7/27/2019 TA5211_New Austrian Tunneling Method

1/19

1

New Austrian Tunneling MethodNew Austrian Tunneling Method(NATM)(NATM)

Ridho K. WattimenaRidho K. Wattimena

Laboratory of Laboratory of

GeomechanicsGeomechanics

InstitutInstitut TeknologiTeknologi BandungBandung

Historical Background of NATMHistorical Background of NATM(Support Systems)(Support Systems)

Use of shotcrete to stabilize squeezing ground intunnelling by Bruner.

1954

Introduction of dual-lining system by Rabcewicz.1948

Development Year

First application of shotcrete in coal mines, Denver.1914

Invention of revolver shotcrete machine by Akeley.1908-1911

Replacement of timber by steel support by Rziha.1872

First attempt to use fast-setting mortar by Wejwanow.1848

Invention of circular shield by Brunel.1811

7/27/2019 TA5211_New Austrian Tunneling Method

2/19

2

Historical Background of NATMHistorical Background of NATM (Cont.)(Cont.)

Development Year

First urban NATM Application in soft ground(Frankfurt am Main).

1969

English form of the term NATM first appeared inliterature produced by Rabcewicz.

1964

Rabcewicz introduced the New Austrian TunnellingMethod in a lecture to the XIII GeomechanicsColloquium in Salzburg.

1962

Recognition of the importance of a systematicmeasuring system by Mller.

1960

Development of ground anchoring by Rabcewicz.1955

Historical Background of NATMHistorical Background of NATM (Cont.)(Cont.)

Development Year

First NATM in Britain at Barrow upon Soar mine.1987

Redefinition of NATM due to conflict existing in theliterature by the Austrian National Committee onUnderground Construction of the International

Tunnelling Association (ITA).

1980

7/27/2019 TA5211_New Austrian Tunneling Method

3/19

3

Characteristics Features andCharacteristics Features andPhilosophy of NATMPhilosophy of NATM

What is NATM?

What are the essential features of NATM?

Is NATM a tunnelling technique or a philosophy ?

These issues gained interest of many scientists,practitioners and technical journalists to determine thetrue concepts of NATM.

Characteristics Features and PhilosophyCharacteristics Features and Philosophy (Cont.)(Cont.)

Prof. L.v. Rabcewicz, the principal inventor (1964):A new method consisting of a thin sprayed concrete

lining, closed at the earliest possible moment by an invert to a complete ring called an auxiliary arch

the deformation of which is measured as a function of time until equilibrium is obtained.

Three key points: The application of a thin-sprayed concrete lining.

Closure of the ring as soon as possible.

Systematic deformation measurement.

7/27/2019 TA5211_New Austrian Tunneling Method

4/19

4

Characteristics Features and PhilosophyCharacteristics Features and Philosophy (Cont.)(Cont.)

Austrian National Committee on UndergroundConstruction of the International Tunnelling

Association (ITA) in 1980:The New Austrian Tunnelling Method (NATM) is based on a concept whereby the ground (rock or soil)

surrounding an underground opening becomes a load bearing structural component through activation of a ring like body of supporting ground.

Characteristics Features and PhilosophyCharacteristics Features and Philosophy (Cont.)(Cont.)

Sauer (1988):A method of producing underground space by using all available means to develop the maximum self-

supporting capacity of the rock or soil itself to provide

the stability of the underground opening. Prof. Dr. Leopold Mller, one of the other

advocates of NATM (1978):The NATM is, rather, a tunnelling concept with a set of

principles Thus in the authors opinion it should not even be called a construction method, since this implies a method of a driving a tunnel.

7/27/2019 TA5211_New Austrian Tunneling Method

5/19

5

The important characteristic features of NATMThe important characteristic features of NATM

amongst the otheramongst the other twentytwenty --twotwo principlesprinciples(M(M ller,1990)ller,1990) ::

1. The surrounding rock mass is the main load bearingcomponent and its carrying capacity must bemaintained without disturbance of the rock mass.

2. The support resistance of the rock mass should bepreserved by using additional support elements.

3. The lining must be thin-walled and necessaryadditional strengthening should be provided by mesh

reinforcement, tunnel ribs and anchors rather thanthickening the lining.

The important characteristic featuresThe important characteristic features (Cont.)(Cont.)

4. The ring closure time is of crucial importance and thisshould be done as soon as possible.

5. Preliminary laboratory tests and deformationmeasurements in the tunnel should be carried out tooptimise the formation of the ground ring.

7/27/2019 TA5211_New Austrian Tunneling Method

6/19

6

Characteristics Features and PhilosophyCharacteristics Features and Philosophy (Cont.)(Cont.)

It is clearly agreed by the Austrian proponents thatNATM is an approach to tunnelling or philosophyrather than a set of excavation and support techniques.

Golser (1979), Brown (1990), Hagenhofer (1990),Barton (1994) are supporters of this idea amongstmany other scientists.

