious defects in thetunnels

• Recommendations: This section will includerecommendations for repair/rehabilitation ofthe tunnel components that were found tohave defects. The recommendations shouldclassify the repair/ rehabilitation in tofollowing categories:

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• Critical• Priority• Routine.

5. Role/Duties of Inspecting Officials

5.1 Inspection by SSE/P.Way or SSE/Works(i) As per Indian Railway Bridge Manual

provisions “Senior Section Engineer in-chargeof tunnel shall inspect every tunnel on hissection once a year during the prescribedmonth after the monsoon season but wherespecified by the Chief Engineer, the structuralpart shall be inspected by the SSE/Works”.But it is advisable that the inspection by SSEin-charge of tunnel or SSE/Works is carriedout before monsoon, as is done in case ofbridges.

(ii) Senior Section Engineer shall record theresults of their inspection in prescribed tunnelinspection register.

(iii) Senior Section Engineer shall submit to theAssistant Divisional Engineer (ADEN) by theprescribed date a list of important defectswith a certificate in duplicate to the effect.“I certify that I have personally carried outtunnel inspection of my section in accordancewith standing orders for the year ending…..and append herewith a list of importantdefects.”

(iv) The ADEN shall issue such orders as deemednecessary to the Section Engineer andcounter sign and forward one copy of thecertificate of inspection to the SeniorDivisional Engineer with remarks if any.

(v) The Section Engineer shall accompany theassistant Engineer on the latter’s annualinspection of tunnels.

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5.2 Inspection by ADEN(i) As per Indian Railway Bridge Manual

provisions ”the ADEN shall inspect everytunnel on the sub division once a year beforethe monsoon during the prescribed monthsand record the results in ink in the tunnelinspection registers”. But it is advisable thatthe inspection by ADEN is carried out aftermonsoon, as is done in case of bridges.

(ii) The tunnel, condition of which warrant specialattention, should be inspected morefrequently.

(iii) The instructions and index as required shouldbe prefixed to each tunnel inspection register.

(iv) The inspection shall be detailed and cover allaspects, entries being made under each ofthe heads given in the register.

(v) The ADEN should make an extract of allremarks concerning repairs required, sendthese to SSE with explicit instructions andensure expeditious compliance

(vi) On completion of his annual tunnel inspectionthe Assistant Engineer shall certify at the endof the register as follow:

“I certify that I have inspected all the tunnelsshown in register during the year ending…..andhave issued detailed orders in writing to theInspectors concerned except the following on whichthe Divisional/Sr. Divisional Engineer’s orders aresolicited”.(vii) These registers should be in the Divisional/Sr.

Divisional Engineer’s office by specified date.

5.3 Inspection by Divisional/Sr. DivisionalEngineer(i) The Divisional/Sr. Divisional Engineers shall

carefully scrutinize the ADEN’s tunnelinspection register and inspect such tunnels

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as called for his inspection. He shall recordhis orders regarding the points which requirea decision by him and initial against everyentry of tunnel in the registers in token ofscrutiny. He should endorse on each register,below the assistant Engineer certificate, asfollows:

“I have personally scrutinized this register and haveissued orders regarding all essential points requiring adecision by me. The following points are submitted tothe Territorial Head of the Department at Head Quartersfor orders.”

(ii) The Divisional/Sr. Divisional Engineer shouldextract the items of inspection registerrequiring attention and send it to the ADENwho should intimate the same to theInspector concerned for expeditiouscompliance.

(iii) The register should be forwarded to thementor Head of the Department atHeadquarters who will examine each register,issue orders regarding matters referred tohim, endorsing the registers to the effect andreturn them to the Divisional/Sr. DivisionalEngineer. Subsequent action taken on thenotes should be entered in the registers bythe ADEN.

5.4 Special Inspection: Understanding and pred-iction of tunnel behavior is a complex subject. Incase regular inspections and/or instrumentationrecords indicate a problem that may affect safety/operation of traffic, special inspections arerecommended to be undertaken with assistance ofexperts on relevant field(s).

It is recommended to get a detailed inspection oftunnel conducted by a tunnel expert at afrequency varying from 1 to 6 years (depending ongeology/tunnel condition).

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Also in case of special unexpected events likerailway accident, earthquake etc. railway officialsshould undertake special inspection of tunnel(s).

6. Maintenance and Repair of Tunnels: Maintenanceand repairs activities to be carried out for variouscomponents of tunnel are as under:

6.1 Portal and Approach Cuttings

(A) Surface Drainage: Maintenance operationscarried out on surface drains usually fall into one ora combination of the following:

(i) Weed control within surface drains.(ii) Removal of debris from other track

maintenance activities.(iii) Removal of sediment.(iv) Re-grading (if required).

(B) Catch water Drain(i) Catch water drain should be pucca with

adequate slope to ensure developmentof self-cleaning velocity.

(ii) Catch water drain should not have anyweep hole.

(iii) The expansion joints should be sealed.(iv) Catch water drains should have well

designed out fall with protection againsttail-end erosion

(C) Removal of loose boulders etc.(i) Loose boulders/loose mass in the portal

area should be removed by loosescaling or by controlled blasting.

(ii) Where necessary, rock fall and debrisdiversion or containment features shouldbe constructed to positively ensure thatno rocks, rock-slides or other debrissuch as soil from mountain slopes above

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the portal area can reach the tracks ordamage equipment or structures.

(iii) Option of constructing false tunnel /cutand cover in tunnel approach should beexplored, if required.

(iv) Weak/leaning trees/branches that mayfall on/near the track should be cut.

(D) Structural Repairs(i) Structural repairs including

strengthening (as per requirements) inportal structure.

(ii) Structural repairs of supports (likeshotcreting, rock bolting etc.) used forstrengthening/ stabilization/erosioncontrol of slopes/tunnel face. Furtherstrengthening of support measures to betaken up as per requirements

(E) Access Road: Road access to portals (ifavailable) should be fit for desired use particularlyfor evacuating passengers and for staff/materialaccess.

(F) Weep Holes: Cleaning of clogged/choked weepholes.

