FLAC3D Theory and Background • Constitutive Models

Undrained Triaxial Test with CYSoil Model

Note

To view this project in FLAC3D, use the menu command Help ? Examples…. Choose “ConstitutiveModels/ UndrainedTriaxialCYSoil” and select “UndrainedTriaxialCYSoil.f3dprj” to load. The project’s main data file is shown at the end of this example.

Tests similar to those in the drained triaxial test with dilation hardening are repeated, but this time under undrained conditions. The model setup and properties are the same as those used in the previous examples. In addition, the groundwater flow configuration is selected, with flow turned off. Porosity is 0.3, and the fluid bulk modulus is 2000 kPa for the runs.

The undrained simulation results for deviatoric stress and volumetric strain versus axial strain are plotted in Figure 1 and Figure 2, respectively. A softening stress-strain response is observed for the loose soil in Figure 1. Also, while the excess pore pressure rises initially for all soils (as indicated in Figure 2), for the medium and dense soils, it is shown to decrease upon further shearing as a result of dilation taking place.

Figure 1: \(\mid \sigma_1 - \sigma_3 \mid\) (in kPa) versus axial strain for dense, medium, and loose sand—undrained triaxial tests.

Figure 2: Pore pressure versus axial strain for dense, medium, and loose sand—undrained triaxial tests.

Data File

; Undrained Triaxial Tests - Dilation Hardening
model largestrain off
model title "Undrained triaxial test - cap-yield-soil"
model config fluid
;
zone create brick size 1 1 1
zone face skin
zone cmodel assign cap-yield
;
zone initialize stress xx -100. yy -100. zz -100.
zone face apply stress-norm -100. range group 'East' or 'West'
zone face apply stress-norm -100. range group 'North' or 'South'
zone face apply velocity-z     0. range group 'Bottom'
zone face apply velocity-z -1e-6  range group 'Top'
;
zone property density 1000 pressure-reference=100. exponent=1.0 beta=0.35 poisson 0.2
zone property flag-dilation=0 friction-mobilized=0 
zone property shear-reference=@_Gref friction=@_friu cohesion=@_coh dilation=@_dilu 
zone property flag-cap=0 pressure-initial=100. 
;
zone fluid cmodel assign isotropic
zone fluid property porosity 0.3
zone gridpoint initialize fluid-tension -1e20
zone gridpoint initialize fluid-modulus 2e3             
model fluid active off
;
fish define _cy_q
    local _z = zone.head
    global _cy_q = zone.prop(_z, 'stress-deviatoric')
    global _pp = zone.pp(_z)
end
zone history displacement-z position (1,1,1)
fish history @_cy_q
fish history @_pp
history interval 50
;
model step 50000
;
history export 2 vs 1 reverse table 'deviatoricStress'
history export 3 vs 1 reverse table 'porePressure'
table 'deviatoricStress' export [string.build('deviatoricStress%1', _IChoice)] truncate 
table 'porePressure' export [string.build('porePressure%1', _iChoice)] truncate 
;
return