Soft Ground Tunnel Construction and Settlement Analysis

Resources and Learning Objectives

• Primary Lecture Material: Week 11 Tunnel Construction Part 1: Soft Ground (CIVL 451 – Construction) by Dr. Yuekai Xie, School of Engineering.

• Core Resources:

  • Introduction to Tunnel Construction by Chapman, Metje, and Stark (E-book available via UOW library).

  • International Tunnelling Association (ITA/AITES) website.

• Key Journal Papers:

  • Mair, R.J., Taylor, R.N., and Bracegirdle, A. (1993). "Subsurface settlement profiles above tunnels in clay," Geotechnique, 43 (2), 315-320.

  • Mair, R.J., Taylor, R.N. (1997). "Bored tunnelling in the urban environment," Proc. 14th Int. Conf. on SMFE.

  • O’Reilly, M.P. and New, B.M. (1982). "Settlement above tunnels in UK – Their measurement and prediction," Tunnelling, 82, 173-181.

  • Atkinson, J.H. & Potts, D. M. (1977). "Stability of a shallow circular tunnel in cohesionless soil," Geotechnique 27, No. 2, 203-215.

• Key Learning Objectives:

  • Comprehend soft rock tunnelling, site investigations, and laboratory testing of soft soils/rocks.

  • Master tunnel terminology.

  • Differentiate between old and modern methods (Open face vs. Close face).

  • Understand equipment (TBMs) and lining installation.

  • Evaluate ground loss and surface settlement using specific conditions and formulas.

Design vs. Construction Philosophy

• Contrasting Nature: Design is often complex and non-straightforward, whereas construction seeks straightforward implementation.

• Assumptions and Realism: Designers must make simplifications but look beyond theoretical conditions to be realistic.

• The Iterative Process: Practicing tunnel construction involves trial and error; answers must be justified by previous experience.

• Attitude: Confidence and optimism are required; solutions always exist.

• Clarity: Construction drawings must be easy for contractors to read to minimize conflict and save time/money.

Site Investigation for Soft Ground Tunnelling

• Rule of Thumb for Exploration Location:

  • Vertical: Exploration should reach a depth of at least $3$ tunnel diameters.

  • Horizontal: Boreholes or tests should be spaced between $50\,m$ and $150\,m$ from the tunnel axis.

• Non-Destructive Methods:

  • Seismic Refraction: Based on waves refracting at material boundaries. Waves bend, travel along deep layers, and return. Used for estimating depths of bedrock or groundwater (Lower resolution).

  • Seismic Reflection: Based on waves bouncing back from boundaries with different properties. Used for mapping detailed subsurface features, faults, or voids (Higher resolution).

  • Electrical Resistivity Method: Uses current electrodes ($C1, C2$) and potential electrodes ($P1, P2$) to measure ground voltage.

  • Ground Penetrating Radar (GPR): Effective in depths of $0.5$ to $5\,m$. Performs poorly in wet, clay-rich, or saline soils.

• Destructive Methods (In-Situ):

  • Boreholes:

    • Cable Percussion Rig: Uses a dropped weight; slow but suitable for soft ground and shallow depth.

    • Rotary Drilling Rig: Rotating drill; fast and suitable for harder, deep ground.

    • Spacing: Typically $30\,m$ spacing for a $300\,m$ long tunnel. Location and ground level must be mapped accurately.

  • Standard Penetration Test (SPT):

    • A standard sampling tool with a weight of $63.5\,kg$ is dropped from a fixed height of $760\,mm$.

    • Blows are recorded for every $150\,mm$ (totaling $450\,mm$).

    • The value $N_{SPT}$ is the sum of the second and third $150\,mm$ increments.

  • Cone Penetration Test (CPT):

    • A penetrometer tip is pushed at a constant rate of $20\,mm/s$.

    • Records cone tip resistance ($q_c$) and sleeve friction ($f_s$).

• Laboratory Tests:

  • Material characteristics include Liquid Limit ($LL$), Plastic Limit ($PL$), Plasticity Index ($PI$), Particle Size Distribution ($PSD$), and unit weight ($\gamma$).

  • Permeability and strength tests (Triaxial, UCS).