6.2 Inside Tunnel• Scaling/Sounding of tunnel• Removal of unwanted material• Cleaning of side drains and sumps• Clearing walkways and refuges• Cleaning of clogged/choked weep holes• Structural maintenance and repairs

(including strengthening)• Track Maintenance (with special

attention to problematic areas)

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• Scheduled maintenance of mechanicaland electrical systems (includingventilation & lighting - by respectivedepartments)

• Scheduled maintenance of instruments• Scheduled maintenance of safety, fire

and `communication equipment andsignage

6.3 Systematic Approach for repairs: Factorsaffecting the repair methods are thedeterioration severity and the structuralimpact of the defect. The cause of the defectshould be determined before remedial worksare undertaken, otherwise the same problemmay recur. It is recommended to adopt asystematic approach to tunnel repairsinvolving following steps:• Determine the cause(s) of damage• Evaluate the extent of damage• Evaluate the need to repair• Select the repair method• Development and approval of detailed

repair methodology• Execution of repairs

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Rapid Transit or Mass Rapid Transit (also known bynames Heavy Rail, Metro, MRT, Subway, Tube, U-Bahnor Underground) is a type of high-capacity publictransport system generally constructed in urban areas.Unlike buses or trams, rapid transit systems are electricrailways that operate on an exclusive right-of-way, whichcannot be accessed by pedestrians or other vehicles ofany sort and it can be on grade separated alignment(elevated alignment), underground alignment orsubways (in tunnels) or on surface (like conventionalRailway system).

1. Metro Tunnels Excavation: Tunnel Boring Machines(TBMs) with features purpose-built to the specific groundconditions are now the preferred mode for boredtunnelling in urban areas. The high capital cost isjustified by the length of tunnel more than 2 km(Sharma, 1998, Ref.: 32). The TBMs offer followingadvantages over the drill & Blast method in the metrotunnels:• Explosives are not used and, therefore, produces

much lower vibrations in built-up areas.• Little or no over break.• Fast excavation, thus saving money.• Less labour cost.• Reduces surface settlement to very low levels

resulting in assured safety to the existing superstructures.

• Reduces risk of life of workers by (i) Rock fallsat face or behind the TBM, (ii) Explosives, (iii)Hit by vehicles; and (iv) Electrocution.

The type of TBM to be used is decided based on thenature of ground to be excavated (i.e. hard rock or softsoil), as discussed in Chapter-10. Now-a-days, dual

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mode shield TBMs have been developed to bore throughall types of soils, boulders and weak rocks (in non-squeezing grounds) under high groundwater table.During tunnelling, the groundwater table is lowered tothe bottom of the tunnel by drilling drainage holes tokeep the ground dry. In dual mode shield TBMs, bothscrapper picks and as well as disc cutters are mountedon the cutter head. Otherwise, these machines are quitesimilar to EPBMs in dimensions and working principles,though progress per day is bit less than the EPBMs(about 6m per day vis-a-vis 10m per day as reported inDelhi Metro).

While working with dual mode shield TBM in Delhi Metro,initially it was found that a large number of scrappersand buckets were getting detached from the cutter head.This was probably because of presence of too manyboulders in the soil strata. As a result, the big boulderswere entangled in the large space between the armsand thereby knocking off the scrapper and buckets. Theprotective plates and deflector strips were added aroundthe buckets to avoid direct impact of boulders on thebuckets, in addition to other modifications. Thereafter,the dual TBMs succeeded (Singh M., Ref.:33).

The advantage of fully shielded TBM with segmenterector is that there is no unsupported ground behindthe shield. That is why TBMs have failed in poor groundsyet dual TBM has succeeded in soils, boulders and weakrock mass in non-squeezing ground conditions. Thereare not much of successful experiences worldwide aboutTBM tunnelling through squeezing grounds. Therefore,TBMs are not recommended in the squeezing groundand flowing conditions.

It is necessary to inject foam along with water at thecutter head which has the following advantages:• Reduced permeability and enhanced sealing at

the tunnel face.• Suppresses dust in rock tunnelling.

427


• Excavation of wet soil or weathered rock iseasier.

• Soil does not stick to the cutters.

2. Pre-cast Lining: Pre-fabricated lining is mostlysuitable in underground metros, for various soils,boulders and rock conditions except squeezing grounds(due to the high overburden pressure) and flowinggrounds within water charged wide shear zones (dueto seepage erosion or piping failure). But generally theseconditions do not occur in suburban tunnels, beingrelatively shallow depth tunnels. In some projects, fiberreinforced precast concrete linings have been adopted.

TBM is capable of placing lining segments in positionall around the circular tunnel with the help of segmenterector. Segment bolts are then tightened by impactwrenches twice. The curved alignment is achieved withthe help of tapering of the lining rings. All the rings aretapered and curvature is obtained by suitably adjustingthe orientation of rings. Before taking inside the tunnel,the segments are checked on ground for any cracks/damage. As water tightness is extremely important forthe durability of the tunnel lining, a double gasketsystem comprising a durable elastomeric gasket and awater sealing made from the hydrophobic material isused. These gaskets are located in grooves cast intothe edges of the precast concrete segments. Togetherwith the high precision casting of the segments achievedby precision steel molds, gaskets will ensure the durableand watertight tunnels. Hydrophobic seals expand 250percent once it comes into contact with water.

Thought should be given to fire-resistant design ofconcrete lining. Extra thickness of concrete cover (about75mm) should be provided over the steel reinforcement.Under-reinforced concrete segments may be used toensure the failure in ductile phase, if it occurs.

Grouting is carried out simultaneously with thetunnelling. There are in-built ports in the tail skin ofthe TBM. These are used in primary grouting of annulus

428

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(void between excavation profile and outer face of theprecast ring). Grouting is continued upto 3 bars (0.3MPa) pressure. Excavation is not commenced until theprevious lining is completed. Secondary grouting is alsodone within 14 days of ring erection. Every third ring isgrouted to pressure of 3 bar (0.3 MPa). Secondarygrouting will fill up any void left during the primarygrout due to its shrinkage.

3. Building Condition Survey and Vibration Limit:Open trenches and shafts are excavated by drilling andblasting method for connection to the undergroundmetro system. The controlled bench blasting method isused in open excavation, under busy and congestedroads which are flanked by old or heavy buildings andmonuments. Before designing the controlled blasting,the entire rock mass is explored thoroughly. The trialblasts are detonated to determine the safe-scaled-distance, according to the nature of structures.

The next step is to assess the condition of buildingsstanding near the blast site to determine how muchvibration can be sustained by these structures, especiallyold buildings and ancient monuments if any. Table 21.02shows the permitted Peak Particle Velocities (PPV) asper German standards. It may be noted that ISRMrecommends almost twice PPV values.

Archaeologists suggest that no surface metro stationshould be built within protected 100m periphery of aprotected (heritage) monument. In such cases, anunderground metro station may be a better choice.

