Geotechnical HKIE Seminar 2003 by huangyuarong

VIEWS: 0 PAGES: 28

									           Managing Geotechnical Risk
            Learning 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                                            1
                 Content of Presentation


l    Describe the Method A/B Problem

l    Comment on Cam Clay model in routine design

l    Highlight Difficulty of modelling piles in 2D Analyses

l    Comments on modelling of JGP




    Asia                                                      2
                   Simplified Soil Behaviour
l    Most engineers are familiar with E and υ

l    Preferable to adopt Shear Modulus (G) and Bulk Modulus (K)

l    Shear strains due to changes in shear stress are proportional to 1/G

l    Volume strains due to changes in mean stress are proportional to 1/K

l    Water has zero G and very high Kw

l    For drained and undrained conditions G is the same

l    For drained conditions K is K for soil

l    For undrained conditions K becomes very high (i.e. is Kw)




    Asia                                                                    3
           Mohr Coulomb Model and Method A/B
l    Most analyses adopt simple Mohr Coulomb model with no dilation

l    For undrained condition no volume change

l    Soil particles are only affected by changes in effective stress

l    No volume change means no change in mean effective stress (p’) in soil

l    Soil is constrained to constant p’ stress path

l    Soil will fail where constant p’ crosses failure line

l    Method A/B refers only to choice of strength criteria in undrained analyses
     using Mohr Coulomb model

l    Method A uses c φand Method B uses Cu




    Asia                                                                           4
       Normally Consolidated
       Clay Undrained Loading
                                 Method A
                                  C, phi




                                 Method B
                                    Cu


           Cam Clay              FE Model Constant p’
           Soil is contractive   Zero dilatancy




Asia                                                    5
       Over-consolidated Clay
       Ko Consolidated Clay




Asia                            6
           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                                           7
       Nicoll Highway M3 Design Section



                             MC
                            Upper
           Soft Clay 40 m


                             MC
                            Lower



                            EC




Asia                                      8
       Effect of Method A on Cu Profile




                         Method A, Ko = 1



                            Method A, Ko = 0.6




             Design Cu
              Profile




Asia                                             9
Method A on Net Pressure Profile Excavation for 6th Strut


            5th Strut           Excavation Level


                         Method A
                         Ko = 0.6
                                                      Net Pressure
                                                           +ve
                         15m                            Pa > Pp
                         Span




                                          Design Cu
                                           Profile




                        Upper JGP Layer
Asia                                                                 10
  Effect of Method A on Wall Displacement

        Method A             Method B




Asia                                        11
   Effect of Method A on Bending Moments

       Method A              Method B




Asia                                       12
           Effect of Method A on Strut Loads
Strut Row    Predicted Strut Load   Design Strut Load   Ratio Method B to
                Using Method B        Using Method A     Design Strut Load

       1             379                  568                 67%
       2             991                 1018                 97%
       3            1615                 1816                 89%
       4            1606                 1635                 98%
       5            1446                 1458                 99%
       6            1418                 1322                107%
       7            1581                 2130                 74%
       8            1578                 2632                 60%
       9            2383                 2173                110%

Design Strut Load may be controlled by backfilling process

Asia                                                                    13
    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                                                      14
       Idealised behaviour of soil using Cam Clay type models




                         Cam Clay               FE Model Constant p’
                         or real Soil




Asia                                                                   15
    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                                              16
       Real Behaviour




Asia                    17
           Parameters for Upper Marine Clay
                                  φ at Peak
    Cu Peak         68 kPa                          25º
                                  undrained

    Cu Large
                    52 kPa      φLarge Strain       34º
     Strain

                      25%
   % Change                       % Change      35% Increase
                   reduction


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                                                       18
           Mohr Coulomb v Modified Cam Clay
l    Modified Cam Clay model includes features of soft clay
     behaviour

l    Some natural soft clays differ from Modified Cam Clay

l    Physically unrealistic values may be required to match
     undrained strength profile

l    For managing risk care must be taken to understand
     implication of differences

l    Possibly simpler to adopt Mohr Coulomb with Method B

    Asia                                                      19
             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                                                                   20
           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                                                          21
                      Modelling JGP
l   Numerical models for design typically adopt Mohr
    Coulomb type model

l   E = 150MPa, Cu = 300kPa (minimum UCS is 900kPa)

l   JGP strength is a factored value used in analyses where
    soil strength is unfactored

l   How are design values justified?




    Asia                                                      22
         USC Results             E50 from UCS Tests




                                          Average
Design                 Average
                                          500 MPa
900kPa                 2000kPa

                                   Design Value 150 MPa




 Asia                                                     23
       Axial strain at failure in UCS tests on JGP
                      Average 0.8%




Asia                                                 24
                 Summary of JGP Properties
                       Model                  Cu    E     Fail Strain
                                                               %

Laboratory              UCS             >1000      500       0.8

Design                  M-C               300      150
Back                    Real              500      70*1       >2
Analyzed
Advanced              Brittle?        500*2/ 200   80         2*2
Analysis
*1 – Non linear response
*2 – Peak to residual at 20% plastic strain

  Asia                                                              25
                   Modelling of JGP

l   Actual mass characteristics of JGP not well understood

l   No direct relationship between lab and field performance

l   Parameters and model presently used for design are
    probably incorrect and may be unsafe

l   JGP is probably a brittle material whereas Mohr Coulomb
    is elastic/perfect plastic

l   Sensitivity analyses with high and low strength and
    stiffness values are essential
Asia                                                         26
                  Concluding Remarks

l    Numerical modelling has an important role in design

l    Numerical modelling requires specialist knowledge

l    For managing risk make sure that the limitations of the
     model are well understood (investigated)

l    Do not rely on preciseness of results

l    Sensitivity/ trends in behaviour more important

l    Always perform sanity checks by alternative means
    Asia                                                       27
       End of Presentation




Asia                         28

								
To top