Large flat jack tests are used to assess rock mass deformability in the design stages of large projects. This test consists in cutting a thin slot in the rock mass in which a flat jack with large dimensions is inserted; oil pressure inside the jack loads the slot walls while the rock mass deformation is measured by displacement gauges located inside the jack and at the surface. The main difficulty regarding interpretation of large flat jack tests has to do with the effect of the tension crack that may develop in the rock mass during the tests. A three-dimensional numerical model was developed for the interpretation of large flat jack test results. The model was used to establish the relations between the in situ stress perpendicular to the slots, the rock mass tensile strength, the applied pressures and the crack depth.

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