4. Impact on the Structures: The blasting worksmay affect the surrounding structures slightly in spiteof the controlled blasting. In worst case, small cracksmay develop in RCC and masonry. The air over-pressuremay also create cracks in glass works of doors andwindows in nearby areas. Appendix-21.1 summarizesvarious types of damages to the structures. Substantialcompensation may have to be paid to the owners ofthe buildings or structures damaged, according to thespecified class of damage.

429


Table 21.02: Permitted PPV on Structures

S.Condition of Structure

Max. PPVNo. (mm/s)

1 Most structures in “good condition” 25

2 Most structures in “fair condition” 12

3 Most structures in “poor condition” 5

4 Water supply structures 5

5 Heritage structures/Bridge structures 5

The traffic is stopped during blasting time for a fewminutes and all the roads, other exits/entries to theblasting site are closed for safety reasons. Flying ofrock pieces during an urban blasting may have severeconsequences.

5. Subsidence: Subsidence of ground and differentialsettlement of nearby structures takes place due tounderground tunnelling. The dewatering due toexcavation causes more widespread subsidence,primarily due to the settlement of overlying loose depositof soil, silt or clay. In totally rocky areas, the subsidenceis very small and does not cause any worry. Followinginstruments are recommended for precision monitoringof structures:• Precise leveling points,• Tilt meters,• Crack gauges embedded in the nearby

structures, and• Vibration monitoring of old/ancient structures.

In case the actual settlement is expected to go beyondthe predicted subsidence, the whole constructionmethodology must be reviewed. Appendix-21.1 maybe used which specifies the maximum tensile straincaused by subsidence (= increment in spacing of

430

Chapter-21

columns divided by the distance between columns,expressed in percentage).

Table 21.03: Preliminary Design of cut Slopes(for Height of cut less than 10m)

6. Portal and Cut Slopes: It is better to locate theportals deeper into the ground or mountain where rockcover of at least equal to width of tunnel is available.The slope of portal should be stable, otherwise the sameshould be reinforced properly with the rock anchors.

* N is 5 for H< 3m; 4 for H= 3 - 4m and 3 for H= 4 -6m

431


Alternatively a thick breast wall (1m) of concrete shouldbe constructed to ensure stability of portals (Singh &Goel, 2002 – Ref.:34).

The side slopes of open trenches should be stable. Deojaet al. (Ref.: 35) have suggested the dip of safe cutslopes with and without protective measures for bothrocks and soils (Table-21.03) for all types of tunnels,including metro tunnels.

432

Chapter-21

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Chapter-21


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435

MISCELLANEOUS

Some topics, which are relevant to tunnels, but havenot been covered so far, are elaborated in this chapter.

1. Ballastless Track in Tunnels: Most of the advancerailways in the world specify laying of Long Welded Rails(LWR) in tunnels. Holding the track structure in place,with LWR, is very competitive in terms of constructionand maintenance practices are well understood. Butthis technique requires:

• Considerable overall track height.• Monitoring of track geometry.• Maintenance in line with actual traffic density.

The alternative to ballasted track is ballastless track(BLT) in which ballast is replaced by foundation layersmade of cement concrete (and asphalt in some cases).This type of track must provide same functions astraditional tracks by superimposing several layers ofdecreasing stiffness. Technological development andresearch in Railway track construction has led tointroduction of BLT usually referred as slab track (Fig.22.01). This system was first introduced in Japan inlate 1960s. Both conventional ballasted track as wellas ballastless track have their advantages anddisadvantages as discussed below:

1.1 Ballasted Track Structure:

Advantages• Known and proven method; suitable for high

speed also.• Relatively low construction cost.• High elasticity.• High maintainability (particularly with

mechanized maintenance) at relatively lowcost.

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MISCELLANEOUS

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Chapter-22

• Good noise & vibration absorption.• Less sensitive to construction defects.• Availability of mechanized construction

technology.

Fig. 22.01: Ballastless Track in Tunnels

Disadvantages• Over the time, the track tends to “float”, in

both longitudinal and lateral directions, as aresult of non-linear, irreversible behaviour ofthe materials. This translates into increasedrequirement for track maintenance.

• Limited non-compensated lateral accelerationoccurs in curves, which is due to the limitedlateral resistance offered by the ballast.

• At high speeds ballast can be churned up,causing damage to rails and wheels.

• Over the time, ballast bed becomes lesspermeable and less elastic due tocontamination, breakage of the ballast andtransfer of fine particles from the subgrade.

• Depth of construction of ballasted track is

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relatively high, which has implications fortunnel size

1.2 Ballastless Track Structure:

Advantages• Less frequent maintenance efforts to maintain

track geometry.• Relatively higher construction cost, but lower

life cycle cost.• Excellent riding comfort even at speed

greater than 250 kmph.• Unlike ballasted tracks where the track tends

to "float" over the time, in both longitudinaland lateral directions as a result of non-linearirreversible behavior of the materials, this isnot the case in ballastless tracks.

• High lateral resistance of track structurewhich allows future increase in speeds incombination with tilting coach technology.

• Relatively low noise and vibration nuisance.• High impermeability.• Lesser dead weight.• Depth of construction is relatively less as

compared to ballasted track, which reducestunnel construction cost.

• The track can be accessible to road vehicles.• Less environment pollution.• Easy and economical maintainability - ease of

replacing parts with minimum dislocation totraffic.

• Electrical insulation for facilitating trackcircuiting control.

Disadvantages• Relatively higher initial construction cost.• Less sensitive to construction defects.• Difficult to repair & restore traffic in case of

438

Chapter-22

accident resulting in damage to ballastless track.• Need for providing transitions.

Considering the reduced maintenance requirement,reduced life cycle cost and enhanced safety,Ballastless track should, by and large, be providedfor all new tunnels.

1.3 Ballastless Track Systems: Many types ofballastless track systems are currently in usearound the world. This includes slab track system,embedded rail system, RHEDA system, Twin-blocksystem etc. Selection of appropriate systemshould be made after consideration of varioustechno-economic factors including contingency planfor repairs & restoration. For details about thesesystems, literature on the subject “Railway Track”may be referred e.g. Modern Railway Track(Second Edition) – By: Coenraad Esveld –Publisher: MRT Productions, The Netherlands,Chapter-9 & Track Compendium – By: Dr.Bernhard Lichtberger – Publisher: Eurail Press,Chapter-10

2. Cut and Cover Tunnelling: This is a common andwell-proven technique for constructing tunnels/underground structures at shallow depths. The methodcan accommodate changes in tunnel width and non-uniform shapes and is very commonly used inconstruction of underground stations.

