Another sample of the same sand tested in Problem 11.30 (consolidated non-hydrostatically, with 1 = 15 kPa

Another sample of the same sand tested in Problem 11.30 (consolidated non-hydrostatically, with σ1 = 15 kPa and σ 3 = 10 kPa) is tested by holding the vertical stress constant and increasing the horizontal stress (this is a lateral compression test). The angle of internal friction is 30° (c = 0).
(a) Draw the Mohr circles for both initial and "at failure" conditions.
(b) What will be the major and minor principal stresses at failure?