A silty sand is tested consolidated-drained in a triaxial cell where both principal stresses at the start of the test were 500 kPa. If the total axial stress at failure is 1.63 MPa while the horizontal pressure remains constant, compute the angle of shearing resistance and the theoretical orientation of the failure plane with respect to the horizontal. The silty sand of Problem 11.29 was inadvertently tested consolidated-un-drained, but the laboratory technician noticed that the pore pressure at failure was 290 kPa. What was the principal stress difference at failure?