Considerations on the Laboratory Measurement of the Basic Friction Angle of Planar Rock Surfaces/Effect of Testing Methods and Surface Polishing

Over last decades, basic friction angle has been considered as one of the essential components of the developed criteria for shear strength of rock joints and thus an important input parameter in numerical modeling of the rock mass. This parameter is either measured by tilt test or by direct shear test on flat surfaces that are mostly saw-cut. While this parameter has been used in the proposed rock shear strength criteria as a constant rock property, researchers have reported a wide range for it. In this paper, the substantial impact of sample preparation and surface polishing on the obtained results of basic friction angle was shown by tilt and saw-cut tests (two common methods for evaluation the basic friction angle with two different levels of normal stresses) on two types of limestone. The results showed, based on the surface polishing, the basic friction angle measured by tilt test is approximately 10 degrees greater than the one measured by the direct shear test. Also, the results displayed that the range of variations of the basic friction angles of the tilt tests was more than the shear test results. Combining the results of this study with the other published works showed that the basic friction angle was found to vary with different factors, instead of being unique and constant.