Slurry MTBM in Alluvial Soils and 25k Rock Chris L. Windley, P.E. – McKim & Creed Inc. Stephen D. Leitch, P.E. – Hazen and Sawyer, P.C. Joshua P. Farmer, P.E. – Hazen and Sawyer, P.C.
• City of Raleigh System-Wide Capacity Study
– 10-year Storm Basis
– Generated Detailed CIP
• Crabtree Parallel Interceptor Required to:
– Correct Identified Hydraulic Restrictions
– Accommodate Future Flows up to 2030
– Minimize SSO’s
Project Background
• 16,000 LF of 72” and 4,000 LF of 60” Gravity Sewer
• 3 Crossings Requiring Trenchless Installation
• Consistent Deep Cuts (20’ - 30’)
• Highly Variable Soil Conditions
• Nearly all Excavations below Water Table
Phase I Interceptor Specifics
Regional Geology - Triassic/Mesazoic Basins
Project Location
Regional Geology - Triassic/Mesazoic Basins
Project Location
• Geologic Conditions formed 200 – 250 million years ago
• Eroded Sediment created Alluvial Fans
• Formed Sedimentary Rock Structure
• Seafloor Spread = Tectonic Shift
• Stress Cracks Form
• Magma intrudes
• Extremely Hard Igneous Rock Intrusions – dikes (vertical)P
– sills (horizontal)
– known as Diabase.
• Result in Extremely Variable & Difficult Tunneling Conditions.
Regional Geology - Triassic/Mesazoic Basins
Pangaea
Regional Geology - Triassic/Mesazoic Basins
• Corrosion Resistance Critical (High Peaking Factor)
• Large Pipe ID Required (60” & 72”)
• Combination = Reduced Carrier Pipe Material Options
• Creek Crossings – Deep Installation
– Proximity to Recharge Source
– Precise Line and Grade (0.008%)
• Alluvial - Loose, Granular Soil Materials Underlain by Fractured and Hard Rock
Phase I Sewer Interceptor Critical Factors
Phase I Sewer Interceptor Critical Factors
• Categories of Excavation Methods Considered – Hand Mining/Traditional
• Segmental Excavation • Shield Support • Pneumatic/Backacter • Drill & Blasting
– Open-Face Mechanized
– Closed Face Mechanized
Trenchless Excavation Methods Evaluated
• Fixed Point Tunnel Profile = Highly Variable Soil Conditions • Structures Potentially Impacted + • High Profile Corridor + • Flat, Tight Hydraulic Design = • Line & Grade Accuracy Required • High Groundwater + • Loose Granular Material =
– Closed Face Methods or – Compressed Air & Segmental Excavation
Trenchless Excavation Methods Evaluated
• Categories of Tunnel Support Methods Considered – Pipe Jacking
• Reinforced Fiberglass Jacking Pipe • Reinforced Concrete Jacking Pipe • Carbon Steel Jacking Pipe
– Assembled-in-Place Support • Steel Liner Plate • Ribs & Lagging • Shotcrete • Rock Anchors/Bolts & Wire Mesh
Trenchless Tunnel Support Methods Evaluated
• Single- Pass vs Two-Pass – Two-Pass
• Excavation Support • Carrier Pipe/System Conveyance
– Single-Pass • System Conveyance + Excavation Support
= One Integrated System
Trenchless Tunnel Support Methods Evaluated
• Single-Pass • Two-Pass − Pipe Jacking − Pipe Jacking
− Assembled-in-Place Systems
• What is Pipe Jacking? – Tunnel Lining: Jacking Pipe or Casing – Thrust/Advancement: Hydraulic Jacking Frame – One-pass or Two-pass is an option
• Types of Pipe Jacking – Jack and Bore (Horizontal Auger Boring) – Compressed Air Hand Tunneling1
– Slurry Microtunneling (MTBM) 1 Can also be installed similar to the TBM process using internal
jacks to advance off the built in place liner plate or ribs and lagging.
