Analysis of velocity and acceleration trends using slope stability radar to identify failure ‘signatures’ to better inform deformation trigger action response plans

Abstract

Ground strata failure is a major hazard for open pit mines as it has the potential to cause damage to property and can result in multiple fatalities. Slope stability radar (SSR) systems are used to continuously monitor pit walls and they can detect slope deformation to sub-millimetre scales. However, geotechnical engineers may have limited prior data to set the required trigger action response plan (TARP) thresholds. As a result, arbitrary values or data from other sites are often used to signify dangerous deformation rates which can ineffectively trigger alarms. Therefore, the primary aim of this investigation was to identify the failure indicator factors which best inform TARP thresholds at a particular mine site. Data from eight open pit failures from the same mine were analysed and then compared with data from other published failures. A secondary aim was to a develop a database of combined failure events that could be used as a reference to set meaningful TARP threshold levels at other mine sites with similar mining conditions. The study site failure events ranged in size from 200 to 200,000 tonnes, with most failures occurring in the upper part of the slopes within highly to completely weathered rock. Geological and geotechnical characteristics of the rock mass for the observed failure modes were also included in the analysis. Multiple peak acceleration and peak velocity plots were used to determine clustering for the different characteristics investigated. It was shown that as the failure size increased so did the peak velocity, suggesting that larger failures can accommodate higher displacement rates. Analysis of the combined dataset showed a clear positive relationship between failure size and failure mode up to approximately 3,000 t. However, failure events greater than 3,000 t do not appear to have clear grouping by failure size, suggesting that other factors may control the peak acceleration and velocity rates. This suggests that the TARP should consider different trigger thresholds based on the expected failure mode and size. However, the accurate recording of all failure data across sites with additional characteristics such as, Rock Mass Rating (RMR89), GSI, weathering and lithology would enable improved analysis of velocity trends to provide further insights into factors influencing potential failure. It is concluded, that back-analysis of slope instability events using log-acceleration and log-velocity plots can refine thresholds used in TARPs for specific sites with the study site TARP presented. However, the consistent collection, processing and filtering of failure data across sites is required to improve analysis and implementation of findings.