The Mohr Failure Criterion:
Mohr presented in 19 00 a theory of rupture of materials that was the result of a combination of both normal and shear stresses. The shear stress at failure is thus a function of normal stress and the Mohr circle is tangential to the functional relationship given by Coulomb
The Mohr-Coulomb F ailure Criterion:
This theory states that: 'a material fails because of a critical combination of normal stress and shear stress, and nott from their either maximum normal or shear stress alone'
This equation is known as the Mohr-Coulomb Failure Criterion.
Laboratory determination of shear parameters - Direct shear test
The shear strength parameters for a particular soil can be determined by means of laboratory tests on specimens taken from representative samples of the in-situ soil. Great care and judgment are required in the sampling operation and in the storage and handling of samples prior to testing, especially in the case of undisturbed samples where the object is to preserve the in-situ structure and water content of the soil. In the case of clays, test specimens may be obtained from tube or block samples, the latter normally being subjected to the least disturbance. Swelling of a clay specimen will occur due to the release of the in-situ total stresses. Shear strength test procedure is detailed in BS 1377 (Parts 7 and 8) .