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Managing Geotechnical Risk Learning from the Failures

Managing Geotechnical Risk Learning from the Failures

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Managing Geotechnical Risk Learning from the Failures

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  1. Managing Geotechnical RiskLearning from the Failures “Issues related to the use of Numerical Modelling in Design of Deep Excavations in Soft Clay” Andy Pickles of GCG (Asia) Ltd. Asia

  2. Content of Presentation • Describe the Method A/B Problem • Comment on Cam Clay model in routine design • Highlight Difficulty of modelling piles in 2D Analyses • Comments on modelling of JGP Asia

  3. Simplified Soil Behaviour • Most engineers are familiar with E and υ • Preferable to adopt Shear Modulus (G) and Bulk Modulus (K) • Shear strains due to changes in shear stress are proportional to 1/G • Volume strains due to changes in mean stress are proportional to 1/K • Water has zero G and very high Kw • For drained and undrained conditions G is the same • For drained conditions K is K for soil • For undrained conditions K becomes very high (i.e. is Kw) Asia

  4. Mohr Coulomb Model and Method A/B • Most analyses adopt simple Mohr Coulomb model with no dilation • For undrained condition no volume change • Soil particles are only affected by changes in effective stress • No volume change means no change in mean effective stress (p’) in soil • Soil is constrained to constant p’ stress path • Soil will fail where constant p’ crosses failure line • Method A/B refers only to choice of strength criteria in undrained analyses using Mohr Coulomb model • Method A uses c φand Method B uses Cu Asia

  5. Normally Consolidated Clay Undrained Loading Method A C, phi Method B Cu Cam Clay Soil is contractive FE Model Constant p’ Zero dilatancy Asia

  6. Over-consolidated Clay Ko Consolidated Clay Asia

  7. Method A at Nicoll Highway M3 Section • Method A/B problem is not unique to Plaxis • Method A was in widespread use in Singapore (and is widely adopted internationally) • Method A was adopted for design of C824 • Method A (and other methods) should be compared with design Cu profile • Excavations at C824 were deepest ever in Singapore Asia

  8. Nicoll Highway M3 Design Section MC Upper Soft Clay 40 m MC Lower EC Asia

  9. Effect of Method A on Cu Profile Method A, Ko = 1 Method A, Ko = 0.6 Design Cu Profile Asia

  10. Method A on Net Pressure Profile Excavation for 6th Strut 5th Strut Excavation Level Method A Ko = 0.6 Net Pressure +ve Pa > Pp 15m Span Design Cu Profile Upper JGP Layer Asia

  11. Effect of Method A on Wall Displacement Method A Method B Asia

  12. Effect of Method A on Bending Moments Method A Method B Asia

  13. Effect of Method A on Strut Loads Design Strut Load may be controlled by backfilling process Asia

  14. Mohr Coulomb and Cam Clay Type Models • For deep excavations Method A can under-estimate wall displacement and BM • For shallow excavations Method A will over-estimate wall displacement and BM • Method B matches the design undrained strength profile and is preferable • Neither Method A or B model the real behaviour of soft clay • Post collapse recommendation to use Cam Clay type models Asia

  15. Idealised behaviour of soil using Cam Clay type models FE Model Constant p’ Cam Clay or real Soil Asia

  16. Actual behaviour of Singapore Marine Clay • Real behaviour of Marine Clay determined from high quality lab tests • Sampling carried out using thin wall with 5 degree cutting angle • Samples anisotropically re-consolidated to in situ stresses prior to testing • Testing carried out undrained in extension and compression Asia

  17. Real Behaviour Asia

  18. Parameters for Upper Marine Clay Design φ adopted in Singapore is 22º (NSF calcs?) To obtain correct design Cu profile with modified Cam Clay model, φ = 17º is required Asia

  19. Mohr Coulomb v Modified Cam Clay • Modified Cam Clay model includes features of soft clay behaviour • Some natural soft clays differ from Modified Cam Clay • Physically unrealistic values may be required to match undrained strength profile • For managing risk care must be taken to understand implication of differences • Possibly simpler to adopt Mohr Coulomb with Method B Asia

  20. Modelling Piles in 2 D Analyses • Structures constructed in deep excavations in Singapore are often founded above soft clay on piles • Piles are often constructed after installation of JGP layers but before commencement of excavation • Piles will be bonded to the JGP • Heave of ground during excavation results in tension in piles • Presence of piles will restrain heave and also restrict wall movements Asia

  21. Comments on modelling of Piles • Modelling piles in 2D analyses as walls connected to the ground can severely restrict the predicted wall movement • Wall displacements will be under-predicted and wall bending moments also under-predicted • If 3D modelling is not available then it may be preferable to carry out sensitivity studies without piles and with piles modelled as “anchors” not connected to the soil mesh • For managing risk you must understand the limitations implicit in simple 2D models – sensitivity analyses Asia

  22. Modelling JGP • Numerical models for design typically adopt Mohr Coulomb type model • E = 150MPa, Cu = 300kPa (minimum UCS is 900kPa) • JGP strength is a factored value used in analyses where soil strength is unfactored • How are design values justified? Asia

  23. USC Results E50 from UCS Tests Average 500 MPa Average 2000kPa Design 900kPa Design Value 150 MPa Asia

  24. Axial strain at failure in UCS tests on JGP Average 0.8% Asia

  25. Summary of JGP Properties *1 – Non linear response *2 – Peak to residual at 20% plastic strain Asia

  26. Modelling of JGP • Actual mass characteristics of JGP not well understood • No direct relationship between lab and field performance • Parameters and model presently used for design are probably incorrect and may be unsafe • JGP is probably a brittle material whereas Mohr Coulomb is elastic/perfect plastic • Sensitivity analyses with high and low strength and stiffness values are essential Asia

  27. Concluding Remarks • Numerical modelling has an important role in design • Numerical modelling requires specialist knowledge • For managing risk make sure that the limitations of the model are well understood (investigated) • Do not rely on preciseness of results • Sensitivity/ trends in behaviour more important • Always perform sanity checks by alternative means Asia

  28. End of Presentation Asia