Testing Procedures

For example, the uniaxial test is the most frequently used rock mechanics test, but provides only elastic properties and a single failure value derived from a very simple stress path.

With computer controlled feedback it is possible to follow different stress paths by varying the axisymmetric confining pressure and the axial compression of a rock cylinder. The use of computed automated controls allows us to perform multiple loading cycles on the same specimen. After each peak load for a given confining pressure the deviatoric stress is reduced to zero and the sample is loaded hydrostatically to the next confining level. Thus, the progressive stress history of a single sample can be monitored instead of using different samples (with different microstructure?) at each stress state and combining the results to estimate the progressive stress behavior of the “intact” rock. This allows a more realistic evaluation of the intact rock strength, and thus the rock mass strength, resulting in more realistic predictions and interpretations of the in-situ rock mass behavior.

There is no way to describe a materials failure criteria without it’s shear strength parameters. However, developments in shear testing and evaluation methods in rock mechanics have largely been ignored. Therefore, to investigate the shear behavior and failure characteristics of both fracture surfaces and intact rock we use automated testing procedures to perform tests with different boundary conditions. This enable the execution of modified shear tests which are behavior specific. For example, stiffness controlled tests can be used to evaluate the ultimate shear strength for different boundary conditions, and also allows the recognition of the different failure modes that occur during shearing. This test method is the most appropriate test method for evaluating a materials shear behavior yielding the shear and normal stiffness, dilation potential, cohesion, and the initial and ultimate friction angles. Multi failure state shear tests (under constant normal loads) as well as various combinations of test control procedures can be performed on a single sample eliminating the effects of sample variability on the failure envelope.

The preparation of and performing tests for a direct evaluation of the tensile strength is difficult and not widely used. Instead many index tests such as the Brazilian test, three or four point bending tests, etc. are used. The correlation between the test results and the direct tensile strength is not often clear since different stress paths are responsible for the rock failure. Using overcored samples we test the direct tensile strength of a sample using compressional loading. This allows a direct correlation between the index tests and direct tensile values creating rock type specific correlations for a given project. In addition, different sample geometries can be used to evaluate different modes of fracture toughness if the Griffith failure criteria is necessary for a given problem.