Survey and geotechnical slope monitoring considerations

Monitoring of the slopes of surface mines can prevent loss of life, equipment, production and possibly the loss of the mine. The effectiveness of such monitoring depends on the extent to which slopes give adequate advanced warning before failing, and on the ability of the monitoring system to detect such warning. There is considerable evidence (for example, Ding et al., 1998) to indicate that slopes do give ample warning, which means that it is well worth the effort to spend time on investigating and implementing appropriate monitoring systems. Figure 1 illustrates the value of survey monitoring at Venetia Diamond Mine, where the exponential movement at Prism M11-3 indicated on the survey charts gave advanced warning of ultimate failure. It is important to note, however, that in the hard rock open pits that are being mined nowadays, failures often occur after very small deformations. Such failures may be very localised and, unless such a localized area is being very closely and accurately monitored, failure may occur apparently without warning. The dictates for such situations are therefore accurate, remote monitoring. This paper gives an overview of the survey and geotechnical considerations for effective monitoring systems as established from a literature survey, various case studies and observation. The monitoring process normally constitutes five steps. It starts with monitoring requirements. This is to establish the objectives, need, advantages (expressed in both economic and health and safety terms), researching historic problems as case studies and finally preparing a report to decisionmakers motivating the need for monitoring along with the expected benefits. The second step is to establish the project requirements, which follows from the pit design and a risk assessment. The outcome of this will state the probability of failure and the variables that will most likely contribute to such failure. The third step will take account of these variables in designing the monitoring system, investigating appropriate instrumentation and implementing the monitoring system (Ding et al., 1998). An overview of these considerations is the subject of this paper. The fourth step is the actual measurement and recording of field data. Issues to be dealt with, which will have been thoroughly considered during the design of the monitoring system, are measuring techniques and frequencies, accuracy, precision and personnel responsibilities. The fifth and final step is interpretation and reporting of monitoring data. This involves processing, analysis, interpretation Survey and geotechnical slope monitoring considerations