In cut-and-cover method, the structure is built insidean excavation and covered over with backfill materialwhen construction of the structure is complete. Cut-and-cover construction is used when the tunnel profileis shallow and the excavation from the surface ispossible, economical, and acceptable. Cut and coverconstruction is used for underpasses, the approachsections to mined tunnels and for tunnels in flat terrainor where it is advantageous to construct the tunnel at ashallow depth.

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Several overlapping works are required to be carriedout in using this method. Trench excavation,underground structure construction and soil coveringof excavated area are three major integral parts of thetunnelling method. Most of these works are similar toother road construction except that the excavation levelsinvolved are deeper. Bulk excavation is often undertakenunder a road deck to minimize traffic disruption as wellas environmental impacts in terms of dust and noiseemissions and visual impact.

For depths of about 10m to 12m, cut-and-cover isusually more economical and more practical than minedor bored tunneling. The cut-and-cover tunnel is usuallydesigned as a rigid frame box structure. In urban areas,due to the limited available space, the tunnel is usuallyconstructed within a neat excavation line using bracedor tied back excavation supporting walls. Whereverconstruction space permits, in open areas beyond urbandevelopment, it may be more economical to employopen cut construction.

Where the tunnel alignment is beneath a city street, thecut-and-cover construction will cause interference withtraffic and other urban activities. This disruption can belessened through the use of decking over the excavationto restore traffic. While most cut-and-cover tunnels havea relatively shallow depth to the invert, depths to 18mare not uncommon; depths rarely exceed 30m.

Disadvantages of this method are:• More dust and noise impact may arise,

though these can be mitigated throughimplementation of sufficient control measures.

• Temporary decks are often installed beforebulk excavation to minimize the associatedenvironment impacts.

• Larger quantity of cut and disposal materialswould be generated from the excavationworks, requiring proper handling anddisposal.

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Chapter-22

Two types of construction methods are employedto build cut and cover tunnels; bottom-up and top-down. Part-A in Fig. 22.02 illustrates “Bottom-UpConstruction” where the final structure isindependent of the support of excavation walls.Part-B in Fig. 22.02 illustrates “Top-DownConstruction” where the tunnel roof and ceiling arestructural parts of the support of excavation walls.These construction types are described in moredetail below.

Fig. 22.02: Cut and Cover Tunnelling Methods

2.1 Bottom-Up Construction: In the conv-entional “bottom-up” construction, a trench isexcavated from the surface within which the tunnelis constructed and then the trench is backfilled andthe surface restored afterward. The trench can beformed using open cut (sides sloped back andunsupported), or with vertical faces using anexcavation support system. In this method, thetunnel is completed before it is covered up and thesurface reinstated.

Various stages involved are as under:

(i) The underground retaining wall is installedbefore excavation commences. The retaining wallcan be a concrete diaphragm wall, a concretebored pile wall or a steel sheet pile wall;depending on the site condition, soil type and theexcavation depth (Fig. 22.03).

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Fig. 22.03: Installation Fig. 22.04: Excavationof retaining wall &

Installation of Steel Strut

(ii) The soil is excavated to the first strut level.The first level strut is installed before theexcavation proceeds further (Fig. 22.04).

(iii) The soil is excavated to the next strut leveland the second level strut is installed. Thiscontinues till the excavation reaches the finaldepth or formation level (Fig. 22.05). The numbersof strut levels depend on the excavation depth.

Fig. 22.05: Fig. 22.06:Excavation & Construction ofInstallation of Underground

Steel Struts structure

(iv) At the formation level, the reinforcedconcrete slab or base slab is constructed, followedby the removal of lowest level strut and the sidewalls are constructed (Fig. 22.06).

(v) The next level of slab is constructed, followedby the removal of the strut near to that slab level.This process progresses upwards till the roof slabis constructed (Fig. 22.07).

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Fig. 22.07: Construction Fig. 22.08: Back fillingof Underground and

structure Reinstatement

(vi) After the roof slab is completed, the soil isbackfilled to the first strut level before the firstlevel strut is removed. This is followed bycompletely backfilling the top of the undergroundstructure (Fig. 22.08). If the retaining wall is adiaphragm wall or a bored pile wall, the top 2m ofthe wall will be removed. If it is a sheet pile wall,the sheet pile will be extracted.

Bottom-up construction offers several advantages:• It is a conventional construction method, well

understood by contractors.• Waterproofing can be applied to the outside

surface of the structure.• The inside of the excavation is easily

accessible for the construction equipment andthe delivery, storage and placement ofmaterials.

• Drainage systems can be installed outside thestructure to divert water away from thestructure.

Disadvantages of bottom-up construction include:• Somewhat larger footprint required for

construction than for top-down construction.• The ground surface cannot be restored to its

final condition until construction is complete.

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• Requires temporary support or relocation ofutilities.

• May require dewatering that could haveadverse effects on surrounding infrastructure.

2.2 Top-Down Construction: In this system,the vertical walls are constructed first, usuallyusing slurry walls, although secant pile walls arealso used. In this method the support ofexcavation is often the final structural tunnel walls.Secondary finishing walls are provided uponcompletion of the construction. Next the roof isconstructed and tied into the support of excavationwalls. The surface is then reinstated before thecompletion of the construction. The remainder ofthe excavation is completed under the protectionof the top slab. Upon the completion of theexcavation, the floor is completed and tied into thewalls. The tunnel finishes are installed within thecompleted structure. For wider tunnels, temporaryor permanent piles or wall elements are sometimesinstalled along the center of the proposed tunnelto reduce the span of the roof and floors of thetunnel.

Various stages involved are as under:

(i) The underground retaining wall is usually aconcrete diaphragm wall, which is installed beforeexcavation commences (Fig. 22.09).

(ii) The soil is excavated to just below the roofslab level of the underground structure. Struts areinstalled to support the retaining wall, which inturn supports the soil at sides (Fig. 22.10).

(iii) The roof slab is constructed, with accessopenings provided on the slab for works toproceed downwards (Fig. 22.11). The roof slab notonly provides a massive support across theexcavation, it also acts as a noise barrier.

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Chapter-22

Fig. 22.09: Installation Fig. 22.10: Excavationof Retaining Wall & Installation of

Steel Strut

(iv) The next level of slab is constructed, and thisprocess progresses downwards till the base slab iscompleted (Fig. 22.12).

Fig. 22.11: Construction Fig.22.12:of Underground Construction of

Structure Underground Structure

(v) The side walls are constructed upwards,followed by removal of the intermediate struts.The access opening on the roof slab are thensealed (Fig. 22.13).

