Describing the mechanical parameters of a rock joint with single values is difficult since rock has a natural spatial variation. In addition to this, the interpretation of the results is dependent on the number of tests. Despite this fact, input data for these parameters are often given with a deterministic value. This paper presents the results of fourteen shear tests tested under a constant normal stress of approximately 0.8 MPa. The samples were taken from a slightly weathered rock joint in granite. Basic friction angle and dilation angle, together with shear and normal stiffness, are evaluated. Based on the results, mean values and standard deviations together with coefficient of variations for the different parameters are presented. After that, different statistical distributions for the parameters are compared. Finally, the results are compared against calculated values derived from commonly used equations.


In contrary to manufactured material like steel and concrete, the characteristics of rock joints have a substantial spatial variationwhich affects their mechanical parameters.Variations in factors such as mineral composition of the intact rock, degree of weathering, surface roughness and the presence of infilling material result in an increased variation. As a consequence, these parameters are better described with a probability density function instead of a single deterministic value.

2.1 Introduction

The shear tests were performed with a shear box at Luleå University of Technology. The shear box has the capacity to perform shear tests of dimensions up to 280 by 280mm according to the methods suggested by ISRM (1981). Totally, fourteen shear tests were performed with samples taken from the rock foundation of Långbjörn hydropower station. Seven of the samples had a dimension of 125 by 125mm and seven of the samples had a dimension of 240 by 240 mm.

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