Hello
I always got smaller q value When using general in soil test than that in experiment. I think there may be the lower initial stress input cause that. So I wander the initial stress input is effective stress or total stress. This is similar with initial unit stress in Triaxial,which can be easily found effective stress. Fig1 shows the clear effective stress, But I can't make sure if the initial stress in General is effective stress or total stress. Any help will be appreciated.
Fig1. Stress in Triaxial
Fig2. Stress in General
Dear Jianshu Liu,
The stresses of a General Test compared to a Triaxial test should be identical, provided that the conditions are set up correctly. For example, I have run a "Drained" and "Undrained" analysis on both tests based on the initial stresses you provided. As you can see, the results are identical.
I think there might be confusion about the cell pressure shown in Fig. 1 of your question. The confining stress σc (cell pressure) is applied by pressurising the cell fluid surrounding the specimen – it is equal to the radial stress σr, or minor principal stress σ3. The deviator stress q is generated by applying an axial strain εa to the soil. The deviator stress acts in addition to the confining stress in the axial direction, with these combined stresses equal to the axial stress σa, or major principal stress σ1. Therefore, when running a General test in isotropic conditions, you need to ensure that the initial stresses (σxx, σyy, σzz) are equal to the cell pressure.
Dear Vasileios Basas
I am using General to simulate the CU-extension test in GDS. The specimens is applied confining pressure(radial pressure) outside and back pressure inside, I wonder the initial stresses (σxx, σyy, σzz) equal to the cell pressure only or cell pressure minus back pressure(σr-σb).
Thanks for your reply. :)