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?