The important characteristic featuresThe important characteristic features (Cont.)(Cont.)

Rabcewicz & Pacher (1975)However, the principle of ring closure as quickly as

possible is only applicable to tunnels in rock with low primary stresses. In tunnels with large overburdens and

poor rock quality only a stress to the largest extent possible will achieve the object. Of course, this stress relief, which will continue for many months, must becontrolled most accurately by measurements.

7/27/2019 TA5211_New Austrian Tunneling Method

7/19

7

The important characteristic featuresThe important characteristic features (Cont.)(Cont.)

In summary, the following major principles, whichconstitute the NATM, can be derived from thefollowing references:

Tunnels & Tunnelling (1990). Will (1989). Brown (1990). Wallis (1995). ICE (1996). HSE (1996). Bowers (1997). Fowell & Bowers (1998).

Major Principles of NATMMajor Principles of NATM

1. The inherent strength of the soil or rock around thetunnel domain should be preserved and deliberatelymobilised to the maximum extent possible.

2. The mobilisation can be achieved by controlled

deformation of the ground. Excessive deformation which will result in loss of strength or high surfacesettlements must be avoided.

3. Initial and primary support systems consisting of systematic rock bolting or anchoring and thin semi-flexible sprayed concrete lining are used to achievethe particular purposes given in (2). Permanentsupport works are usually carried out at a later stage.

7/27/2019 TA5211_New Austrian Tunneling Method

8/19

8

Major PrinciplesMajor Principles (Cont.)(Cont.)

4. The closure of the ring should be adjusted with anappropriate timing that can vary dependent on thesoil or rock conditions.

5. Laboratory tests and monitoring of the deformation of supports and ground should be carried out.

6. Those who are involved in the execution, design andsupervising of NATM construction must understandand accept the NATM approach and react co-operatively on resolving any problems.

7. The length of the unsupported span should be left asshort as possible.

NATMNATMExcavation Method & Support SystemExcavation Method & Support System

Excavation Method (Rabcewicz, 1964): the sections were usually not driven full face but divided into subsequently opened out

Support Systems (Rabcewicz, 1973):

The first is a flexible outer arch-or protective support-design to stabilize the structure accordingly, and consists of a systematically anchored rock arch with

surface protection mostly by shotcrete, possibly reinforced by additional ribs and closed by the invert

The second is an inner arc consisting of concrete and is generally not carried out before the outer arch reached equilibrium

7/27/2019 TA5211_New Austrian Tunneling Method

9/19

9

Excavation Method & Support SystemExcavation Method & Support System (Cont.)(Cont.)

The ground response curve (Remember?) shows therock/support interaction and deformations in time. Itprovides a tool to idealise support stiffness and time of installation.

When a stiffer support is chosen, it will carry a largerload because the rock mass around the opening hasnot deformed enough to bring stresses intoequilibrium. Thus, the safety factor will sharply

decrease. If the support is installed after a certain displacement

has taken place, then the system reaches equilibrium with a lower load on the support.

Excavation Method & Support SystemExcavation Method & Support System (Cont.)(Cont.)

Rabcewicz (1973):It is a particular feature of NATM that the intersections always take place at the descending branch of thecurve.

7/27/2019 TA5211_New Austrian Tunneling Method

10/19

10

ShotcreteShotcrete & Secondary Lining Design& Secondary Lining Design (Cont.)(Cont.)

Additional support elements such as rock bolts,spiling, lattice girders, steel welded mesh or steelfibre reinforcement should be used to increase thestrength of the shotcrete. Shotcrete materials must beconsidered in the lining design to optimise time-dependent behaviour to answer the necessaryflexibility and load bearing capacity.

Monitoring of the stresses in/on the lining and thedeformation must be provided.

Design Criteria of NATMDesign Criteria of NATM

7/27/2019 TA5211_New Austrian Tunneling Method

11/19

11

Design CriteriaDesign Criteria (Cont.)(Cont.)

Rabcewicz (1965):A design of shotcrete should attain a high carrying capacity as quickly as possible, and it must be rigid and unyielding so that it seals off the surface closely and almost hermetically.

Vavrovsky (1995):The scope of the design is consequently not to

support itself but a package of measures including sealing, reinforcement and support of the rock mass during the redistribution process

Design CriteriaDesign Criteria (Cont.):(Cont.):ShotcreteShotcrete & Secondary Lining Design& Secondary Lining Design

Ground characteristics, such as strength and stand uptime must be determined. The ground supportinteraction curve obtained accordingly.

Ground water must be taken into consideration andrequired drainage or sealing should be maintained. If drainage is considered, the long-term stability of the

drainage holes must be preserved and the quantity of theseholes in respect to the water intake must be determined.

When sealing is considered, water pressure must be takeninto account in the design to calculate the loads on the lining.The long-term stability of the waterproof membrane shouldalso be considered.

