ABSTRACT:
The caving of stope backs in high stress conditions creates severe safety and economic problems in blasthole stopes. When this instability started to occur at Xstrata’s Kidd Mine, a solution was sought as a matter of urgency. The back and hangingwall failures appeared to be related to high horizontal stress that initiated failure in locally weaker wall rock units. This caused post-failure displacements large enough to "push" the failed material from the back into the stopes, creating instability and dilution problem. A field programme was undertaken to identify characteristics that accentuated the failures, and in parallel, a FLAC3D model was developed to mimic the observed failures. This FLAC3D model is unusual in that the geometry of the model changes during the simulation. This allows one to follow the evolution of failure and the progression of the caved material up into the back. Several innovative algorithms (using FLAC3D’s built-in FISH language) were developed to allow the model to behave in the same way as the observed failures. After the model was calibrated, it was then used to investigate the most important controlling factors that govern the back stability problem. This calibration and forward prediction exercise allowed the mine to make the most cost-effective changes to the planned stope layouts to ensure that subsequent stopes could be extracted in a controlled fashion. Based on the work, a simple design tool was developed to assist in the design of stope shape and back support. In the two years following development of the tool it has been used successfully in the design of many stopes, and has largely eliminated the stope back caving problem.
1 INTRODUCTION
Xstrata’s Kidd Mine currently produces from some of the deepest blasthole stopes in the world, where little or no empirical geomechanical design data exist.
