Tunnel Support Design

Eric Wang, PE

University of Denver
Tunneling Short Course
September 11, 2018
 Agenda

1.0 Introduction
2.0 Tunnel Support Design Principles
3.0 Input for Design
4.0 Tunnel Support Systems
5.0 Hard Rock Tunnel Support Design Example
6.0 Soft Ground Tunnel Support Design
7.0 Summary

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 1.0 Introduction

Design Flowchart sample:

(THE Tunnel project)

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 2.0 Tunnel Support Design Principles

Rock Support Interaction:

• Optimize support installation - acceptable displacement.

Ground vs Support Reaction Curves Ground Reaction Curves based on Overburden Depth

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 3.0 Geologic Input for Rock Tunnel Design

• Rock Mass Classifications:

– Rock Mass Rating (RMR) System (Bieniawski, 1989)
– Modified RMR (Laubscher and Page, 1990)
– NGI’s (Q) System (Barton et al., 1974, 2015)
• Rock Mass Discontinuity Orientation and Properties
• Rock Hardness, Strength and Abrasiveness for Excavation

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 Initial Ground Support

• Rock Bolts / Dowels • Shotcrete

– Type – Thickness
– Length – Type, Dry vs Wet
– Pattern • Lattice Girders
– Anchorage

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Rock Mass Classification – Q system

After NGI 2015

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RMR Classification

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 Tunnel Support Design

Support Estimate:
• Empirical Approach
• Analytical Approach
– Kinematic (Block stability)
– Rock Reinforcement (Bischoff and Smart)
• Numerical Modeling

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 Empirical Methods

• Terzaghi’s Rock Load (conceptual)

– More conservative
– Modified by Deere et al., 1970

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Empirical Methods

Graphical – Ubiquitous Joint Method (No. 7 Subway Line Extension) 11

 Kinematic Approach

Jointed Rock Mass

• DIPS and UNWEDGE software (Rocscience)

Stereonet of structural Cavern cross-section – wedge

discontinuity planes formation and initial support pattern
(No. 7 Subway Line Extension) (No. 7 Subway Line Extension)

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Station Cross section (THE Partnership)

Station Longitudinal section (THE Partnership)

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Excavation Sequence:
Stage 1: Excavation of four TBM tunnels
(TBM1, 2, 3, and 4) through the station
cavern

Installing initial ground supports behind TBM1 TBM2

TBM shield as required.

TBM3 TBM4

Station Cavern Excavation (THE Partnership)

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Excavation Sequence:
Stage 2: Excavation of Top-heading ( Side-drift 1,
Side-drift 2, and Center-drift 3)
Stage 3: Excavation of Benches and Inverts (4, 5,
6, 7, 8 and 9)

Installing initial ground support after excavation of TBM1 TBM2

each blast round including rock bolting and
shotcreting.

TBM3 TBM4

Full excavation sequence

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

• Partially release of stress at the face of excavation at each

excavation round before installing initial support.
• Evaluate stress release considering forces on ground and lining:
– Size of Excavation Face
– Ground Stiffness (Elastic Modulus, Poisson’s ratio)
– Initial Support Stiffness
– Length of Unsupported Excavation Round

(after Hoek)
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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Stress release for each excavation step before installing initial

support in the model:

TBM1 = 10%, TBM2 = 10%, TBM3 = 10%, TBM4 = 10%

Side-Drift1 = 30%, Side-Drift2 = 30%

Center-Drift3 = 40%

Bench4 = 50%, Bench5 = 50%

Bench6 = 50%, Bench7 = 50%

Invert8 = 50%, Invert9 = 50%

Final Lining = 100% relaxation

At the final stage: Eliminating the initial supports

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

• Simulate excavation sequence

– Consider ground strain due to excavation at each
Stage (Estimate stress release at each stage)

Eground = Stress / Strain

Installing initial support for each stage after simulating

the strains in the numerical model

(THE Partnership - ILF)

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Top heading / bench / invert excavation at Final Stage

(prior to removing the initial support)

General Station Cavern Excavation combining TBM and SEM Enlargement

Top heading / Bench / Invert
(THE Partnership - ILF)

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 Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

