The purpose of the triaxial test is to determine the shear strength and the deformation behaviour of the soil. The test is used for many reasons, but mainly to determine bank stability and for deformation calculations for sheet piles. A cylindrical soil sample is tested whilst confined between a rigid bottom and top plate covered with a rubber membrane. A vertical load is then placed on the sample. Horizontal pressure is exerted on all sides of the soil sample by applying pressure to the water around the sample. The rubber membrane around the sample prevents the water from penetrating.
The water pressure is maintained at a constant level throughout the test. By increasing the vertical force at a constant velocity the sample gradually compresses until shearing occurs. The shear strength parameters usually derived from the triaxial test are the cohesion and the angle of internal friction. The test can however also be used to determine rigidity parameters such as Young's modulus E.