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The torsional shear test consists of applying a torque to a cylindrical core of rock isolated from a rock mass, by means of a diamond drill. The core is restrained by means of an axial rock bolt which is used to apply a controlled normal load. The core is gripped in an apparatus and twisted with a known torque. The twist is measured and the point of failure determined. Failure occurs on a well-defined plane, which can be made to coincide with indigenous planes of weakness. Test results give the shear strength directly in terms of the cohesion and the angle of friction. Residual shear strength after prolonged sliding can also be determined.
rock mechanics, foundation investigation, shear strength, instruments, testing, rocks, planes of weakness
Operations manager, Teledyne Geotech/Terrametrics Division, Golden, Colo.