Total displacement contours with deformation Contours of yielded elements

vectors and deformed boundaries

(THE Partnership - ILF)

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Numerical Modeling Example – THE Tunnel 34th Street Station Cavern

After Elimination of Initial Support

Contours of yielded elements

Axial Forces Bending Moments

(THE Partnership - ILF)
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 Ground Support

• Ground support classes

• Pre-support
• Ground Treatment / Ground improvement

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 Ground Support Classes

Contract - Typically 2 to 3 initial support classes

• Hard rock TBM example:

Support Class I: Pattern rock dowels Support Class II: Additional mine-straps Support Class III: Steel rib support
and/or shotcrete support
TUNNEL CENTER TUNNEL CENTER TUNNEL CENTER

ROCKBOLT ROCKBOLT

W6X25 STEEL RIB

6'- 4'-0
00
" 0"

160° TUNNEL AXIS

120°

6' TOE BOLT

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 Tunnel Support Systems

• Hard Rock – Available Tunnel Support Systems

• Rock bolts
• Rock Anchors
• Rock Dowels
• Mine straps
• Shotcrete – plain and reinforced (either steel fibers or WWF)
• Soft Ground – Available Tunnel Support Systems
• Soil Nails
• Rebar / Pipe Spiling
• Lattice Girders
• Shotcrete
• Cast-In-Place

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 Tunnel Support Systems

• Open (Gripper) TBM – Available Tunnel Support Systems

• Rock bolts / Rock dowels
• Steel Ribs with channel lagging or minestraps
• Shotcrete (limited to extremely poor ground)
• CIP Lining

• Double shield EPB / Slurry Face – Available Tunnel Support Systems

• Segmental lining
• Annular and Secondary Grouting

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 Ground Support through Shear Zone

Steel mat lagging

WWF and minestraps

Spiling and shotcrete

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 Pre-support Systems

Spiling (Fore-poling) Pre-grouting ahead of face Double-roof pipe canopy arch

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Canopy spiling at Portal

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 Ground Treatment
Ground freezing
Pre-Grouting

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 Groundwater Control

Tunneling – infiltration control and waterproofing systems

Pre-excavation grouting of open fracture (South River Tunnel, Atlanta, GA., 2011)

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 Tunnel Grouting

CEMENTITIOUS vs. POLYURETHANE GROUTS

CEMENTITIOUS:
• Dry, open joints
• Long-term strength
POLYURETHANE:
• Wet conditions or relatively narrow fissures
• Time dependent properties
• Flow behavior (unreacted & reacting)

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 Tunnel Grouting

Pre-excavation Combined Cement + Water-reactive

Polyurethane Grout:
Steps
1. Polyurethane (TACSS – single component pre-
polymerized polyurethane) reaches permeable rock
mass – forms barrier upon reacting with water
2. Subsequent Cement grout able to begin filling crack
and curing with dilution from leaky crack.

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 5.0 Example - Pillar Stability Evaluation

• Phase 2D -Pre-support considered

• Staged Excavation
Results
• No significant yielding & deformations at 60% gripper pressure
• Localized spalling of Starter Tunnel shotcrete lining at 30%
gripper
• No impact on Global Stability

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Pillar Stability Evaluation

Proposed
T302

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Pillar Stabilization Measures

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Starter Tunnel Rock Reinforcement

Brow and gripper wall support

Cradle for TBM launch

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Starter Tunnel Rock Reinforcement

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Starter Tunnel Rock Reinforcement

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 Shotcrete Support Design

In Blocky ground
• Prevent rock mass raveling and
loosening between bolts
• Typical failure modes:
– Adhesive
– Direct Shear
– Flexural
– Punching Shear

Applied Load Model -Shotcrete Support

(after Barrett and McCreath, 1993)

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 Shotcrete Support Design

Failure modes (blocky ground):

• Adhesive failure

Failure mode (after Barrett and McCreath, 1993)

Adhesive failure model
(after Barrett and McCreath, 1993)
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 Shotcrete Support Design

Failure modes (blocky ground):

• Direct Shear Failure

Failure mode (after Barrett and McCreath, 1993)

Direct Shear failure model
(after Barrett and McCreath, 1993)

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 Shotcrete Support Design

Failure modes (blocky ground):

• Flexural failure

Failure mode (after Barrett and McCreath, 1993)

Flexural failure model
(after Barrett and McCreath, 1993)
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 Shotcrete Support Design

Failure modes (blocky ground):

• Punching Shear failure

Failure mode (after Barrett and McCreath, 1993)

Punching Shear failure model
(after Barrett and McCreath, 1993)

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 Shotcrete Support Design

Good bond between rock and

shotcrete
• Direct Shear Failure unlikely
• Adhesion Failure > 4m spacing
Type equation here.