Laboratory Testing and Material Strength

• Unconfined Compressive Strength (UCS):

  • Measures strength where confining stress $\sigma_3 = 0$.

  • Failure modes include shearing, brittle failure, longitudinal cracks, or barreling.

  • Young's modulus ($E$) is determined from the middle third of the unload/reload part of the stress-strain curve.

  • $E = \frac{\Delta\sigma}{\Delta\epsilon} = \tan(\alpha)$

• Triaxial Test:

  • Analyzes failure under confining pressures ($\sigma_3$) using Mohr-Coulomb failure criteria.

  • $\tau = c + \sigma \tan(\phi)$

• Consolidated Undrained (CU) Triaxial Tests:

  • Skempton's B-parameter: $B = \frac{\Delta u}{\Delta \sigma}$

  • Major principal stress at failure (total): $\sigma_1 = \sigma_3 + (\Delta\sigma_d)_f$

  • Major principal stress at failure (effective): $\sigma'_1 = \sigma_1 - \Delta u_f$

  • Poisson Ratio (Lateral strain / Axial strain): $\nu = \frac{\epsilon_{lateral}}{\epsilon_{axial}}$

• Unerosion/Saturated Conditions: For fully saturated cohesive soil, the total stress failure envelope angle is $\phi = 0$.

Case Study: Pudong Airport

• The Problem: Observed settlement after applying surcharge was more than twice the initial prediction.

• Data: 15 years post-construction, accumulated settlement reached approximately $900\,mm$.

• Geology: A $30\,m$ clay layer exists underneath a silt layer, likely an ancient creek. The clay layers are nearly saturated.

• Conclusion: Major settlement resulted from the consolidation of the clay. Aircraft movement (take-off and landing) contributed only minor settlement compared to the long-term consolidation process.

Tunnel Terminology

• Crown: The topmost part of the tunnel cross-section.

• Shoulder: The upper side region between the crown and springline.

• Springline: The horizontal axis where the curve transitions into the wall.

• Knee: The area where the tunnel wall bends toward the floor.

• Invert: The bottommost part of the tunnel cross-section.

• Intrados: The inner surface of the tunnel lining.

• Extrados: The outer surface of the tunnel lining.

• Lining: The structural shell protecting the tunnel interior.

• Top Heading: The upper portion of the tunnel excavated first in staged methods.

• Face of Top Heading: The front surface of the excavated top heading.

• Bench: The lower portion excavated after the top heading.

• Face of Bench: The front surface of the excavated bench.

Historical and Old Methods of Tunnelling

• Sequential Excavation: The face is excavated in small sections or "headings" rather than full-bore.

• Forepoling: Driving timber poles into the ground ahead of excavation to prevent collapse in soft ground.

• Masonry Supports: Thick rings of brick or stone masonry installed post-excavation.

• Brunel’s Shield: One of the earliest shield forms developed for sequential support.

• Specific Sequential Variations:

  • German Method: Provides roof support and face stability while other parts are excavated. The unexcavated central portion acts as temporary support; suitable for firm ground.

  • Italian Method: Difficult to execute due to unusual shapes. The thin central core is prone to buckling under high loads.

  • Austrian/English Method: Uses fewer sections, making it fast and efficient. Works well in cohesive soils (clays) that can stand briefly unsupported; simpler geometry.

Modern Tunnel Construction: Open vs. Closed Face

• Open Face Tunnelling:

  • The tunnel face is directly accessible and visible.

  • Allows non-circular sections and divided faces.

  • Offers flexibility in construction sequence based on observations.

  • Easier to apply ground treatment (grouting) within the tunnel.

• Closed Face Tunnelling:

  • Seals the front of the excavation area.

  • Required for unstable ground conditions requiring constant support.

  • Uses pressurized fluid (air, slurry, or excavated soil) to support the face.

  • Lining segments are usually installed as the machine progresses.

Tunnel Boring Machines (TBMs)

• Slurry TBM (Slurry Shield):

  • Cutterhead: Rotates to excavate soil.

  • Plenum (Chamber): Holds slurry under pressure against the face.

  • Bulkhead: Isolates the pressurized face from the working chamber.

  • Slurry Feed/Return Lines: Pumps fresh slurry in and removes spoil-mixed slurry to a separation plant.

