Session 3 Mohr-Coulomb Soil Model & Design (Part 2)

Transcription

1 Mohr Coulomb Model Session 3 Mohr-Coulomb Soil Model & Design (Part 2) Time Session Topic 09:00 10:30 1 Overview 10:30 11:00 Coffee Break 11:00 12:30 2 Design (Part 1) 12:30-01:30 Lunch 01:30 03:00 3 Mohr-Coulomb Soil Model & Design (Part 2) 03:00 03:30 Coffee Break 03:30 05:00 4 How to reduce wall deflection Mohr Coulomb Model 1 Things you should know about the Mohr Coulomb Soil Model Plastic Elasticplastic Elastic ε ε Mohr Coulomb Model 2 Wong Kai Sin 1

2 Mohr Coulomb Model Can Mohr Coulomb Model simulate Real Soil Behaviour? UU Test on Clay Plastic c u > 0 φ u = 0 Elasticplastic Elastic ε ε CD Test on Clay or Sand Plastic c' 0 φ' > 0 ε Elastic ε Real Soil Mohr Coulomb Soil Mohr Coulomb Model 3 Can a Elastic Model simulate Real Soil Behaviour? Elastic Model Shearstress produces Normalstress produces shear strain: volumetric strain: τ γ ε v τ no ε v Mohr Coulomb Model 4 Wong Kai Sin 2

3 Mohr Coulomb Model Can a elastic soil simulate undrained behaviour of clay? τ Plastic Elastic ε ε Real Soil Behaviour τ γ τ no ε v no ε v (undrained) Stress independent Elastic Model (ν=0.5) τ γ τ no ε v no ε v (undrained) Stress independent Mohr Coulomb Model 5 Can a elastic soil simulate undrained behaviour of clay? τ Plastic Elasticplastic Elasticplastic Elastic ε ε Yes! If we use c u and E u. Can we use c' φ' and E'? Mohr Coulomb Model 6 Wong Kai Sin 3

4 Mohr Coulomb Model CU Test 100 ESP TSP p or p (kpa) Mohr Coulomb Model 7 CU Test Consolidated Undrained Triaxial Compression Test = 100 kpa Real Soil Mohr Coulomb 1 3 ε curve q K f q 2c u K f c u from c' φ' 2c u ESP TSP ESP TSP c u measured p or p p or p c' φ' over predicted c u!!! Mohr Coulomb Model 8 ε 1 Wong Kai Sin 4

5 Mohr Coulomb Model Is the pore pressure response correct? Let s look at CU test on a normally consolidated clay. Real Soil Elastic Soil q K f q Kf U f U f ESP TSP ESP TSP p or p p or p The predicted pore pressure is much smaller than the measured! Mohr Coulomb Model 9 Method A Effective stress Mohr Coulomb Method using c and φ It over estimates the undrained shear strength and under estimates the excess pore pressure of a normally consolidated clay. Real Soil Elastic Soil q K f q 2c u Kf 2c u U f U f ESP TSP ESP TSP p or p p or p Mohr Coulomb Model 10 Wong Kai Sin 5

6 Mohr Coulomb Model Over estimation of c u at a Reclaimed Site Depth (m) Undrained Shear Strength (kpa) Method A (qt-po)/nkt 0.22*p'o corr. FVT Consol tests Cu based on phi=22 & p'o Mohr Coulomb Model 11 Nicoll Highway Results of Undrained Analysis using Method A Computed using Method A Measured Reduced Level (m) Reduced Level (m) Level mm Formation = 118 mm Final = 145 mm Wall Deflection (mm) Mohr Coulomb Model 12 Wong Kai Sin 6

7 Mohr Coulomb Model Does Method Aalways over estimate c u for NC clay? ( 1 3 ) f τ φ' 1 3 This site has a constant c u. ε c u A B C φ u =0 ' For NC Clay, it under estimates c u at low stress and over estimates it at high stress. Mohr Coulomb Model 13 Method B Effective stress Mohr Coulomb Method using c u and φ u =0 It forces the soil to fail at a specified undrained shear strength. Real Soil Elastic Soil q K f q 2c u 2c u K f ESP TSP ESP TSP p or p p or p Mohr Coulomb Model 14 Wong Kai Sin 7