Adhesive failure model

(after Barrett and McCreath, 1993) 44
 Shotcrete Support Design

Poor bond between rock and shotcrete

• Direct Shear Failure unlikely
• Adhesion Failure > 2.25 m spacing

(after Barrett and McCreath, 1993) 45

 Soft-ground Tunneling

• Continuous support of face and periphery (invert)

• Timely groundwater control

Vacuum Drain

Horizontal vacuum system (after Taiwan HS Rail, 2001-2003)

Flowing Ground 46
 Soft-ground Tunneling

• 3D tunnel convergence monitoring – vertical and transverse

• Reference THSR:
– 40-ft diam SEM tunnel
Vertical

Backfilled Invert
Lateral

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 Soft-ground Tunneling

• Continuous tunnel convergence monitoring - longitudinal

Longitudinal

Reference THSR: 40-ft diam SEM tunnel

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Face bolting and Ring Cut in Sandy Ground

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 Face bolting and Ring Cut in Sandy Ground

1. Face Bolting 2. Partial Excavation

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 Face bolting and Ring Cut in Sandy Ground

Running Ground
• Periodic sealing (accelerator)
• Contact Grouting of Arch

3. Temp. Sealing

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 Protection of Adjacent Structures

• Identify reinforcement/ underpinning needs

• Instrumentation & Monitoring Program
• Control Ground movement /Subsidence
• Rigid Water-tight Excavation Support under
construction as well as permanent conditions

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 Building Impact Assessment - Procedure

1. Initial screening
2. Develop settlement profile
3. Evaluate settlement profile per project criteria (max surface
settlement, impact to structures, etc.)
4. Identify sensitive structures within influence zone - more
comprehensive analysis
5. Mitigation measures – construction restrictions

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 Settlement Contour Map at East Portal
Protection of Existing Structures – Potential TransPak
Impacts
1. TransPak (Shipping depot - single-story
warehouse bldg.) – settlement shallow (35-ft)
cover
2. Bayshore Freeway (Route 101) – adjacent
highway slight embankment shallow (30-ft)
cover
3. Lower Silver Creek – shallow cover
settlement cracking of liner leaking shallow
(30-ft) cover

0.8 0.8
0.8 0.8
1.1 1.1

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 Settlement Contour Map at West Portal

DEKA BATTERIES ALL WORLD FURNITURE

Protection of Existing Structure
1. I-880 Nimitz Freeway Overpass, Shallow cover (+/-
40 FT) embankment footing and adjacent pile
foundation.
2. Shallow cover (+/- 40 FT) overlying single-story
building i.e., All-World Furniture and Deka
Batteries settlement potential.

0.9 1.4

0.9 1.4

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Building Impact Evaluation

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 Potential Mitigation Options
Further evaluation of specific existing structures based upon ground conditions and as-built
foundation data, potential mitigation options could feature:

• Structural Underpinning,
• Grouted Canopy spiling and lattice girder
• Rigid Support of Excavation Systems (Secant pile walls, etc.)
• Ground treatment

“A critical issue in design of mitigation measures is avoiding the creation of hard points in
the building that can focus and amplify building response or damage.” (Boscardin and
Walker, 1998)

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 Tunnel Support Design Summary

Summary
• Project-specific ground characterization
• Compatible excavation methodology
– Hard rock tunnel: rock mass discontinuities, strength and abrasion
– Soft-ground tunnel: continuous face and periphery support/ GW control /
monitoring
• Contingency – Risk Mitigation measures: pre-support / ground
treatment/ temporary invert support
• Optimize support design evaluate results from several analytical
approaches

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 Questions?

Mile-high thank you for your attention!

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