Triaxial tests are conducted on a rock sample with a planar joint inclined at an

Question

Triaxial tests are conducted on a rock sample with a planar joint inclined at an angle of 39 degree to the horizontal. The properties of the rock and the joint are given as follows. For rock: c = 10 MPa and phi = 36 degree; For joint: c_j = 6 MPa and phi_j = 27 degree The confining pressure for the tests is 12 MPa. Determine the range of joint angles within which the joint will fail first. For the given joint angle, determine the axial stress sigma_1 at failure. Estimate the joint angle that will cause sliding of the joint at the smallest sigma_1, and estimate the value of sigma_1.

Explanation / Answer

6.1 The range of joint angle within which the joint will fail first is determined to be an approximate value.

For the rock the angle is 36degrees and for joint the angle is 27degrees.

From 27degrees till 40degrees the joint would be strong after which a failure would occur.

At the range 27±13 degrees the first failure may occur as the angle inclination is till 39degrees in horizontal direction.

6.2 The axial stress at that point is determined using the angle of joint and the pressure in the joint.

Stress=(confining pressure+pressure at joint)/angle

=(12+6)/27=0.66×10^12Pa

6.3The value of sigma 1 is estimated in the previous step as 0.66×10^12 and it is estimated as the lowest value and the joint angle causing sliding is =27degrees.

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