This paper is based on the collection of geotechnical and mining data relating to slope instabilities in British surface coal mines over a period of seven years. This research programme aims to develop a statistical approach to assess slope stability in British surface coal mines. Towards this aim the characteristics of slope instabilities have been analysed and appropriate probability distributions selected to describe them. This paper summarizes the development of an Instability Database, presents the distributions of certain instability characteristics and considers the manner in which a statistical approach will be applied to slope stability assessment.
Production of coal from surface mines in the United Kingdom is currently running at some 14 million tonnes annually and this production is achieved From 58 mines. The average seam width is approximately lm with up to 25 working seams in some mines. Generally British surface coal mines are small operations in very difficult geotechnical settings near urban environments and with very hith stripping ratio (sometimes up to 50:1). In order to exploit coal economically by surface mining techniques it is necessary to paydue attention to the stability of the excavations. Therefore between 1976 and 1984 a programme of research into slope stability problems in surface coal mines was carried out. Initial studies concentrated on the identification of mine slope characteristics in an effort to establish Factors influencing stability. During this period many records of slope failures were collected (Cobb, 1981, Scoble, 1981, Stead, 1984) to establish an Instabiliky Database (Skead, 1984), which records details of slope failures in rock slopes, loose- walls and external kips. This paper considers only mine rock slope Failures and not those in the other two mine slope catempties. One of the major aims of establishing the Database was to model the probabilistic nature of the input parameters to enable the development of a probabilistic model for British surface coal mine slope stability assessment. Whilst more data would be of assistance in the development of a probabilistic model, sufficient information is available to enable the development of a statistical approach to stability assessment, that would be of real value in the British surface coal mining environment. As a first stage to this work the geometric factors relating to rock slope instabilities have been analysed and their probability distributions defined.
2. THE INSTABILITY DATABASE
Information on slope failures in surface coal mines, over the last seven years, has been collected using standard reporting forms concerning the factors which defined slope instabilities. A range of quantitative and qualitative information was collected on some 240 instabilities during this period. This information included factors such as slope failure geometry and dimensions, structural geology, failure mechanisms, groundwater and other influencing parameters. Whilst the data recorded does not encompass the total number of instabilities occurring during the research period, and may include some regional bias, it does represent a reasonable sample and forms a comprehensible database on UK surface coal mine slope instabilities. The information was stored on a computer file to facilitate statistical analysis and efficient data retrieval. Much of the information was invaluable as the input parameters for back-analysis of past failures using limit equilibrium techniques.