Question – 6.3. Triaxial testing on clay soil is conducted using modified triaxial testing machine. The sample is in equilibrium under the all-round pressure of 300 kPa, deviator stress of 100 kPa, pore-air pressure of 200 kPa, and pore water pressure of 100 kPa. Illustrate the state of stress using Mohr's circle and find the maximum shear stress.

Solution : As we discussed earlier, the state variables need to be defined in unsaturated soils for defining the stress state. The two state variables viz. net normal and matric suction for the present problem are computed here for the representation of stresses.

The minor principle stress is equal to all-round pressure in this case. Thus, kPa. Major principle stress kPa.

Net normal stresses: kPa

matric suction kPa

The Mohr circle for the state of stress is shown in the following figure

Fig. 6.25 Mohr's circle representation of the stresses for problem 6.3