The relation between effective principle stresses and the shear strength parameters can be obtained using the Mohr's circle shown in Fig. 6.2.
Fig. 6.2 Mohr-Coulomb's failure criterion
The relationship may be written as
.........................................(6.3)
which is referred to as Mohr-Coulomb failure criterion.
The aforementioned shear strength parameters are often determined in the laboratory using direct shear and triaxial test apparatus on representative soil samples obtained from the field. The testing procedure is summarized briefly here.
Direct shear is a simple test conducted routinely in the geotechnical engineering laboratory. The soil sample is confined in a square split metal box. The metal box can split into two halves at mid-height during the application of shear as shown in Fig. 6.3.
Fig. 6.3. Direct shear apparatus