Large flat jack tests are used to assess rock mass deformability in the design stages of large projects. The paper introduces a methodology for interpretation of the tests, with which it is also possible to obtain the in situ normal and shear stress components on the flat jack plane. The main problem regarding interpretation of large flat jack tests has to do with the effect of the tension crack that may develop in the rock mass, depending on the loading pattern, the rock mass tensile strength and the in situ stress normal to the jack. The three-dimensional numerical model that was developed simulates the test with the actually applied loading and accounts for the development of the tension crack. Model results show that the extent of the tension crack is irrelevant for the determination of the in situ stresses. Therefore, the in situ stress values can be used as an important parameter for interpretation of the test results, and furthermore constitute one more output of this reliable in situ test.


The deformability of rock masses plays an important role in the design of several types of structures, because their behaviour depends on the displacements undergone by the rock mass. This is, in particular, the case of concrete dams, large bridge foundations, underground caverns and tunnel linings. For design of these types of important structures built in or on rock masses, it is not adequate to characterize the rock mass deformability by only using laboratory tests on intact rock specimens, and extrapolating their results to the rock mass based on some indices such as the RMR, the Q or the GSI values. For these cases, in situ deformability tests are essential. They should be used for zoning the rock mass according to a deformability parameter, usually the dilatometer modulus, but they often do not supply reliable estimates of the rock mass deformability at a representative volume. Plate loading tests are widespread in situ deformability tests, but in many cases they do not yield satisfactory results, because the rock mass in the tested zone is often disturbed by the excavation, and the tested volumes are still not representative of the rock mass [1]. To avoid both these shortcomings, large flat jacks (LFJ) are preferably used, as they allow testing relatively large volumes of rock mass and determining the deformability inside the rock mass, in less disturbed conditions [2].


LFJ tests consist in cutting a thin slot in the rock mass, by means of a disk saw, and inserting a flat jack that is then pressurized in order to load the slot walls while measuring the rock mass deformation with several displacement transducers. In order to obtain a mean value of the modulus of deformability in large rock volumes, as well as information about the rock mass heterogeneity, a group of two co-planar contiguous slots is usually opened for each test.

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