Undrained Triaxial Test with Soft-Soil Model
Note
The project file for this example is available to be viewed/run in FLAC3D.[1] The project’s main data files are shown at the end of this example.
The Soft-Soil model is used to simulate the triaxial consolidated-undrained (CU) tests with the consolidated stresses at 100, 80, 50, and 25 kPa. The pre-consolidated pressure (cap pressure) is assumed 100 kPa for all four cases, which corresponds to OCR = 1.00, 1.25, 2.00, and 4.00, respectively. The triaxial undrained compression is realized by setting zero volumetric strain, or the horizontal strains are setting to -0.5 times of the axial strain. The axial strain is up to 20%. The input parameters are summarized in the table:
\(\lambda^*\) | 0.106 |
\(\kappa^*\) | 0.016 |
\(v\) | 0.25 |
\(\phi\) | 30° |
OCR | 1/1.25/2/4 |
Figure 1 plots the deviatoric shear stress (q) vs. the axial strain. All deviatoric stresses of the 4 cases increase and then remain constant values. The saturated deviatoric stress values are due to the load paths reaching the Mohr-Coulomb shear failure line, as shown in Figure 5, which plots the curves of q vs. p.
From Figure 2, for the case of OCR = 1, p is decreasing immediately after the undrained triaxial loading until it hits the Mohr-Coulomb shear failure line and the cap is expanding when q is increasing. For the case of OCR = 1.25, p remains constant until it reaches the cap; then p decreases until it reaches the Mohr-Coulomb shear failure line. For the cases of OCR = 2 and 4, p remains constant because the load paths reach the Mohr-Coulomb shear failure line before the cap. For the last two cases, the high OCR values prevent the undrained loading paths from reaching the cap.
Data File
model new
model large-strain off
;
[global cs0 = -100.0]
[global totalsteps = 10000]
[global rate = 0.2/totalsteps]
[global ocr = 1.00]
;
zone create brick size 1 1 1
zone face skin
zone cmodel assign soft-soil
zone property lambda-modified 0.106 kappa-modified 0.016 poisson 0.25
zone property coefficient-normally-consolidation 0.62 ...
over-consolidation-ratio [ocr]
zone property friction=30.0 cohesion 0 dilation 0
zone property stress-1-effective=[cs0] stress-2-effective=[cs0] ...
stress-3-effective=[cs0]
;
zone face apply velocity-x=0.0 range group 'west'
zone face apply velocity-y=0.0 range group 'south'
zone face apply velocity-z=0.0 range group 'bottom'
zone initialize stress xx [cs0] yy [cs0] zz [cs0]
;
[global zp = zone.head]
[global gp = gp.find(8)]
fish define q_
global q_ = zone.stress.xx(zp) - zone.stress.zz(zp)
global a_ = -gp.disp.z(gp)
global strain_ = zone.strain.shear.inc(zp)
global p_ = -(zone.stress.xx(zp) + zone.stress.yy(zp) + ...
zone.stress.zz(zp))/3.0
end
;
model history name 'steps' timestep
fish history name 'q' q_
fish history name 'a' a_
fish history name 'shearstrain' strain_
fish history name 'p' p_
;
zone face apply velocity-z [-rate] range group 'top'
zone face apply velocity-x [0.5*rate] range group 'east'
zone face apply velocity-y [0.5*rate] range group 'north'
;
model solve cycles [totalsteps]
;
history export 'q' vs 'a' table 'qa_ocr1p00'
history export 'q' vs 'p' table 'qp_ocr1p00'
table 'qa_ocr1p00' export 'qa_ocr1p00' truncate
table 'qp_ocr1p00' export 'qp_ocr1p00' truncate
;
model save 'triaxial_cu_ocr1p00_ss'
Endnotes
[1] | To view this project in FLAC3D, use the program menu.
⮡ FLAC |
⇐ Single Zone Loading-Unloading Test with MohrT Model | Undrained Triaxial Test with NorSand Model ⇒
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