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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. Introduction. Reasonable mesh type in association with CPU time

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

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

14th Crisp user meeting at UCL

introduction
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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mesh a

Plane Strain Mesh

Mesh A
  • Total 639 nodes
  • Total 1160 elements:

1132 LSTs + 28 LSQs

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mesh b

Plane Strain Mesh

Mesh B
  • Total 195 nodes
  • Total 176 elements:

4 LSTs + 172 LSQs

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parameters drained condition
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
Analysis conditions:

DCM

1. Simulation of pile loading

Pile head settlements from the pile

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

4. Number of increments

320 increments

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increment block parameters
Increment Block Parameters

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displacement norm convergence check for the mesh b
Increment size effect

(based on  = 20°)

Dilation angle effect

(based on total 320 increments)

Displacement norm convergencecheck for the Mesh B

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comparison of cpu times
Comparison of CPU times

More than 1hr

Less than 12min

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comparison of modified newton raphson methods
ICFEP (by Potts et al, 1999)

The MNR results are insensitive to increment size

e.g. Pile problem:

SAGE CRISP

The MNR results are dependent on increment size

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

Comparison of Modified Newton-Raphson methods

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conclusions
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
Results of plastic stage (20 – 30Kg)
  • Vector movements
  • Horizontal displacement contours
  • Vertical displacement contours
  • Volumetric strain contours
  • Max. shear strain contours
  • Major principal strain directions
  • 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

photo image processing (Scale:15)

SAGE CRISP (M.F.=10) based

on the mesh B ( = 20°)

1. Vector movements

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2 horizontal displacements
2. Horizontal displacements

Experimental result

SAGE CRISP

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3 vertical displacements
3. Vertical displacements

Experimental result

SAGE CRISP

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4 dilatant volumetric strains
4. Dilatant volumetric strains

Experimental result

SAGE CRISP

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5 max shear strains
5. Max. shear strains

Experimental result

SAGE CRISP

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6 major principal strain directions
6. Major principal strain directions

Experimental result

SAGE CRISP

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7 zero extension line directions or slip line directions
7. Zero extension line directions(/or Slip line directions)

Experimental result

SAGE CRISP

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slide21

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