(vi) After the underground structure is completed,the soil is backfilled to the top strut level beforethe strut is removed. This is followed bycompletely backfilling the top of the undergroundstructure and finally reinstating the surface areas(Fig. 22.14).

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Fig. 22.13: Construction Fig. 22.14: Backfillingof Underground & Reinstatement

Structure

Top-down construction offers several advantages incomparison to bottom-up construction:• It allows early restoration of the ground

surface above the tunnel. The temporarysupport of excavation walls are used as thepermanent structural walls.

• The structural slabs will act as internalbracing for the support of excavation thusreducing the amount of tie backs required. Itrequires somewhat less width for theconstruction area.

• Easier construction of roof since it can be caston prepared grade rather than using bottomforms.

• It may result in lower cost for the tunnel bythe elimination of the separate, cast-in-placeconcrete walls within the excavation andreducing the need for tie backs and internalbracing.

• It may result in shorter construction durationby overlapping construction activities.

Disadvantages of top-down construction include:• Inability to install external waterproofing

outside the tunnel walls.• More complicated connections for the roof,

floor and base slabs.

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Chapter-22

• Potential water leakage at the joints betweenthe slabs and the walls.

• Risks that the exterior walls (or centercolumns) will exceed specified installationtolerances and extend within the neat line ofthe interior space.

• Access to the excavation is limited to theportals or through shafts through the roof.

• Limited spaces for excavation andconstruction of the bottom slab.

It is difficult to generalize the use of a particularconstruction method since each project is uniqueand has any number of constraints and variablesthat should be evaluated when selecting aconstruction method. The following summarypresents conditions that may make oneconstruction method more attractive than theother. This summary should be used in conjunctionwith a careful evaluation of all factors associatedwith a project to make a final determination of theconstruction method to be used.

Conditions favourable to Bottom-Up Construction:• No right of way restrictions.• No requirement to limit sidewall deflections.• No requirement for permanent restoration of

surface.

Conditions favourable to Top-Down Construction:• Limited width of right-of-way.• Sidewall deflections must be limited to

protect adjacent features.• Surface must be restored to permanent

usable condition as soon as possible.

3. Micro-tunnelling: Micro-tunnelling is a name forremotely-operated, small-diameter tunnelling. It istypically achieved by jacking pipes of concrete or other

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MISCELLANEOUS

suitable materials, behind a tunnelling machine, froma launch shaft into a reception shaft. MTBMs areavailable in the 30-114 inches Outer Diameter range.Micro-tunneling operations are managed by an operatorin an above-ground control container. Cutter heads canbe customized for specific ground conditions (Fig.22.15). In slurry shield machines, soil excavation takesplace by way of infusing the soil with slurry at the faceof the bore and cuttings are forced into slurry inlet holesin the MTBM’s crushing cone for circulation to and froma separation plant through a closed system. The launchshaft is outfitted with a pit seal to prevent shaft floodingand a project specific thrust block to distribute jackingforce.

Fig. 22.15: MTBM Cutter Heads

MTBMs feature high pressure jetting nozzles, anarticulated steering joint with three-point steeringcontrol and hydraulically activated dirt wings to minimizeMTBM roll. Additional MTBM attributes include a live,one-way audio system & camera allowing for systemmonitoring during operation, gas detector andsubmersible pump.

A full installation system may typically include followingcomponents:• Shield machine• Control cabin• Hydraulic jacking rig and the power pack to

operate it• Guidance System, including laser

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Chapter-22

• Keyhole jacking frames and series of pumps• Slurry system with pumps and separation plant• There may also be pipe lubrication equipment

3.1 Guidance System: The Micro-tunnelingGuidance System includes an active target andthree inclinometers that read and transmit data tothe control console in the control container. Twoinclinometers in the target and one at the rear ofthe MTBM track roll and incline to ensure accuracy.Another inclinometer, mounted in the front of theMTBM, assesses the incline. The proprietarysoftware programs display the current andprojected cutter head location, MTBM incline androll to the operator’s control console.

The Total Guidance System (TGS) provided inrecent MTBMs is a monitoring system for extendedlengths and alignments with curves (Fig. 22.16). Itcomprises of individual, self-leveling, station unitsthat maintain a surveyed connection throughoutthe alignment without the need for continuousmanual surveying. The target is axially mountedbehind the machine’s articulation cylinders at lineand grade, and registers the position and angles ofincidence of the red laser emitted from theguidance system. The combined stationscommunicate a continuous electronic distancemeasurement for the operator to monitor exactmachine X and Y positioning, real-time cutter headlocation and horizontal and vertical deviation withthe proprietary software program. This state-of-the-art guidance system can be utilized as astandalone guidance system for any tunneling,pipe jacking or micro-tunneling application,regardless of equipment manufacturer.

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MISCELLANEOUS

Fig. 22.16: Total Fig. 22.17: ControlGuidance System Container

3.2 Control Container: Control Container housesthe operator’s control console, motor controlcenters for the slurry pumps, MTBM drive motorand bulkhead panel for electrical andcommunication connections (Fig. 22.17). Theoperator monitors and controls all facets of themicro-tunneling operation from a dual-monitordisplay console and control station including:MTBM’s pitch and yaw, rotation, torque, jetting,jacking thrust, steering, slurry flow and pressure,as well as the MTBMs anticipated position at thecutter face using proprietary software programs.

3.3 Remote Hydraulic Power Pack: The RemoteHydraulic Power Pack is a power distribution centerfor the jacking frame and micro-tunneling auxiliaryfunctions (Fig. 22.18).

Fig. 22.18: Remote Fig. 22.19: Main DriveHydraulic Power Power container

Pack

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Chapter-22

3.4 Main Drive Power Container: The MainDrive Power Container offers increased voltage torun the periphery drive and face access MTBM’smain drive (Fig. 22.19).

3.5 Procedure of Micro-tunneling: Theprocedure used in micro-tunnelling, with slurryshield machine, will typically comprise of followingsteps:(i) The MTBM and jacking frame are set up in a

shaft at the required depth.(ii) The MTBM is pushed into the earth by

hydraulic jacks mounted and aligned in thejacking shaft.

(iii) The jacks are then retracted and the slurrylines and control cables are disconnected.

(iv) A product pipe or casing is lowered into theshaft and inserted between the jacking frameand the MTBM or previously jacked pipe.

(v) Slurry lines and power and control cableconnections are made, and the pipe andMTBM are advanced for another drive stroke.