8 Mohr Coulomb Model Nicoll Highway Results of Undrained Analysis using Method B Computed using Method B Measured Mohr Coulomb Model 15 Can Method Abe used for Overconsolidated Clay? ( 1 3 ) f C τ CU 1 φ' B 3 A c φ u =0 u UU This site has a c' constant c u. ε A B C ' For a given layer of OC Clay, it under estimates c u at low stress and over estimates it at high stress. Mohr Coulomb Model 16 Wong Kai Sin 8

9 Mohr Coulomb Model Using Method A for Undrained Analysis in OC Clay Real Soil Elastic Soil q 2c u K f q 2c u K f U f U f ESP TSP ESP TSP p or p p or p 1. Make sure the measured stress path is similar to that of Elastic Soil. 2. Divide the stratum into sub layers with different c and φ for each layer. 3. Compute c u from c and φ for each layer. Make sure the values are reasonable. Mohr Coulomb Model 17 Using Mohr Coulomb model for Undrained Analysis Method A c' and φ' produces wrong c u for NC clay, but it may produce correct c u for OC clay Method B or C Forces Plaxis to use specified c u Method A Method B Method C Stress Type Effective Effective Total Strength c and φ c u and φ u c u and φ u Modulus E E E u Poisson s Ratio ν ν = 0.35 ν u = K o or K ot K o K o K ot Mohr Coulomb Model 18 Wong Kai Sin 9

10 ) Mohr Coulomb Model Can MC model simulate undrained behaviour of clay? τ Elastic plastic Plastic Inelastic Elastic ε ε 1. It produces the correct strength with c u specified. 2. It cannot simulate non linear and inelastic behaviour. 3. It may not generate reliable pore pressure response. Mohr Coulomb Model 19 Can M C model generate accurate deflection profiles at every stage of excavation? Constant E ε Wall Deflection (mm) Depth (m Mohr Coulomb Model 20 Wong Kai Sin 10

11 Mohr Coulomb Model At early stage of excavation, Mohr Coulomb, Linear E larger δ Hyperbolic, Non linear E smaller δ E t Mohr Coulomb Model 21 At final stage of excavation, Mohr Coulomb, Linear E smaller δ Hyperbolic, Non linear E larger δ E t Nonlinear Linear Mohr Coulomb Model 22 Wong Kai Sin 11

12 Mohr Coulomb Model Mohr Coulomb E u /c u ~ 100 to 500 Constant E ε Conclusion M C model may not produce good match at every stage of excavation. Advanced Soil Model Mohr Coulomb Model 23 ε How reliable are the results generated by the MC model? Soft MarineClay Fill ε δ V,MAX = 33 mm δ H,MAX = 28 mm Is the mode of deformation reasonable? Mohr Coulomb Model 24 Wong Kai Sin 12

13 Mohr Coulomb Model Results using Hyperoblic Model Soft MarineClay Fill ε δ V,MAX = 72 mm δ H,MAX = 59 mm Is the mode of deformation reasonable? Mohr Coulomb Model 25 Linear vs Non Linear Soft Marine Clay Fill ε Mohr Coulomb Model 26 ε Wong Kai Sin 13

14 Mohr Coulomb Model Check plastic points and relative shear stress! Fill Soft Marine Clay Lesson learned: Correct analysis may not produce correct results. Mohr Coulomb Model 27 Linear vs Non Linear Model Mohr Coulomb Model Real Soil Behaviour E 2 E 3 E4 E 2 E 1 Constant E ε ε You must understand the shortcomings of the soil model used! Mohr Coulomb Model 28 Wong Kai Sin 14

