A triaxial test is performed on a normally consolidated clay. The sample is further consolidated by the test of all-around confining pressure prior to application of the axial load. The axial force is...

A triaxial test is performed on a normally consolidated clay. The sample is further consolidated by the test of all-around confining pressure prior to application of the axial load. The axial force is then applied very slowly. The equipment pore pressure lines remain open so that drainage of soil pore water can occur during the test. (The described conditions outline the procedure for a consolidated–drained-type test.) The sample fails (shears) when the σ3 pressure is 21 kPa and the σ1 value is 61 kPa. From these data, determine the angle of internal friction for the effective stress strength–failure envelope (that is, determine ɸ_CD).