  • Agitators: Keep the excavated material mixed.

  • Tail Seal: Prevents slurry/water backflow into the machine.

• Earth Pressure Balance (EPB) TBM:

  • Screw Conveyor: Key component; regulates pressure by controlling the rate of soil removal (extraction).

  • Conditioners: Added to spoil to achieve the correct consistency.

  • Suitability: Less suitable for soils with low fines content (prefers clay/silt).

  • Belt Conveyor: Transfers excavated material to the rear for disposal.

• Forces on the Shield:

  • Thrust Force ($P$) is calculated based on tunnel diameter ($D$) and a constant ($k$).

  • $P = kD^2$

  • Requirement: kD^2 > W_1 + W_2 + W_3

  • $W_1$ = Shear stress between skin and ground.

  • $W_2$ = Shear stress between skin and linings.

  • $W_3$ = Passive earth resistance at the cutting edge.

  • $k = 20\,MPa$ for soft ground; $k = 70\,MPa$ for hard ground.

  • Mobility rule: Optimal shield length $L$ is roughly $2D$.

Installation of Concrete Lining

• Reinforced concrete segments are delivered via shafts.

• Segments are assembled inside the tail of the TBM shield.

• Erector System: Mechanical arm that places segments to form a ring.

• Advance: Hydraulic jacks push against the newly installed lining ring to propel the TBM forward.

• Waterproofing: Segment hinges must be waterproofed or a final in-situ concrete shell must be provided.

Ground Loss and Surface Settlement

• Total Displacement ($S_C$): Sum of various loss components.

  • $S_C = S_f + S_h + S_s + S_t + S_l$

  • Face effect ($S_f$): Soil moving inward just ahead/around the crown during cutting.

  • Heading effect ($S_h$): Soil moving inward at the crown/heading during advance.

  • Over-cut shield loss ($S_s$): Empty space because the shield is slightly larger than the final lining.

  • Tail loss ($S_t$): Void between the soil and the newly installed lining ring.

  • Lining Deformation ($S_l$): Compression of the lining caused by external earth pressure.

• Volume Relationships:

  • $V_S$ = Volume of surface settlement.

  • $V_L$ = Volume loss at tunnel level.

  • Deep tunnels: $V_S \approx 0$.

  • Clay/Sandy soils: V_L > V_S (compression and plastic deformation).

  • Undrained saturated clay/dense sands: $V_L = V_S$.

  • Loose sands: V_L < V_S (loose sand contracts during shear).

Prediction Formulas for Settlement

• Overload Factor ($O_F$): Used to predict yield.

  • $O_F = \frac{P_z - P_i}{C_u}$

  • $P_z = \gamma z$ (overburden pressure).

  • $P_i$ = internal pressure from linings.

  • $C_u$ = undrained shear strength.

  • O_F > 1 indicates yielding; O_F < 1 is elastic.

• Peck and Schmidt Method (Gaussian Curve):

  • $S(y) = S_{max} \exp\left( -\frac{y^2}{2i^2} \right)$

  • $i$ = horizontal distance from center to the point of inflection.

  • $S_{max}$ = Maximum vertical settlement above centerline.

  • $y$ = distance from centerline.

• Estimating Trough Width ($i$):

  • $i = \alpha z_0$

  • $\alpha = 0.5$ for clays.

  • $\alpha = 0.25$ for sand and gravel ($6-10\,m$ depth).

  • Atkinson and Potts (Sandy): $i = 0.25(z_0 + a)$.

  • Atkinson and Potts (Clayey): $i = 0.25(1.5z_0 + 0.5a)$.

  • Clough and Schmidt: $i = R \left( \frac{z_0}{2R} \right)^{0.8}$.

Longitudinal Volume Loss and Twin Tunnels

• Longitudinal Settlement: Surface displacement is approximately $0.5 \times$ crown settlement.

• Interaction of Twin Tunnels:

  • Interaction occurs if the distance between tunnels d < 4a ($a$ is radius).

  • The second tunnel often experiences higher ground loss because the ground is already "softer" (previously loaded).

  • Calculation Assumption: Use an effective radius for the second tunnel where a_{2ef} > a_2.

  • Final settlement profile: $S_F = S_1 + S_2$.