Numerical analysis of pile-load test in granular material using

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					Numerical analysis of a piled foundation in
  granular material using slip element




       Yongjoo Lee
       Soil Mechanics Group
       Department of Civil and Environmental Engineering
       University College London
       Gower Street, London WC1E 6BT


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               Introduction
• Reasonable mesh type in association with CPU
  time
• Number of increments for displacement norm
  convergence in connection with MNR
  (Modified Newton-Raphson)
• Values of dilation angle () for displacement
  norm convergence under New Mohr-Coulomb
  soil model (Non-associated flow rule applied)


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Laboratory test using ideal material
        (Aluminium rods)
2D model pile-load test                            P-S curve




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                          Plane Strain Mesh
                                                         Mesh A



   Total 639 nodes

   Total 1160 elements:
    1132 LSTs + 28 LSQs




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                        Plane Strain Mesh
                                                    Mesh B



   Total 195 nodes

   Total 176 elements:
    4 LSTs + 172 LSQs




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                     Parameters (drained condition)

   Granular material: Hypothetical
    elastoplastic material based on New
    Mohr-Coulomb model – Linear
    elastic perfectly plastic model
     C = 0.1Kpa,  = 30°,  = 20°,  =
    0.35, E0 = 1600Kpa, mE = 40000Kpa,
    bulk = 24KN/m3 , Y0 = 0.72m
   Slip model:
     C = 0.005Kpa,  = 5°, Kn =
    16000Kpa, Ks=8000Kpa, Ksres =
    0.8Kpa, t = 0.1m
   Concrete pile: Isotropic elastic
    model
     E = 1.55e7Kpa,  = 0.2, bulk =
    23KN/m3



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                  Analysis conditions:
1. Simulation of pile loading
                                                                       DCM
 Pile head settlements from the pile
                                                                        applied y
 load test applied to the centre node
 of the pile head (i.e. DCM)

2. Iterative solution scheme
 MNR (Modified Newton-Raphson)
 Tolerance: 0.05, Max. iteration: 40

3. In-situ stress condition
 K0 = 0.5
                                                                     Pile element

4. Number of increments                                        Slip element

                                                      Soil element
 320 increments

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            Increment Block Parameters
Increment    Increment      Pile head settlement     Time-               Number of Increments
Block No.    Block List             (mm)           Step (sec)
                                                                Case 1     Case 2   Case 3      Case 4

   1         Install pile            0                 1          5          5         5          5

   2        y1 = 0.08mm       0+0.08=0.08             1          5          10       20          5

   3        y2 = 0.6mm        0.08+0.6=0.68           1          5          10       20         20

   4        y3 = 0.32mm       0.68+0.32=1             1          5          10       20         40

   5        y4 = 1.34mm       1+1.34=2.34             1          5          10       20         50

   6        y5 = 1.95mm      2.34+1.95=4.29           1          5          10       20         50

   7        y6 = 3.71mm       4.29+3.71=8             1          5          10       20         50

   8        y7 = 3.83mm       8+3.83=11.83            1          5          10       20         50

   9        y8 = 8.56mm     11.83+8.56=20.39          1          5          10       20         50

                Total              20.39               9         45          85       165        320




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       Displacement norm convergence
            check for the Mesh B

   Increment size effect                          Dilation angle effect
    (based on  = 20°)                               (based on total 320 increments)

    Number of            convergence                 Dilation angle          convergence
    increments                                         (degrees)
        45                   No                                 0                  No
                                                                5                  No
        85                   No
                                                            10                     No
       165                   Yes
                                                            15                     No
       320                   Yes                            20                    Yes




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                        Comparison of CPU times

                 4000
                           More than 1hr
                 3500

                 3000
CPU time (sec)




                 2500

                 2000

                 1500

                 1000
                                                               Less than 12min
                  500

                    0
                             Mesh A                               Mesh B
                                            Types of mesh


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              Comparison of
    Modified Newton-Raphson methods
   ICFEP (by Potts et al, 1999)                   SAGE CRISP
    The MNR results are insensitive to              The MNR results are dependent on
    increment size                                  increment size
e.g. Pile problem:
                                                    The MNR solution was not fully
                                                    implemented in connection with
                                                    relationship between load and
                                                    displacement norms, being based
                                                    only on the displacement norm
                                                    convergence checking system at the
                                                    moment

                                                   There is no detailed information of the
                                                    MNR iterative solution in the Crisp
                                                    technical manual



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                        Conclusions
   CPU time can be improved through the reasonable mesh type
    using the Linear strain quadrilateral elements (i.e. LSQs).