7/27/2019 TA5211_New Austrian Tunneling Method

12/19

12

Design CriteriaDesign Criteria (Cont.):(Cont.):ShotcreteShotcrete & Secondary Lining Design& Secondary Lining Design

Preliminary design of the initial lining should beconducted using available means of analysis such asempirical methods based on stochastic and/orobservations, computational methods and small or full-scale physical models.

The secondary lining is usually a precast concretelining and they are placed after shotcrete has beenapplied. These concrete slabs are generally

connected to each other with joints, which may beplane, or helical joints, concave/convex joints,convex/convex joints, and tongue and groove joints(Craig & Muir Wood 1978).

Design CriteriaDesign Criteria (Cont.):(Cont.):Geotechnical CriteriaGeotechnical Criteria

Appropriate geotechnical design parameters must bechosen to fulfil analytical or computational preliminarydesign for eligible excavation patterns and geometry,and face advance in each round, as well as optimum

support design. It has been noted earlier that geomechanical

investigation of the ground, in which NATM will beused, is vital to the understanding of the modellingmethodology for rock/soil.

7/27/2019 TA5211_New Austrian Tunneling Method

13/19

13

Design CriteriaDesign Criteria (Cont.):(Cont.):Geotechnical CriteriaGeotechnical Criteria

Rock mechanics and the soil mechanics fall intoregions four and two, which are data-limitedproblems.

Design of NATM Applications inDesign of NATM Applications inSoft GroundSoft Ground

The main concern pointed (Mller, 1978) is that theshotcrete ring must be closed as early as possible inany soft ground application of NATM.

One of the reasons for rapid ring closure is to preventsurface buildings suffering damage from settlement.

Another reason is that the shorter stand-up time of softground is due to the bond between soil particles being

weaker and cohesion is also lower than for rocks. Inthe near surface soft ground case, the insitu stress willbe relatively low, the ground relatively weak andunable to support redistributed loads.

7/27/2019 TA5211_New Austrian Tunneling Method

14/19

14

NATM Applications in Soft GroundNATM Applications in Soft Ground (Cont.)(Cont.)

Brown (1990):In a near surface tunnel excavated in soft ground, it will be generally necessary to close the invert quickly to

form a load-bearing ring and to leave no section of theunexcavated tunnel surface unsupported eventemporarily

The length of the unsupported span must be leftshorter compared to tunnelling in rock.

The stability of the working-face must be maintained. The geometry and the size of the excavation section in

one round should be optimised accordingly.

Safety Measures and Design CriteriaSafety Measures and Design Criteriafor NATM Tunnelsfor NATM Tunnels

Ground investigation:This investigation must be carried out to reduce thelikelihood of encountering unexpected geologicalconditions.

Engineering technology:Technological improvement in tunnelling equipment.

A comparison between new and previous technology.

Universities, research groups can contribute to theevaluation and investigation of new and/or untriedmethods of working.

7/27/2019 TA5211_New Austrian Tunneling Method

15/19

15

Safety for NATM TunnelsSafety for NATM Tunnels (Cont.)(Cont.)

A risk-based approach to NATM design:In tunnel design and construction, there has alwaysbeen some degree of uncertainty.

Monitoring:

Monitoring should be undertaken to ensure safety of design and construction. Data assessment andinterpretation must be done by the geological/

geotechnical specialists, tunnel designers,construction managers (including quality and safetymanagers)

Safety for NATM TunnelsSafety for NATM Tunnels (Cont.)(Cont.)

Stability of the tunnel heading:Most failures occur during or soon after excavation of this part of the tunnel. Therefore, to secure the safetyof those who work within the tunnel and in buildings,

structures and utilities above the tunnel, stability of theface must be maintained using additional supportssuch as forepoles, faster excavation, draininggroundwater and reducing the face size or advanceper round.

7/27/2019 TA5211_New Austrian Tunneling Method

16/19

16

Safety for NATM TunnelsSafety for NATM Tunnels (Cont.)(Cont.)

Ground settlement control measures:Settlement due to tunnel excavation must becontrolled by proper construction of the tunnelheading, under-pinning existing structures, andcompensation grouting.

Sprayed concrete lining design:The physical properties of the shotcrete must be

designed according to the project requirements.Necessary computational design as well as small scaletrial works and past experiences should beconsidered.

NATM Process on SiteNATM Process on Site

7/27/2019 TA5211_New Austrian Tunneling Method

17/19

17

11

Cutting a length of tunnel, here with aCutting a length of tunnel, here with a roadheaderroadheader

22

Applying layer of Applying layer of shotcreteshotcrete on reinforcement meshon reinforcement mesh

7/27/2019 TA5211_New Austrian Tunneling Method

18/19

7/27/2019 TA5211_New Austrian Tunneling Method

19/19

55Completed tunnelCompleted tunnel

See you in the next lectureSee you in the next lecture