A triaxial specimen of loose sand is first consolidated non-hydrostatically, with 1 = 15 kPa and

Question:

A triaxial specimen of loose sand is first consolidated non-hydrostatically, with σ1 = 15 kPa and σ 3 = 10 kPa. The sample is then failed by holding the vertical stress constant and decreasing the horizontal stress (this is a lateral extension 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?