   In numerical analysis using the slip element, the MNR iterative
    solution result is very sensitive to the number of increments (or
    increment size) in contrast to the comment by Potts et al. (1999).

   In the New Mohr-Coulomb soil model (i.e. linear elastic perfectly
    plastic model), the value of dilation angle () is a key factor in
    order to satisfy the displacement norm convergence.




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Results of plastic stage (20 – 30Kg)
     1.   Vector movements
     2.   Horizontal displacement contours
     3.   Vertical displacement contours
     4.   Volumetric strain contours
     5.   Max. shear strain contours
     6.   Major principal strain directions
     7.   Zero extension line directions

Note that these displacements are associated with
strain fields in soil mechanics problems

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             1. Vector movements
Experimental result from the                SAGE CRISP (M.F.=10) based
photo image processing (Scale:15)           on the mesh B ( = 20°)




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                      2. Horizontal displacements
                  Experimental result                                                    SAGE CRISP
                      Horizontal displacement contour
                      (Pile loading: 200 - 300N)

700.00



                                                                       6.00
                                                                       5.50
600.00
                                                                       5.00
                                                                       4.50
                                                                       4.00
                                                                       3.50
                                                                       3.00
                                                                       2.50
500.00                                                                 2.00
                                                                       1.50
                                                                       1.00
                                                                       0.50
                                                                       0.00
                                                                       -0.50
                                                                       -1.00
400.00                                                                 -1.50
                                                                       -2.00
                                                                       -2.50
                                                                       -3.00
                                                                       -3.50
                                                                       -4.00
                                                                       -4.50
300.00




         300.00   400.00      500.00        600.00       700.00   800.00




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                            3. Vertical displacements
                  Experimental result                                                    SAGE CRISP
                        Vertical displacement contour
                        (Pile loading: 200 - 300N)

700.00




600.00                                                                 6.50
                                                                       6.00
                                                                       5.50
                                                                       5.00
                                                                       4.50
                                                                       4.00
                                                                       3.50
500.00                                                                 3.00
                                                                       2.50
                                                                       2.00
                                                                       1.50
                                                                       1.00
                                                                       0.50
                                                                       0.00
400.00                                                                 -0.50
                                                                       -1.00
                                                                       -1.50
                                                                       -2.00
                                                                       -2.50
                                                                       -3.00
                                                                       -3.50
300.00




         300.00    400.00      500.00       600.00       700.00   800.00




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                    4. Dilatant volumetric strains
                  Experimental result                                                    SAGE CRISP
                           Volumetric strain contour
                           (Pile loading: 200 - 300N)

700.00



                                                                  0.10

                                                                  0.00
600.00
                                                                  -0.10

                                                                  -0.20

                                                                  -0.30
500.00
                                                                  -0.40

                                                                  -0.50

                                                                  -0.60

400.00                                                            -0.70

                                                                  -0.80




300.00



         300.00   400.00        500.00         600.00    700.00      800.00




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                                         5. Max. shear strains
                  Experimental result                                                    SAGE CRISP
                           Shear strain contour
                           (Pile loading: 200 - 300N)

700.00



                                                                  0.80

                                                                  0.70
600.00
                                                                  0.60

                                                                  0.50

                                                                  0.40

500.00                                                            0.30

                                                                  0.20

                                                                  0.10

                                                                  0.00
400.00
                                                                  -0.10




300.00



         300.00   400.00        500.00         600.00    700.00    800.00




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6. Major principal strain directions
Experimental result                                    SAGE CRISP




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7. Zero extension line directions
    (/or Slip line directions)
Experimental result                               SAGE CRISP




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Numerical analysis of a piled foundation in
 granular material using the slip model




       Yongjoo Lee
       Soil Mechanics Group
       Department of Civil and Environmental Engineering
       University College London
       Gower Street, London WC1E 6BT


                    14th Crisp user meeting at UCL         21