15 Mohr Coulomb Model Using Method B at Reclaimed Site Fill Soft Marine Clay Sandy Silt Method B is an effective stress method. K o = 1 sin φ' If clay is still consolidating, the computed relative shear stress will be > 1, i.e. the clay is in failure state prior to excavation. Mohr Coulomb Model 29 Using effective K o at a site still undergoing consolidation Plastic points Mohr Coulomb Model 30 Wong Kai Sin 15

16 Mohr Coulomb Model Method B (c u φ u ) and K o (1 sin φ) Reduced Level (m) B Current effective stress Effective overburden pressure A Current Effective Stress (kpa) Fill Soft Marine Clay Sandy Silt At A, (' V ' H ) = ' V (1 K o ) = 74 kpa At B, (' V ' H ) = ' V (1 K o ) = 37 kpa Current c u = 22 kpa ( 1 3 ) f = 2 c u = 44 kpa Mohr Coulomb Model 31 Need to set the correct initial stresses! Fill Soft Marine Clay Sandy Silt Check plastic points after generating the initial stresses! Mohr Coulomb Model 32 Wong Kai Sin 16

17 Mohr Coulomb Model Mohr Coulomb Model 33 Mohr Coulomb Model 34 Wong Kai Sin 17

18 Mohr Coulomb Model Stress Dependent Behaviour of Soil under Drained Condition ε Mohr Coulomb Model 35 Stress Paths in an Elastic Medium D C K o B F E Δ 1 Δ 3 A E Questionable Zone F Danger Zone 3 Mohr Coulomb Model 36 Wong Kai Sin 18

19 Mohr Coulomb Model Typical Stress Paths in Excavation A B B A Mohr Coulomb Model 37 Stress Path in Zone F under Drained Condition rubber soil ε 1 (%) ε v (%) Mohr Coulomb Model 38 Wong Kai Sin 19

20 Mohr Coulomb Model Stress Path in Zone E under Drained Condition 1 =300 3 =300 Mohr Coulomb Model 39 A drained analysis can produce incorrect results under certain stress path. Which one is correct? A B A B ε Measured Computed Lesson learned: Correct analysis may not produce correct results! Mohr Coulomb Model 40 Wong Kai Sin 20

21 Mohr Coulomb Model Some problems may be sensitive to Poisson s Ratio ν=0.2 c =5 kpa φ =35 o E =8000 kpa H=9 m ν=0.4 M max,knm/m Strut 1, kn/m Strut 2, kn/m Strut 3, kn/m D epth ( m ) Wall Deflection (mm) ν=0.2 Pois. Ratio = 0.2 Lesson learned: Drained analysis can produce many surprises. ν=0.4 Pois. Ratio = 0.4 Mohr Coulomb Model 41 Can MC model simulate drained behaviour of soil? 1. It gives correct strength τ f = c + tan φ 2. Modulus is not stress dependent. 3. It cannot simulate non linear and inelastic behaviour. 4. It may produce wrong response in certain stress path. 5. Results may be sensitive to Poisson s ratio. Mohr Coulomb Model 42 Wong Kai Sin 21

22 Mohr Coulomb Model Can MC model simulate drained behaviour of soil? ε v Elastic Plastic ε 6. It may not produce correct pore pressure response. 7. When using c' φ' in consolidation analysis, it may generate the wrong undrained strength at end of construction. 8. There is no dilation until after the soil reaches failure. Mohr Coulomb Model 43 Mohr Coulomb Model 44 Wong Kai Sin 22

23 Designing Temporary Work Design & Analysis Instrumentation Monitoring Construction Control Designing Temporary Work is a Continuous Process Initial Design (Working Drawings) Start Excavation Final Design (As-Built) Finish 1 Types of Analysis in TERS Design 1. Analysis for preliminary design 2. Analysis for working design to be adopted in construction 3. Back-analysis 4. Re-analysis Prelim. Design Working Design Back-Analysis & Re-analysis Final Design (As-Built) Start Excavation Finish 2 Wong Kai Sin 1

