ABSTRACT
The paper is concerned with the geotechnical assessment of the instability of high wall faces in jointed rock masses. The method of assessment of slope instability was based on joint surveys conducted on the face of surface mining excavations. The analysis of the pit walls indicated that small scale toppling and wedge failures are the most likely modes of instability. This approach requires the laboratory determination of the shear strength of discontinuities, to assess the nature of an instability. The paper presents the application of this technique to the stability assessment of two roadstone quarries in limestone and basalt respectively. The paper concludes with a general discussion on the effectiveness of this method of evaluating the stability of jointed hard rock slopes.
1 INTRODUCTION
Instability of hard rock slopes is mainly governed by the open pit geometry and the presence of structural discontinuities along which slope failures may develop. Fractures, joints, faults, and bedding planes are the most important structural features, (collectively grouped as discontinuities) which are responsible for the potential pit slope instability. Thus, geological discontinuities, their size, shape, frequency and orientation together with the rock mass strength parameters control the size and the shape of unstable blocks. Although intact rock strength is an important parameter to classify rock types within the unstable block, the instability of surface mining slopes is controlled by the shear strength of discontinuities which act as potential planes of weakness. The study described in this paper was concerned with the geotechnical assessment of the instability of high wall faces in jointed rock masses. The method of geotechnical assessment of slope instabilities was based on joint surveys conducted in surface mining excavations. The results were interpreted with the aid of stereographic techniques for predicting the size, shape and mode of failure of a potential unstable block. This approach requires the laboratory determination of the shear strength of the discontinuities in order to assess the nature of unstability. The shear strength of natural discontinuities was obtained by direct shear testing in the laboratory. The paper presents the application of this approach to the stability assessment of two roadstone quarries in Limestone and Basalt respectively.
2 RESEARCH APPROACH AND PROGRAMME
The research approach for the stability evaluation of surface mine slopes is based on the following discrete steps :-
- field investigations
- laboratory evaluation of strength characteristics of joints and intact rock
-theoretical slope stability analysis
The field study is aimed at the acquisition of the geotechnical and geological data, pit and excavation geometry, visual observations of instability and collection of specimens for laboratory studies. Spatial distribution of the strength properties of discontinuities and the density is determined in relation to lithology and major geological structures and mean shear strength and density evaluated. The data regarding slope geometry, field geological structure and strength parameters permit the assessment of failure. This study enables an assessment to be made of the original mine design against the risk of major structural instability. If the risk of failure is considered minor, a probablistic analysis of risk of failure is made and a strict inspection and examination routine for the slopes is established.
