A consolidated-drained triaxial test is carried out on a sand specimen that is subjected to 100 kN/m² confining pressure. The vertical deviator stress was increased slowly such that there is no build-up of pore water pressure within the specimen. The specimen failed when the additional axial stress Ds reached 260 kN/m².

a. Find the friction angle of the sand.

b. Another identical sand specimen is subjected to 200 kN/m² confining pressure. What would be the deviator stress at failure?