24 Analysis for preliminary design To assess feasibility of proposed TERS configuration and construction sequence. To assess effect of excavation on surrounding structures To conduct analysis using moderately conservative design parameters 3 Analysis for working or Final design to be adopted in construction To conduct sensitivity studies assessing the effect of variable uncertainties To finalise the strut forces and wall bending moments for structural design To assess the risk of damage to adjacent structures 4 Wong Kai Sin 2

25 Back-Analysis during Construction To be carried out when the field performance is much better or worse than anticipated. To calibrate the design parameters against field measurements Wall Deflection (mm) Computed Depth (m) Measured Re-Analysis during Construction To be carried out after back-analysis To assess potential final outcome using calibrate design parameters To revise the design where appropriate Wall Deflection (mm) Wall Deflection (mm) Depth (m) Computed Design Measured Back-Analyzed Depth (m) Wong Kai Sin 3

26 Overview of Design Process 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output 7 Design Step 1: Site Investigation Plan View 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output Sectional View Designer must be actively involved in the site investigation. Get the best S.I. company to do the job! Do enough borings and CPTs. 8 Wong Kai Sin 4

27 2. Pre-Construction Survey To check pre-existing conditions of surrounding structures Things you can see.. Cracks Patches under new paint Settlement of aprons & driveway Constructions in the vicinity 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output A comprehensive pre-con survey provides the designer with a proper perspective of the surrounding and issues that must be considered in the design Wong Kai Sin 5

28 Pre-Construction Survey Pre-existing Conditions Things you can t see.. Ongoing movements Seasonal fluctuations Ground settlement profile Invest in Instrumentation Settlement marks Paper prisms Water standpipes Inclinometers Evaluation of Soil Conditions Things to check.. Fill thickness and variations Soft clay thickness and variations State of consolidation of soft clay Depth to hard stratum & variations Ground water table 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output Fill Soft Marine Clay Stiff Silty Clay Dense Silt Sand 12 Wong Kai Sin 6

29 Design Soil Profile & Parameters Fill Upper Marine Clay Intermediate Stiff Clay Lower Marine Clay Old Alluvium Extract only the reliable facts from Factual Report. Is the soil condition uniform? Can we use half mesh? 13 Example on Idealised Soil Profile ABH-30 M3010 Worst soil condition ABH-32 Instrumented section AC-3 ABH-84 ABH-31 Soil Profile at ABH-32 adopted in Original Design 14 Wong Kai Sin 7

30 Example on Soil Profile -- Half-mesh based on ABH-32 Fill E upper UM C F2 upper LMC RL (m) JGP1 JGP2 E lower F2 lower 61. OA N = OA N = Example on Soil Profile Full-mesh at Instrumented Section ABH-84 Fill E M3010 Fill E RL (m) UMC F2 upper UMC F2 upper 85.4 LMC LMC F2 lower OA N = 20 OA N = 30 OA N = 70 JGP1 LMC JGP F2 F2 lower JGP3 OA N = 20 OA N = OA N = OA N = Wong Kai Sin 8

31 Example -- Results can be very sensitive to variations in soil profile C B Cross-Over at Newton MRT Station A A B C 17 Results can be very sensitive to minor variations in soil profile Cross-Over at Newton MRT Station A B A B C 18 Wong Kai Sin 9

32 Results can be very sensitive to minor variations in soil profile Cross-Over at Newton MRT Station 19 Design Step 4: Selection of TERS We need to know Site constraints Dimensions Adjacent buildings MRT & CST tunnels δ h,max allowable? Slab elevations Ramp locations 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output 20 Wong Kai Sin 10

33 Preliminary Design Configuration Wall type & size Penetration depth Strut size and spacing JGP/DCM slab thickness Preloading This is where experience comes in! 21 Need to Establish the Excavation Sequence 22 Wong Kai Sin 11

