Impacts of Tunnelling on Ground and Groundwater and Control Measures Part 1: Estimation Methods

Transcription

1 Impacts of Tunnelling on Ground and Groundwater and Control Measures Part 1: Estimation Methods Steve Macklin Principal Engineering Geologist GHD Melbourne

2 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in coarse & fine grained soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

3 1. Introduction, scope of lecture 2. Terminology and concepts 3. Immediate settlement in soils the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures

4 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.

5 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

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7 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

8 VOLUME LOSS (V s ) V s = additional exc. Vol. = A exc A o (m 3 /m, or m²) or % 100% V o = theoretical exc. Vol. Also C/D, P/D ratios and internal support pressure (σ T )

9 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

10 THE SETTLEMENT TROUGH exp i= trough width point of inflection between hogging and sagging parts G function required for settlements in between (Attewell & Woodman (1982)

11 x/i NORMALISED FORM OF THE TRANSVERSE AND LONGITUDINAL SETTLEMENT PROFILES settlement/maximum settlement settlemet/maximum settlement y/i

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

13 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

14 DEWATERING SETTLEMENT IN PERMEABLE SOILS

15 CONSOLIDATION SETTLEMENT IN FINE GRAINED SOILS Coode Island Silt case study.

16 Coode Island Silt case study.

17 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

18 NON GAUSSIAN Simple elastic analysis Numerical Finite or Distinct element modelling..

19 1. Introduction, scope of Part 1 2. Terminology and concepts 3. Tunnelling settlement the Gaussian model 4. De pressurisation effects in soils 5. Settlement in rock 6. Staged assessment of settlement effects on structures 7. Concluding remarks

20 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

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

22 2 ND STAGE ASSESSMENT Model of a building as a beam undergoing bending and shear deformation after Burland et al (1978) bmax = L L 12t + 2y 3I LHG E dmax = L 1+ HL2 18I G E

23 aspect ratio and bending and diagonal strain effects.

24 relative stiffness effects, ρ Potts and Addenbrooke (1997).

25 Utilities (and tunnels) 2 2 Δ R, trans = S max /(Z o Z pipe ) and apply reduction factors for SSI

26 Don t forget differential settlement effects on services

27 DETAILED EVALUATION 2D FE mesh of the WRB facade

28 Tunnel

29 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 Bolted segment lined tunnel

30 I&M AND BACK ANALYSIS e.g. JLE St James s Park data (Nyren et al, 1996)

31 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.