A triaxial test is performed on a cylindrical core soil or rock specimen to evaluate its shear strength. The triaxial test attempts to replicate the in-situ stresses (stresses in the original place from where the soil sample was taken) on the core soil or rock specimen.
Shear strength is the maximum shear stress that can be resisted by the soil. Soil doesn’t fail by the normal stress, no crushing failure in the soil. Soil always fails by shear before it reaches its crashing end. More contact force on the soil/rock indicates more shear stress. More number of particles in contact indicates more shear stress.
To have the shear failure, the interlocking surface has to be broken down. Two things to be known-
What is the shear stress at failure, so that we can design for that stress?
At which plane is it going to failure, so that we can strengthen the plane?
Procedures
The cylindrical soil sample is prepared in the lab using a proper mould. The specimen is generally saturated and has a depth ranging from 38 mm to 100 mm. The height to diameter ratio of the sample is generally 2:1.
The cylindrical soil/rock specimen is vertically sealed within a thin rubber membrane and placed between two porous discs at the top and bottom end.
The cylindrical soil sample is then positioned between loading plates and placed on a pedestal inside a triaxial pressure chamber.
The pressure chamber is filled with water/fluid which applies fluid pressure to the sides of the cylindrical soil sample.
Vertical or axial stress is applied to the cylindrical sample by applying force to the moveable top-loading plate.
This is the most widely used test and used for all types of soils. Some of the important specifications are-:
Drainage conditions can be controlled and pore water pressure can be measured.
Volume change can be measured.
The principal planes do not rotate.
The soil can fail at the weakest plane or simply bulge.
In stage-I, apply the confining pressure and in stage- II, the direction is called cell pressure, for which the Mohr circle is a point on the horizontal axis (Drainage conditions can be controlled as per requirement). In Stage-II, apply the deviator stress at the top along with the cell pressure.
I know, you are very eager to know how to calculate shear strength from the triaxial test. This video has it all, do watch it out.
Types
Stage I- Cell pressure application: because of this, equal amounts of pore water pressure would develop. Either we can open the drainage then consolidation will occur hence the volume change will occur. If the drainage is closed then unconsolidated, no volume change.
Stage II- Actual application of load: Shear stress developed here due to deviator stress. If the drainage is open, drained condition, if not then undrained condition.
Depending upon the condition, 3 types of tests are there.
UU test
This is the Unconsolidated Undrained test. This is a quick test, which takes around 15 mins. Minimum 3 samples with identical void ratio and water content are taken.
Apply cell pressure and deviator stress with the undrained condition and no volume change, no consolidation.
Shear stress will be the same for all the samples as the void ratio and water content would be the same.
If we conduct the test for different cell pressures, the same increase in pore pressure would be there. Hence the effective stress in the soil remains the same.
In the field to know the immediate strength of the soil or to know the immediate stress during construction we use this test.
CU test
This is the Consolidated Undrained test. It takes 24 hrs for consolidation and 2 hrs for stageII.
After the consolidation, no excess pore pressure, but after this when we apply the deviator stress again the pore water pressure will develop.
It can either be positive or negative depending upon the stress history of soil.
For Normally Consolidated soils, the pore water pressure would be positive and tends to decrease in volume and pore water tries to move out. (C=0 for NC clays)
For overconsolidated soils, the pore water pressure would be negative and volume tries to increase and pore water tries to enter. Effective stress would be more than total stress.
CD test
When the loading is so gradual, then no pore water pressure exists. This field condition would be simulated to find long term stability.
For NC Clays Cohesion =0.
For OC Clays there exists some cohesion.
So, let us close the blog here and I hope you got a good grasp on the triaxial test, if yes solve the question below and if not, let us know by commenting. I will come up with another article very soon. Bye-bye!
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