34 Design Step 5: Basic Stability Checks Before conducting FEA, check Basal Heave Stability Uplift or Blowout Stability Toe Kick-in Stability 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output 23 Basal Heave Stabillity Which method should we use? Terzaghi Bjerrum & Eide Eide et al. Tschebotarioff Goh Chang Wong and Goh O'Rourke Su et al. Ukritchon et al. Plaxis Does FOS 1 mean failure? 24 Wong Kai Sin 12

35 Uplift Stability B Fill E UMC F2 LMC H w W = γ d B E / F2 R=αc u d R d Sand U = γ w H w B W + 2R Fs = U Check permeability & connectivity of sand layer! 25 Toe Kick-in Stability M L p L a P p P a How do we check toe stability? 26 Wong Kai Sin 13

36 Design Step 6: Preparation for FEA 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5 Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output 1. Selection of software 2. Selection of soil models 5. Assessment of 3. Selection of type of analysis 4. Evaluation of soil parameters 5. Generation of FE mesh 6. Preparation of data input Plaxis? Mohr-Coulomb? Undrained? Total stress? 27 Design Step 7: Assessment of Computed Output Tons of data can be generated with a few clicks. But what are the relevant ones? 1. Site investigation 2. Pre-construction survey 3. Evaluation of soil conditions 4. Selection of TERS configuration 5. Assessment of system stability 6. Preparation for FEA 7. Assessment of computed output Generating thick reports with not-soimportant graphs reflects badly on the engineer. It is a reflection of he/she not knowing what s important! 28 Wong Kai Sin 14

37 What are the relevant results? Relevant Results Wall deflections Ground settlement Pore pressure Strut forces Wall moment and shear Plastic points Displacement vector plots 29 Interpretation of Computed Output Check Mode of Deformation Expected Unexpected Is the mode of deformation reasonable? 30 Wong Kai Sin 15

38 Interpretation of Computed Output Check extend of soil yielding Plastic point plot 31 Plastic Points Relative Shear 32 Wong Kai Sin 16

39 Plastic points in JGP/DCM layer Residual stress ε Lesson learned: Plastic point and relative shear plots provide insight to the extend of soil yield and overall stability of the system. 33 Plot wall deflections for construction control Deflection Profiles Max. Wall Deflection computed measured 34 Wong Kai Sin 17

40 Change in Pore Pressure with Excavation Depth 35 Ground Settlement at End of Excavation 50 nd Settlement (mm) Groun Distance (m) 36 Wong Kai Sin 18

41 Plot ground settlement vs excavation depth at selected locations 50.0 Settlement (mm) 0.0 5/24/02 9/1/02 12/10/02 3/20/03 6/28/03 10/6/03 1/14/04 4/23/ Plot maximum strut forces with depth RL (m) Fill Fill E E Computed MC MC F2 F Measured MC MC JGP F2 OA (20) OA (30) OA (70) LMC JGP F2 F2 OA (20) OA (30) OA (70) OA (100) Wong Kai Sin 19

42 Plot development of strut forces during excavation S1 Strut Force (kn) De epth below ground (m) S1 S1 39 Bending Moment at Different Stages of Excavation Wong Kai Sin 20

43 Displacement Vectors Showing Movements at End of Excavation 41 Displacement Vector Plot after Strength (φ-c) Reduction Analysis FOS=1.30 False alarm? 42 Wong Kai Sin 21

44 Are the computed wall deflections acceptable? 43 Comparison of Strut Forces with Published Apparent Pressure Diagrams Peck s Apparent Earth Pressure Diagrams (1969) CIRIA s CIRIAs Characteristic Pressure Diagrams (1996) Local Experiences on Apparent Pressure Diagrams 44 Wong Kai Sin 22

45 Mohr-Coulomb model Can t match all stages of excavation! Constant E E 1 E 2 E 3 E 4 ε ε Wall Deflection (mm) Depth (m) Sensitivity Study to Finalise Design Sand Marine Clay Old Alluvium JGP Surcharge 10 and 20 kpa Soil Modulus (E u /c u ) 300 and 200 Over-excavation 0.5 and 1 m JGP Thickness 1.5 and 1.0 m JGP modulus 150 and 100 MPa Wall stiffness 1.0EI and 0.7EI Modelling of bored piles Included and excluded Preload 100, 50 and 0% 46 Wong Kai Sin 23

