Hi dear researchers,
To the best of my knowledge, in a triaxial test, the cell pressure represents the confining stress (σ₃). However, I have access to some triaxial test data in which the confining stress is considered as "cell pressure minus back pressure". In my understanding, cell pressure minus back pressure gives the effective confining stress (σ₃′), not the total stress.
Could anyone please clarify which definition should be considered as the total confining stress and effective confining stress?
Interestingly, when I use the conventional assumption (i.e., cell pressure = σ₃ and cell pressure - back pressure = σ₃′), the calculated total cohesion becomes negative in several cases in the provided sheets. However, when applying the sheet author's approach (i.e., cell pressure - back pressure = σ₃), the calculated total cohesion turns out to be positive and reasonable.
(It is worth mentioning that in both approaches, the values of effective cohesion and effective friction angle remain unchanged.)
I would greatly appreciate any insights or references on this issue.
In a triaxial test, confining stress (also called cell pressure or lateral pressure) is the uniform pressure applied all around the cylindrical soil sample, except on the top and bottom ends.
Imagine the soil sample inside a chamber filled with water or oil that pressure around the sides of the sample is the confining stress, usually denoted as σ₃.
The cell pressure represents the confining pressure (σ₃), when you carry out unconsolidated undrained tests. But in the case of Cu+u, the concept changes, because firstly, the sample must be saturated using a back pressure. in order to maintain a constant pressure around the sample, the cell pressure and the back pressure must be increased simultaneously.
For example, to achieve 95% saturation, the cell pressure has reached a value of 500 KPa and the back pressure has reached a value of 480 KPa, so the confining pressure (σ₃) around the sample is 20 Kpa.
Now we move on to consolidation. to achieve the desired confining pressure for shearing the sample (e.g. σ₃=100 KPa), we need to increase the cell pressure from 500 KPa to 580 KPa, so the confining pressure surrounding the sample is really 100 KPa (σ₃= Cell pressure- Back pressure= 580-480) KPa. The back pressure remains constant because it was applied just to saturate the sample.
Therefore, the stress surrounding the sample is total confinement stress and not effective stress. The ffective stress is the resultant of total stress minus the variation in pore pressure within the sample (σ₃′ = σ₃-Du). And Du= U-U0. The variation in pore pressure is obtained during the shear test.
We can also consider that the total stress is equal to the effective stress when we are in a consolidated drained test (CD) because the pore pressures are drained during the consolidation phase.
Summarizing:
- The UU test gives only total stresses.
- The CU+u test gives total and effective stresses.
- The CD test gives only effective stresses.
We have tried to reply to these questions in the clearest way possible and hope these replies will satisfy you.