(vi) This process is repeated until the MTBMreaches the reception shaft.

(vii) Upon drive completion, the MTBM and trailingequipment are retrieved and all equipmentremoved from the pipeline.

3.6 Utility in Railway: When any pipe linecrossing has to be made under the track (in majoryards or under very high embankments), executingthe work by conventional methods of usingrelieving girders or using conventional box pushingtechnique is either not possible due to siteconditions or it may be very time consuming. Insuch situations, using the MTBM (as trenchlesstechnology) is highly advantageous. In IndianRailways, MTBM has been used successfullyrecently in Bhubaneshwar Yard for constructing apipe culvert crossing of more than 100m length

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MISCELLANEOUS

across the whole yard to address the problem ofdrainage during rainy season, in Igatpuri Yard forconstructing a pipe culvert crossing of more than90m length across the whole yard, to address theproblem of drainage during rainy season andconstruction of pipe culverts below very high banksin lieu of damaged existing minor bridges inIgatpuri – Kasara section of Central Railway.

4. Provision of OHE: Planned location of Over HeadEquipment (OHE) fixtures to be fitted in tunnel walls/roof, in electrified territory, in plan and cross-sectionshould be obtained from electrical department andincorporated in construction drawings. Self-anchoredbolts of High Tensile steel/Stainless steel shouldpreferably be used for fixing arrangements. It shouldbe ensured that bolts do not puncture the waterproofinglayer (if provided). Planning for fixation of OHE fixturesshall be kept in consideration while designing tunnellining.

452

Abrasivity- 28Cerchar Abrasivity Index- 28

Access Control- 348Access Control Systems- 317Action Level- 255ADECO-RS Method of Tunnelling-216

Stages of- 225Advance Supporting- 287Air Column Method- 180Alarm Level- 255Alert Level- 255Artificial Ventilation- 342Austrian Method- 279

Ballastless Track in Tunnels- 435Bedding Planes- 6Belgian Method- 277Borehole Extensometer- 237, 238Bottom-Up Construction- 440Blasting- 179, 330Brazilian Tensile Strength Test- 21Buckles and Kinks- 401Building Condition Survey- 428Burn Cut Drilling Pattern- 176

Cables- 328, 345CCTV System- 324Chimney Formation- 307Compaction Grouting- 283Compressed Air TBM- 197Compressive Strength- 18, 25, 33,59Concrete Lining- 130

Cast in-situ- 131Precast Segmental- 134

Condition Rating System-407Cone Cut Drilling Pattern- 175Connection Bolts- 403Controlled Blasting- 179Control Systems- 360Control Limits- 254, 255, 257, 258Conventional Tunnelling- 203Corrosion- 400Cracking- 398Cracks- 401Cross Passages- 355Cut and Cover Tunnelling- 438

INDEX

Daylighting- 307Density- 27Detailed Design Consultant (DDC)-144Diamond Cut Drilling Pattern- 176Differential Settlement- 257, 397,429Digital Ortho Mapping- 94Digital Topographic Mapping- 94Dimpled Mats- 264Direct Shear Test- 23Displacement Grouting- 283Displacement Velocity- 256Drainage during Construction- 263Drainage Mats- 264DRESS Method- 290Drill and Blast Method- 173Drilling- 175, 330

Earth Pressure Balance TBM- 199Efflorescence- 399Electric Resistivity Survey- 73Electronic Distance Measuring- 97Emergency Tunnel Lighting- 344,362Engineering Seismograph- 246English Method- 279Escape Distance- 101, 354Escape Routes- 352Excavation Support Ratio- 127Explosives- 334

Accountal- 334Choice of- 177Disposal- 333Requirement of- 178

Extrusion- 219

Face Collapse- 308Face Consolidation- 286Face Plugging- 309Failure Criteria of rock materials-31

Mohr Coulomb Criterion- 31Hoek-Brown Criterion- 33

Faults- 5Fire Extinguishing System- 350Fire Protection- 324Fire, Smoke and Gas detection-350

453

Floor Bolting- 130Folds- 6

Asymmetric, Symmetric- 6Anticline, Syncline - 6

Forepole- 292Full Face Excavation- 182Fully Drained Tunnel- 270

Geological Strength Index (GSI)-47, 48, 49Geothermal Gradient- 312German Method- 278Glass-fiber Reinforced PlasticAnchors- 124Global Positioning System- 98Grouted (Friction) Bolt- 121Gripper TBM- 194Ground Freezing- 262, 285Ground Improvement- 279Ground Reaction Curve (GRC)-205Ground Support Interaction-Grouting- 281

Compaction Grouting- 283Jet Grouting- 284Location for- 282Materials- 284Permeation Grouting- 283

Hardness- 28Rebound Hammer Test- 28

Hard Rock TBMs- 192Heading and Benching- 183Hollow Area- 400Honeycomb- 400Horizontal Ribs and VerticalLagging- 383Housekeeping- 327

Inclinometer- 238In-situ stresses- 10

Horizontal- 11Vertical- 10

Instrumentation and Monitoring-231

Purpose of-Items to consider in-

Inspection by SSE/P.Way or SSE/Works- 410Inspection by ADEN- 411

Inspection by DEN/Sr.DEN- 411

Jet Grouting- 284Joint Alteration Number- 45Joint Roughness Number- 45Joint Set Number- 45Joint Water Reduction Factor- 45Joint Volume Count- 45

Lab Test Samples- 17Lateral Exits- 102, 355Lattice Girder- 117Leakage- 400Leaky Feeder- 346Liner Plates- 381Lines of Influence- 252Load Cell- 243Loading of Explosives- 177

Maintenance and Repair of Tunnels-413Maximum Unsupported Span- 157Measuring Anchor- 245Mechanical Bolts- 121Mechanical Pre-cutting- 222, 287Mechanical Support TBM- 197Mechanized Tunnelling- 187Medical Facilities- 320Micro-tunnelling- 446Mock Drills- 366Modified Terzaghi’s Theory- 147Mortar- 402Mucking- 181, 333Multiple Drift Advance- 184Multiple Point BoreholeExtensometer- 237

Natural Ventilation- 342New Austrian Tunnelling Method(NATM)- 212

Advantages of- 215Construction Sequence- 214Definition of- 212Limitations of- 214Pre-requisites for- 215Principles of- 213

Noise Protection- 322Normal Tunnel Lighting- 344

454

Norwegian Method of Tunnelling(NMT)- 215

Observational Method ofTunnelling- 210Onboard Fire Detection- 362Onboard Fire ExtinguishingEquipment- 362Optical Targets 3-D- 242

