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Direct shear strength tests are commonly recognized as a valuable tool in rock engineering. However, numerous doubts exist, regarding the real mechanism of failure in the tested specimen, as well as the meaning of the shear strength parameters. It is especially important, when the specimen is built of complex rock medium. Results of numerical simulation of direct shear tests, using two codes, i.e. UDEC and FLAC, are presented in the paper. These show progressive failure process, and mechanism, which is far from this assumed, when the interpretation of the test results is made.
The aim of the direct shear tests is determination of the shear strength parameters, i.e. cohesion and angle of friction of rock medium. Most frequently the tests of discontinuity shear strength are carried out in the laboratory and field conditions. However, there are also examples, in which forced shear surface does not cover the discontinuity. Such a tests are performed e.g. if the slip surface of the potential landslide slope crosses discontinuity sets or if the general strength of dam foundation should be determined (in such a case sheared specimen "imitates" foundation). Examples from praxis show, that failure mode in above mentioned tests depends first of all on the orientation of discontinuities in the specimen, but also on other factors, such as quantitative relation of rock and discontinuity mechanical characteristics, tensile strength of rock, etc. Large-scale tests are performed to avoid the effect of local heterogeneity. However, position of single discontinuities influences both strength and failure mode. Above disadvantages of some realizations of large- scale shear tests cause, that assumptions, which are taken in the interpretation of the results - i.e. homogeneous stress distribution along forced shear plane, shear failure without any tension phenomena - are far from reality. The "external observer" of the test can only measure directly the displacements of the specimen. Their analysis allows to recognize only some elements of the failure mechanism. For more accurate explanation of this mechanism additional informations are needed. Therefore numerical simulation of direct shear tests was undertaken; the paper presents selected results from this simulation. Calculations were done using two numerical codes: UDEC (Universal Distinct Element Method) and FLAC (Fast Lagrangian Analysis of Continua), (Itasca 1991 a,b). The model of tested rock medium was elaborated on the base of properties of flysch mass, which built of Carpathy Mountains in Poland (Thiel 1989). It should be finally mentioned, that the research programme is not finished yet. Therefore, results described in the paper do not have a final character. It seems, however, that results obtained hitherto are worthy of presentation.
Carpathian Flysch is sedimentary rock mass composed of sandstone layers, interbedded with clay shales. It is intersected with two sets of joints, which are almost perpendicularly inclined to the layers (Thiel 1993). In many cases rock mass is strongly tectonically disturbed. There is very high variability of thickness of sandstone layers and shale interbeddings. Extremely, sandstone or shale contents reaches 100%.