Tunneling with Pipe Jacking Methods
• Assembled-in-Place Tunnel Support – Many Types of Tunnel Lining – Thrust/Advancement: Hydraulic from Support or Rock – Two-pass only – Requires Consistent Man-entry
• Types of Excavation Systems using Assembled-in-Place Tunnel Support – TBMs
• Hard Rock - Open-Faced – Single Shield, Gripper, & Double Shield • Soft Earth - Closed-Face – EPBM & Slurry Shield
– Compressed Air Hand Tunneling1
Tunneling with Assembled-in-Place Methods
• Jack and Bore (Horizontal Auger Boring) – Limited Availability at Size Required – Inability to Control Fluidized Material – Line and Grade Control – Length of Drive (400’+ Crossing) – Two-Pass Only
• Single Shield TBM – Inability to Control Fluidized Material – Worker Safety (workers in the tunnel) – Two-Pass Only
• Slurry Shield TBM – Eliminated due to equipment availability
Trenchless Methods Eliminated
• Compressed Air Hand Tunneling vs
• Earth Pressure Balance Machine
vs
• Slurry Microtunneling
Final Trenchless Method Decision
• Advantages – Inexpensive Over Short Distances – Dewatering not Required – High Line and Grade Accuracy – Smaller Footprint – Can Handle Mixed Face Conditions – Capability to Easily Identify and Clear Larger Obstructions – Capability to Install One-Pass Tunnel Option
• Disadvantages – Worker Safety Risk – Not Widely Available/Reduced Contractor Availability – Not Cost Effective Over Long Distances – Uncontained Spoil Removal
Compressed Air Hand Mining
• Advantages – Dewatering Not Required – Potential for Subsidence and Surface Heave is Lowered – High Line and Grade Accuracy – Can Handle Mixed Face Conditions
• Disadvantages – Safety Concerns/Worker Entry Required – Expensive – Reduced Equipment Availability – Largest Operational Footprint – Spoil Removal Limits Production – One-Pass Option Not Available
Earth Pressure Balance Machine (EPBM)
• Advantages – Remote Controlled/ no Worker Entry Required – Dewatering not Required – Potential for Subsidence and Surface Heave Minimized – Reduced Skin Friction by Lubrication – Contained Spoil Removal – High Line and Grade Accuracy – Medium-sized Footprint/Flexible – Can Handle Mixed Face Conditions – Capability to Install One-Pass
• Disadvantages – Expensive – Can Have Issues with Large Cobbles and Boulders
Slurry Microtunneling
• Key Decision Criteria for Selection of Slurry Microtunneling – Remote Controlled/ No Worker Entry Required – Capability to Install One-Pass Tunnel Option – Medium-sized Footprint/Flexible – Contained Spoil Removal
Slurry Microtunneling Selected
• Horizontal Positioning and Vertical Limitations • Launch Shaft Location • Pipe Material Selection for Single-pass Tunneling • Design Calculations • Regulatory (NCDOT) Approval
Microtunnel Design
Horizontal Positioning and Vertical Limitations Horizontal Positioning • Maximize separation with existing utilities, structures & creek • Allow room for shafts/pits
Vertical Limitations • Maintain design grade in inconsistent material • Achieve vertical clearances with existing utilities and creeks
Microtunnel Design
Launch Shaft Location • Work with spatial constraints • Minimize location in hard rock
• Shaft in rock vs. extended tunnel • Accessibility
Microtunnel Design
Pipe Material Selection for Single-pass Tunneling • Consistency with overall pipe material for project • Water-tight joint system • Pipe stiffness • Corrosion resistant • Regulatory requirements
Microtunnel Design
Design Calculations • Jacking forces vs. allowable • Thrust blocking • Pipe deflection (Long-term) • Buckling
Microtunnel Design
Regulatory (NCDOT) Approval • FRP for Pipe Jacking • Microtunneling • Single-pass Installation • Launch and receiving shafts near embankments Solution: Comprehensive submittal packet and close coordination w/NCDOT
Microtunnel Design
Project Conditions that May Warrant Slurry MTBM
• Limited / No Ability to Effectively Dewater Tunnel Area in High Groundwater Areas
• Dewatering in Areas with High Conductivity and/or Cohesionless Soils Near Roadways Or Railroads – Subsidence or Heave is Critical
• Gravity Sewer with Critical Line and Grade and Minimal Allowance for Variation
• Tunnel Installations with Varying Subsurface Conditions – Mixed Face Conditions
Is Microtunneling the Best Option For You?
Slurry Microtunneling has proven to be a good choice for the City of Raleigh Crabtree Project
• NCDOT Approved Single-pass Tunnels
• All Three MTBM Installations are Complete • No Subsidence, Settlement or Heave Reported • Ability to Handle Unexpected Subsurface Conditions
Is Microtunneling the Best Option For You?
Questions?