Parallel Cut Drilling Pattern- 176Partial Face Excavation- 182Partially Drained Tunnel- 269Permanent Drainage System- 268Permeation Grouting- 283Permeability- 29Personal Protective Equipment(PPE)- 316Piezometer- 246Point Load Index Test- 25Poisson’s Ratio- 21Pop-outs- 399Porosity- 27Portal- 346, 367

Construction of- 368Design of- 367Need for- 367

Portal Collapse- 312Pyramid Cut Drilling Pattern – 176Pre-drainage- 261Pre-supporting- 287Preliminary Survey- 70Pressure Cell- 243Pressure Transient Hazards- 341Principal Monitoring Sections- 249Project Safety Plan (PSP)- 315Protection System- 401Protective Intervention- 221

Qualifications of InspectingOfficials- 393

Raises- 389Reinforced Rock Arch- 125Repair Priority Classification- 408Rib Reinforced Shotcrete (RRS)-156Road header- 185

Rocks- 1Discontinuities- 4Igneous- 1Joints- 4Mass- 7Material- 7Metamorphic- 3Origin of- 1Sedimentary- 3

Rock Bolt- 117Length- 157

Rock Bolting Pattern- 129Rock Bolts Reinforcement- 124Rock Burst- 305Rock mass- 7

Compressive Strength- 59Squeezing Behaviour of- 57Tensile Strength- 59

Rock mass classification Systems-37

Geological Strength Index(GSI)- 47Rock Mass Quality (Q)- 44Rock Mass Rating (RMR)- 43

Rock Quality Designation (RQD)-39Ruling Gradient- 104

Safety Management and Training-319Safety walkway- 346Scale Effect- 8Scaling- 181Seismic Refraction Survey- 71Service and Safety Tunnel- 356Shaft excavation in Soft and WetGround- 386Shaft excavation in Soft Ground-376Shaft excavation in Rock- 388Shafts- 373

Design of- 374Location of- 373Lining of- 390Need for- 373Shapes of- 374

Shapes of tunnels- 107Circular- 109D-Shape- 107Elliptical- 109Horse Shoe Shape- 108

Shear strength- 23

455

Shielded TBM- 195Shotcrete- 137Shotcrete Strain Meter- 245Sidewall Drift- 185Signage- 317Slurry Shield TBM - 197Slurry Walls- 384Soft Ground TBMs- 196Soldier Piles and Lagging- 380Spalling- 398Special Inspection- 412Special Purpose TBMs- 200Special Rocks- 11

Crushed Rock- 12Weathered Rocks- 11Soft Rocks and Hard Soils- 12Swelling Rocks- 12

Speed Monitoring System- 346Spiles- 288Sprayed Waterproofing layer- 275Stabilization of cavity- 111Stable Core Face- 223Staining- 399Standard Monitoring Section- 248Stand-up time- 334Steel Fiber Reinforced Shotcrete(SFRS)- 139Steel Ribs (sets)- 115Steel Sheet Piling- 378Stress Reduction Factor (SRF)- 45,46Supplementary Tunnel InformationRegister- 396Support Reaction Curve (SRC)- 205Support Design- 145, 169

Analytical Method- 169Empirical Method- 145Numerical Method- 170

Surface Monitoring- 250Swellex Bolts- 122

Tape Extensometer- 240Telephone System- 324Tensile Strength- 22Tri-axial Shear Test- 19Terzaghi’s Rock Load Factors- 145Thermal Environment Hazard- 341Threshold Values- 235Tilt- 243Tilt meter- 243Timber Sheet Piling- 377Time Displacement Diagram- 251

Top Down Construction – 443Toxic Gases- 312Train Control- 348Train Detection- 347Train Radio- 347Trend Lines- 253Tunnel Boring Machine (TBM)-189Tunnel Illumination System- 343Tunnel Inspection Register- 396Tunnel Inspection Unit- 393Tunnelling in Seismic Area- 301Tunnelling in Squeezing Grounds-297Tunnelling in Swelling Grounds-294Tunnelman’s GroundClassification- 49Tunnel Rescue Train- 360Tunnel Seismic Prediction- 87Tunnelling Shield- 188

Umbrella Arch- 289Uniaxial Compressive StrengthTest- 18Unstable Core Face- 224Utility Surveys- 95

“V” Cut Drilling Pattern- 176Vector Diagrams- 251Ventilation- 321, 340, 350, 405Ventilation System- 342Vibration Limits- 428

Warning Signs- 323Water Content- 27Water Ingress- 309Water Level Sounder- 246Waterproofing Systems- 273Watertight (Waterproof) Tunnels-268Wave Velocity- 30Wedge Cut Drilling Pattern- 176Wellpoint System- 387

Young’s Modulus- 21

456

1. “Tunnelling in Weak Rocks” – By: Bhawani Singh& Rajneesh K. Goel – Elsevier Geo-EngineeringBook Series Volume -5.

2. “Engineering Geology and Tunnels Engineering” –By: Jaafar Mohammed - Online publication onResearchgate.

3. “Hoek-Brown Failure Crierion-2002 Edition”- By:E. Hoek, C. Caranzza Toress and B. Korkum -Proceedings NARMS-TAC Conference, Toronto,2002, 1, 267-273.

4. “Rock Mass Classification-A practical approach inCivil Engineering” – By: Bhawani Singh & R. K.Goel – Published by Elsevier.

5. Lecture notes of Dr. K. G. Sharma, EmeritusProfessor, IIT Delhi, on “Tunnels”.

6. 3D – TSP – Advanced geological predictionduring construction of Hydro tunnels inHimalayas – Paper by Kripal Choudhary andThomas Dickman.

7. UIC Code 779-9 R: Safety in Railway Tunnels –1st Edition August’2003.

8. “Design and Construction of Tunnels – Analysisof controlled deformation in rocks and soils(ADECO-RS)” – By: Pietro Lunardi – Publisher:Springer.

9. “Introduction to Tunnel Construction” – By:David Chapman, Nicole Metje and Alfred Stark –Publisher: Spon Press.

10. “Technical Manual for Design and Constructionof Road Tunnels – Civil Elements” – PublicationNo. FHWA-NHI-10-034 (December’2009) – by:US Department of Transportation – FederalHighway Administration.

BIBLIOGRAPHY AND REFERENCES

457

11. “Practical Tunnel Construction” – By: Gary B.Hemphill – Publisher: John Wiley & Sons.

