The Angle Between the Sample Surface and the Shear Plane; Its Influence on the Direct Shear Strength of Jointed Granitic Rocks and Opalinus Claystone

Buocz, Ildikó (Budapest University of Technology and Economics) | Rozgonyi-Boissinot, Nikoletta (Budapest University of Technology and Economics) | Török, Ákos (Budapest University of Technology and Economics)

OnePetro 

Abstract

Shear strength along rock discontinuities by means of direct shear strength tests were performed on granitic and clayey rocks. Both rock types represent potential host rocks of radioactive waste. The paper focuses on the effect of one parameter in particular influencing the shear strength: the angle enclosed by the plane of the sample surface and the shear plane, in the direction of shear. 3D surface roughness measurements were carried out on 18 Opalinus Claystone sample (Switzerland) and 6 on granitic rock sample (Hungary) surfaces. An interval of angle was analyzed in the direction of shear and against the direction of shear. Direct shear strength tests were carried out under constant normal loading (CNL) conditions, the influence of the angle on the shear strength values were determined. Additionally, a percentage value was calculated for how much the range of both peak and residual shear strength values change, with taking into consideration the effect of upslope and downslope shear.

1. Introduction

Designing and constructing in rock masses require a detailed and sound knowledge of the mechanical properties of the rock materials involved. Failure of rocks and determination of appropriate failure envelope has been studied for a long time [1, 2], however one of the most important parameters controlling the strength of rock masses is the shear strength of the discontinuities [3]. The discontinuities are usually a weak point of the rock mass; in particular, the features of their surfaces, such as roughness, joint strength, brittleness, humidity, mineralogy, (etc.) determine its strength. Mechanical, test related factors i.e.: loading conditions, type of testing machines, (etc.) influence the shear strength as well. The mechanical parameters defining the shear strength, i.e. friction angle and apparent cohesion, are investigated by means of laboratory direct shear strength tests. The basic principles of such analyses and test methods have been already published [4, 5, 6].

In this paper one parameter influencing shear strength was investigated in particular: the angle between the sample surface and the shear surface in the direction of the shear, resulting in “upslope” or “downslope” shearing. The paper compares the results of upslope and downslope shearing for granites and claystones. Both rock types represent target formations for hosting radioactive waste in Hungary [7, 8] and in Switzerland [9, 10, 11], respectively.