Historically, large infrequent collapses of hanging wall at Western Chrome Mines posed a major risk to the safety of underground employees as well as disrupting mine production. Such collapses may extend for up to 14m into the hanging wall of stope excavations and weigh in excess of 1000t. At Western Chrome Mines these collapses are most frequently associated with joints or dome-like structures. The normal examination method (visual inspection and sounding of the hanging wall rock) proved to be ineffective in detecting these structures. In many cases conventional detection for the presence of such structures is impossible as they rarely have a surface expression on the hanging wall. Since the introduction of a GSSI SIR-3000 Ground Penetrating Radar (GPR) in late 2002, major successes have been achieved regarding the early detection of problematic dome-like structures. This system has proved so effective that it has become an integral part of the monthly mine planning, with regular scans of the working panels now being conducted. Changing the location of support pillars, to optimally place them below such structures for support, is now a daily occurrence. Confidence in the system is so high that GPR data and the interpretations made by the operators form one of the building blocks of a newly designed Panel Risk Rating System, categorizing panels into different levels of anticipated risk.
1.1. Locality
Samancor Western Chrome Mines (WCM) is located in the western portion of the Bushveld Complex and is situated between Brits and Rustenburg approximately 80 km north west of Johannesburg South Africa, refer figure 1 for locality plan.
1.2. Geotechnical Environment
Chromium ore is produced from the Middle Group MG1 and MG2 chromitite layers which is situated in the Critical Zone of the Bushveld Complex. Both these layers are tabular deposits, dipping at an average of 9º south to north. The average stoping width for the MG1 layer is 1,35m and 2,6m for the MG2 seam. Middling between these two seams are on average some 13m refer figure 2 for a stratigraphic section[1].
The orientation of the major principal stress within the Bushveld Complex is horizontal with a K-ratio of between 1.8 and 2. This stress environment posses it?s own unique challenges, negatively influencing stability of flat dipping geological weaknesses.