46 Sensitivity Study on Wall Deflection Deflection (mm) Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP P(1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design δ H,max = 200 mm 47 Sensitivity Study on Wall Bending Moment 5000 Bending Momen nt (knm/m) Reference e case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design M max = 3400 knm/m 48 Wong Kai Sin 24

47 Sensitivity Study on Wall Shear Forces 3500 Shear Forc ce (kn/m) Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design V max = 2200 kn/m 49 Sensitivity Study - Maximum Strut Load (S1) d (kn/m) Strut loa Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design S1 = 420 kn/m 50 Wong Kai Sin 25

48 Sensitivity Study - Maximum Strut Load (S2) (kn/m) Strut load ( Reference case Surcharge 20 kpa E=200Cu 1.0m mover excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelle d No preload Design S2 = 780 kn/m 51 Sensitivity Study - Maximum Strut Load (S3) (kn/m ) Strut load Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design S3 = 960 kn/m 52 Wong Kai Sin 26

49 Sensitivity Study - Maximum Strut Load (S4) (kn/m ) Strut load Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile enot modelled No preload Design S4 = 880 kn/m 53 Sensitivity Study - Maximum Strut Load (S5) (kn/m) Strut load Reference case Surcharge 20 kpa E=200Cu 1.0m over excav. JGP (1.0m) E(JGP) = 100MPa Preload 50% 0.7EI D-Wall Bored pile not modelled No preload Design S5 = 500 kn/m 54 Wong Kai Sin 27

50 Best Estimates and Design Values Best Estimates Design Values based on Sensitivity Study Deflection mm Diaphragm Wall Moment knm/m Shear kn/m Strut S1 Force kn/m Strut S2 Force oce kn/m Strut S3 Force kn/m Strut S4 Force kn/m Strut S5 Force kn/m Bending Moment and Shear Forces at Various Stages Elevation (m) Elevation (m) Bending moment (kn.m/m) Bending Moment (knm/m) Shear force (kn/m) Shear Force (kn/m) 56 Wong Kai Sin 28

51 From the results of sensitivity studies, we can proceed to finalize the design: Wall design Strut design Waler/stiffer design Set alert levels Instrumentation plan Contingency plan Design drawings 57 Analysis of Control Section for Construction Control Fill E UMC F2 upper LMC Fill E UMC F2 upper LMC RL (m) 85.4 Use best estimated parameters to compute: Wall deflection profiles Deflection vs Excav. depth Strut forces Wall bending moments LMC 72.1 F2 F2 lower 69.4 OA N = 20 Wall shear forces F OA N = 30 lower 64.7 Ground settlement 63.7 OA N = 20 OA N = OA N = 30 OA N = 100 Pore pressures 59.2 OA N = Results are to be compared with field measurements. 58 Wong Kai Sin 29

52 How reliable is your design? sand Benchmarking Exercise in Germany 59 Benchmarking Exercise in Germany Measurement Five worst results were OMITTED! 60 Wong Kai Sin 30

53 Prediction Exercise in Singapore Maximum Wall Deflection vs Excavation Level Elevation Level (RL in m) E Maximum Wall Deflection (mm) Particpant # 7 Particpant # 10 Particpant # 1 Particpant # 5 Particpant # 3 Particpant # 9 Particpant # 8 Particpant # 11 Particpant # 12 Particpant # 6 Particpant # 13 Particpant # 4 Particpant # 14 Particpant # 12 Measured 61 Design vs As-Built Construction Sequence As-Built Design 62 Wong Kai Sin 31

54 Over- Excavation (Clough & O Rouke, 1990) 63 Excessive Surcharge q = 20 kpa 64 Wong Kai Sin 32

55 Don t be over-confident about your analysis! Be prepared to face a few surprises. Implement Observational Method diligently. If in doubt, get a second opinion Wong Kai Sin 33