Behaviour of sparsely fractured rock in low confining stresses

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The initiation and propagation of failure in rock is a matter of fundamental importance in rock engineering. In order to understand the characteristics of rock and rock masses as engineering materials, it is necessary to start with the behaviour of intact rock. From an engineering point of view, this involves studying laboratory scale samples, such as diamond drill core, with dimensions in the range of 50 mm in diameter. For many rock types, the grain size is small enough that samples of this scale can be considered representative of larger intact rock volumes. When establishing the intact strength of rock, several laboratory tests are employed. The more common tests are the Brazilian test to establish the tensile strength, the uniaxial test to establish the uniaxial compressive strength when confinement is zero, and the triaxial test to establish the shear strength under confined conditions. For decades, the results from shear strength were considered the most important tests in establishing the failure envelope for rock. It is now accepted that the tensile strength of intact rock is a fundamental material property that controls the ultimate behaviour of intact rock. The widely used Hoek-Brown empirical failure criterion is a shear criterion based on the triaxial compressive failure of rock. The parameters for the criterion are derived from laboratory compression tests. Preliminary results show that the Hoek-Brown is not adequate for predicting the strength in the tensile region when only the shear strength is used as input. In this proposal, we plan to develop a confined-tension test that will be used to track the behaviour of the intact rock from tension to compression. The results from those tests will be used as the bases for modifying the Hoek-Brown criterion for those conditions where tensile stresses can dominate the failure process. The stability of large rock slopes is dominated by the cohesion of the rock mass. This cohesion can be destroyed by tensile stresses, which are created by the natural heterogeneity of the intact rock and rock mass. Quantifying the magnitude of the rock cohesion can be effectively carried out using the tensile strength as a proxy. Another proxy for cohesion is the ratio of the compressive strength to the tensile strength, and this proxy is used in the Hoek-Brown criteria. Recent research suggests that this ratio is in fact a material property. The confined tension tests will be used to compile this ratio for various homogeneous and anisotropic rocks. A newly developed slope stability numerical program that utilises this ratio as an input parameter will be used to evaluate several case histories where the stability of the slope has been difficult to assess. These case histories have extensive documentation, and the results from traditional analyses have not provided adequate explanations of their behaviour. The laboratory test results, in combination with the newly developed slope model and case histories will provide guidance on the application of this methodology for assessing the stability and deformation behaviour of rock slopes. The results from this research will enable both mining and civil industries to develop rational design approaches for slope excavations in jointed rock masses.