tunnelling on ground groundwater and control measures 1

THE ATS TUNNEL DESIGN & CONSTRUCTION SHORT COURSE 2012

Impacts of Tunnelling on Ground and Groundwater and Control Measures – Part 1: Estimation MethodsSteve MacklinPrincipal Engineering GeologistGHD Melbourne


1. Introduction, scope of Part 12. Terminology and concepts3. Tunnelling settlement – the Gaussian model4. “De‐pressurisation effects” in coarse & fine grained

soils5. “Settlement” in rock6. Staged assessment of settlement effects on structures7. Concluding remarks


1. Introduction, scope of lecture2. Terminology and concepts3. Immediate settlement in soils – the Gaussian model4. “De‐pressurisation effects” in soils5. “Settlement” in rock6. Staged assessment of settlement effects on structures


Scope of today’s presentation :

• principles of the “Gaussian curve” empirical method• Tunnel excavations at depths typical for civil engineering (e.g. for sewers, roads, railways etc…)

• Tunnelling in both soil and rock, emphasis on soil• time dependent consolidation and “de‐pressurisation” effects• effects of settlement on structures on/within the ground • more rigorous analytical methods, 2D and 3D numerical modelling, centrifuge testing and “1G” scaled modelling are not discussed

• “Control measures” are discussed in the Part 2 companion paper.

References to be found in the handouts.


Tunnelling movements:

• Face take; • Radial take into annular gap; • distortion of the tunnel lining; • Alignment variation during

drive (radius, pitch, yaw); • changes in groundwater

pressures and time dependent consolidation in fine soils;

• re‐compaction in coarse grained soils.

• Transient outward movements or “heave”

• Rapidly changing ground conditions


1. Introduction, scope of Part 12. Terminology and concepts3. Tunnelling settlement – the Gaussian model4. “De‐pressurisation effects” in soils5. “Settlement” in rock6. Staged assessment of settlement effects on structures7. Concluding remarks


VOLUME LOSS (Vs)

Vs = additional exc. Vol.

= Aexc – Ao (m3/m, or m²)

or

%∗ 100%

Vo = theoretical exc. Vol.

Also C/D, P/D ratios and internal support pressure (σT)


THE SETTLEMENT TROUGH

exp 2

∗

2.507

i = trough width point of inflection between hogging and sagging parts

G‐function required for settlements in between (Attewell & Woodman (1982)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-3 -2 -1 0 1 2 3y/i

settl

emet

/max

imum

set

tlem

ent

NORMALISED FORM OF THE TRANSVERSE AND LONGITUDINAL SETTLEMENT PROFILES

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-3 -2 -1 0 1 2 3x/i

settl

emen

t/max

imum

set

tlem

ent


HANDY EQUATIONS….

Where, i = 0.5*Z (clay soils); 0.3*Z (granular soils) typically…...


DEWATERING SETTLEMENT IN PERMEABLE SOILS

∆ ∗ ′

∆ ∗ ′


CONSOLIDATION SETTLEMENT IN FINE GRAINED SOILS

Coode Island Silt case study….


Coode Island Silt case study….


4 1 1 4 1 2

4 1 2 1 2 2

NON GAUSSIAN

Simple elastic analysis…..

Numerical Finite or Distinct element modelling……..


Preliminary assessment

Second stage assessment

Detailed evaluation

• Contours of settlement• <10mm, 1:500 slope• e.g. Rankin (1988)

• Sum of tensile ground and structure strains

• Deflection ratios/angular distortions

• Relative stiffness effects

• “Moderate” risk or greater• structure details, movement history and condition in detail

• 2D and 3D, numerical analysis of SSI if appropriate


Damage Risk classification after Burland et al (1977), Rankin (1988) and Boscardinand Cording (1989).


2ND STAGE ASSESSMENT

Model of a building as a beam undergoing bending and shear deformation after Burland et al (1978)

dmax = ∆L

1+HL2

18IGE

bmax = ∆L

L12t+

3I2y LH

EG


aspect ratio and bending and diagonal strain effects….


Potts and Addenbrooke(1997).

,ρ ∗

relative stiffness effects


22 ∗ Δ

, trans = 0.446 Smax/(Zo – Zpipe)and apply reduction factors for SSI

R’

Utilities (and tunnels)


Don’t forget differential settlement effects on services


2D FE mesh of the WRB facade

DETAILED EVALUATION


Tunnel


Bolted segment lined tunnel

Detailed model of lining skin and flanges:

• 4 elements through the thickness

• 6 elements across segment width

Radial joints modelled explicitly:

•Bolt Shear Capacity•Bolt Play in Shear•Bolt Tensile Capacity •Bolt Play in Tension


… e.g. JLE St James’s Park data (Nyren et al, 1996)

I&M AND BACK‐ANALYSIS


KEY POINTS

1. Tunnelling method, heading geometry (C/D, P/D), and stress ratio (N, LF) are important considerations when using the empirical Gaussian method.

2. Assumption of radial movements towards the tunnel axis are OK and generally conservative for near surface settlement assessment in uniform soil. They fall down when looking close to the tunnel however.

3. “de‐pressurisation” (effective stress) settlement can be important when tunnelling in or near compressible fine grained soils.

4. Long term “consolidation settlement” can be important, especially in soft fine grained soils, even if water pressures balanced during tunnelling.

5. A phased approach to risk assessment is typically undertaken with a preliminary assessment based on settlement and slope – often all that can be done with limited data.

6. Second stage assessments may be undertaken based on an understanding of the structural form (e.g. Burland’s /L and h % method); simple modification factors can be applied for SSI effects.

7. Detailed evaluation of critical structures should take into account relative stiffness effects, 3‐D effects and self weight of the structure. FE/FD modelling usually required.

8. Simple back‐analysis of inexpensive I&M data is recommended to validate your design assumptions and improve the case history database.

tunnelling on ground groundwater and control measures 1

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