Assessment of basic friction angles of various rock types from Turkey under dry, wet and submerged conditions and some considerations on tilt testing

The shear strength of discontinuities has a prime importance in the assessment of structurally controlled stability problems. Natural discontinuity surfaces are usually not smooth and show irregularities, which have an important effect on their shear strength. The peak shear strength of natural, unfilled, rough discontinuities is estimated by a non-linear failure criterion and one of the main inputs of this criterion is the basic friction angle, which is determined via a tilt test or direct shear test. The limited number of studies on tilt testing and the need to develop a standard and/or suggested method for estimating a reliable basic friction angle value from tilt tests have given rise to some recent research efforts on different aspects of this test. In this study, an attempt was made to contribute to previous works on the tilt test with the aid of an experimental program on a wide range of rock types. The study mainly focused on investigating the dependence of the basic friction angle on different testing conditions (dry, wet and submerged), the suitable number of tests (repetitions of sliding on the same surface) which does not cause a wear effect, and a more detailed assessment on the influence of water head on the basic friction angle determined under submerged conditions. Additional considerations in the study include a general evaluation of the influence of the testing surface's mineralogical composition on test results. For this purpose, 22 different rock types were collected from different parts of Turkey and their mineralogical compositions were determined via a total of 415 tilt tests under three different conditions. The results obtained from the experimental studies are presented and compared with the results of previous studies and some recommendations are made.

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