What is the theoretical inclination of failure planes predicted for purely cohesive material during an unconfined compression test?
The correct answer and explanation is :
The theoretical inclination of failure planes predicted for a purely cohesive material during an unconfined compression test is 45 degrees.
Explanation:
In an unconfined compression test, a cylindrical specimen of a material is subjected to axial compressive stress. For purely cohesive materials, where cohesion is the only contributing factor to the material’s strength and there is no internal friction (i.e., the material has no friction angle), the failure behavior is primarily governed by the material’s cohesion (denoted by (c)).
The failure of a material under compression is typically associated with the development of shear failure planes, which are the planes along which the material will fail. These planes occur when the material cannot resist the applied shear stress, and as a result, the material fractures along these planes.
To understand the inclination of these failure planes, we can refer to Mohr’s Circle, a graphical representation of stress states. In the case of an unconfined compression test, the material is subjected to a compressive normal stress (denoted by (\sigma_1)) along the vertical axis of the cylinder, while the lateral stresses are zero. The principal stresses in the material are given by:
- (\sigma_1) (major principal stress)
- (\sigma_3 = 0) (minor principal stress, as there is no lateral confinement)
For purely cohesive materials, there is no frictional resistance, so the failure criterion is defined by the cohesion (c). Under this condition, the maximum shear stress occurs at a 45-degree angle to the principal stress direction. This results from the material’s inherent cohesion, which allows failure to initiate at an inclination where the material reaches its shear strength.
Therefore, the theoretical inclination of the failure planes for a purely cohesive material under unconfined compression is 45 degrees, as it corresponds to the direction of maximum shear stress in the material. This aligns with the results predicted by classical soil mechanics theories and the Mohr-Coulomb failure criterion for purely cohesive materials.