4. Four additional compression tests are performed on the sandstone at successively higher

confining pressures (Table 2). In each case, the rock sample failed when the differential

pressure reached a value > 420 MPa. Examination of the core samples showed that failure

occurred by ductile shear along a small shear zone oriented at a consistent angle to 0₁. Using

the data in Table 2, draw a second set of Mohr circles for these experimental runs. Next, draw

a failure envelope tangent to the four Mohr circles and draw a Mohr radius for each circle

using the point of tangency. Estimate the angle a of the shear zones relative to 0₁. What is the

significance of the intersection between the Coulomb and ductile failure envelopes?