Problem: A triaxial test was performed on saturated sand. The sample was first consolidated under a confining pressure of 70 kN/m2. Then, the test conditions were changed to undrained loading and a deviator stress was gradually increased to 90 kN/m2 when the sample failed. The pore water pressure in the sample was measured to be 47 k N/m- at failure. Estimate the effective friction angle of this sand sample.