12. “Hand Book of Tunnel Engineering – Structuresand Methods, Vol. I & II” – By: Bernhard Maidl,Markus Thewes and Ulrich Maidal – Publisher:Ernst & Sohn, A Wiley Brand.

13. “Pir Panjal Railway Tunnel – A Dream Realized”–Published By: Northern Railway.

14. “Specification and Guidelines for the use ofspecialist products for Mechanized Tunnelling(TBM) in Soft Ground and Hard Rock” –April’2005 - Published By: EFNARC (EuropeanFederation of National Associations Representingfor Concrete).

15. Lang T. A. (1961) – Theory and Practice of RockBolting, A.I.M.E. Trans., 220.

16. Lang T. A. (1966) – Theory and Practice of RockReinforcement – 45th Annual Meeting, Highwayresearch Board, Washington D. C.

17. Barton N., Lien R, and Lunde J. (1974) –Engineering classification of rock masses fordesign of tunnel supports, Rock mechanics, 6,189-236.

18. “A Treatise on Konkan Railway” (1998) –Published by: Konkan Railway CorporationLimited, Navi Mumbai, India.

19. RDSO Guidelines No. RDSO/2009/GE:G-0015,June 2012 - Guidelines for Civil EngineeringInspection, Maintenance and safety in existingTunnels.

20. RDSO Guidelines No. RDSO/2011/GE:G-0016,June 2012 – Guidelines for Safety in Tunnelsduring construction.

21. RDSO Guidelines No. RDSO/2012/GE:G-0017,June 2012 – Guidelines for Design andConstruction of Tunnels.

458

22. UIC Code 779-10R - Maintenance of existingRailway Tunnels.

23. Harbour, Dock and Tunnel Engineering – By: R.Srinivasan – Charotar Publishing House Pvt.Ltd., Anand (India).

24. Goel R. K. and Jethwa J. L. (1991) – Predictionof support pressure using RMR Classification –Proceedings of Indian Geotechnical Conference,Surat, India (Page 203-205).

25. Information from website “www.railsystem.net/drill-and-blast-method/”

26. IRC:SP:91-2010 Guidelines for Road Tunnels –Published by: Indian Road Congress, New Delhi

27. “Tunnelling and Tunnel Mechanics – A RationalApproach to Tunnelling” – By: DimitriosKolymbas – Publisher: Springer.

28. “Tunnel Engineering Handbook (Second Edition)”– By: John O. Bickel, Thomas R. Kuesel andElwyn H. King - Publisher: Kluwer AcademicPublishers.

29. British Standard BS EN 1997-1:2004 – Eurocode7: Geotechnical Design, Part 1: General Rules.

30. OeGG (Austrian Society for Geomechanics)Handbook for “Geotechnical Monitoring inConventional Tunnelling” – 2014.

31. NSW Transport (T MU CI 12140 GU) –Geotechnical instrumentation and MonitoringGuidelines – Ver. 1, Dec’ 2016.

32. “Underground or aboveground? Making thechoice for urban mass transit systems” - AReport by International Tunnelling Association(ITA) – Prepared by Working Group no. 13(WG13) – “Direct and indirect advantages ofunderground structures” (Year 2003).

http://www.railsystem.net/

459

33. Sharma, V. M. (1998): Metro stations and railtunnels of metro projects – National Workshopon Underground Space Utilization, ISRMTT, NewDelhi, 131-163.

34. Singh, M. (2003): Delhi Metro Line No. 2 –Tunnelling by shield boring machines – Seminaron productivity and speed in tunnelling,Dehradun, India, 207-218.

35. “Software for Engineering Control of Landslideand Tunnelling Hazards” – By: Singh, Bhawaniand Goel, R. K. (2002) – Published by: A. A.Balkema, The Netherlands, 334.

36. Deoja, B. B., Dhittal, M., Thapa, B. and Wagner,A. (1991) – Mountain Rock EngineeringHandbook – international Centre of IntegratedMountain Development, Kathmandu, Part-II,Table 22.15.

37. AFTES (French Tunnelling and UndergroundSpace Association) Recommendations – RailTracks and Track Beds in Tunnels (No.GT40R2A1).

38. Einstein, H. H. and Bischef, N. (1975) – Designof tunnels in swelling rock, Design methods inrock mechanics – Proceedings of 16thSymposium on Rock Mechanics, University ofMinnesota, U.S.A., 185-195

39. Barla, Giovanni – Full-face excavation of largetunnels in difficult conditions – Journal of RockMechanics and Geotechnical Engineering 8(2016) 294-303.

40. Jaw-Nan (Joe) Wang (1993) - Seismic Design ofTunnels, A Simple State of the Art DesignApproach

41. Goel, R. K. – Tunnel Boring Machines in theHimalayan Tunnels – A paper published onResearchGate (2014).

460

42. Sharma H. K., Sharma J. K., Chauhan R. S. &Kuthila Sudhir – Challenges in Design andConstruction of HRT of Nathpa JhakriHydroelectric Project (1500 MW) – A Case Study: World Tunnel Congress 2008 – UndergroundFacilities for better Environment and Safety(India).

43. IS:9143-1979 (Reaffirmed-2001): Method forthe determination of Unconfined CompressiveStrength of Rock Materials.

44. IS:13047-1991 (Reaffirmed-2016): Method fordetermination of Strength Of Rock Materials InTriaxial Compression.

45. IS:10082-1981 (Reaffirmed-2016): Method oftest for determination of Tensile Strength byIndirect Tests on Rock Specimens.

46. IS:12634-1989 (Reaffirmed-2015): Rock JointsDirect Shear Strength Laboratory Method ofdetermination.

47. ASTM:D5731-2016: Standard Test Method fordetermination of the Point Load Strength Indexof Rock and Application to Rock StrengthClassi?cations.

48. ASTM:D7625-2010: Standard Test Method forLaboratory Determination of Abrasiveness ofRock Using the CERCHAR Method.

49. IS:4348-1973 (Reaffirmed-2017): Methods ofTest for determination of Permeability of NaturalBuilding Stones.

50. Bieniawski Z. T. (1973); Engineeringclassification of jointed rock masses. The CivilEngineer in South Africa, 15, 335-344.

51. Grimstad E. and Barton N. (1993): Updating theQ-System for NMT. International Symposium onSprayed Concrete, Fagernes. Eds: Kompen,Opsahll and Berg, Norwegian ConcreteAssociation, Oslo.

461

52. Marinos P. G., Marinos V. and Hoek E. (2000):The Geological Strength Index (GSI) – ACharacterization Tool for assessing Engineeringproperties of Rock Mass.

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