ABSTRACT:

The science and art of rock mechanics applied to stability of "soft rock" slopes in open pit coal mines is not as well developed as for "hard rock" metalliferous mines. The reasons for this are discussed. Because slopes are very steep, failure processes are often rapid with little warning. Safety and stability issues for excavated pit walls may involve block fall, multi-block toppling, or composite material-defect mechanisms. The rock mechanics literature is not well endowed with models for rational slope stability assessment under such conditions. This paper is based on several years of practice and observation. It sets out guiding concepts for stability assessment, including defect orientation mapping, continuity and persistence observations, mechanism definition, and analytical tools. Needs for research on visualisation, rock mass strength assessment, and paths to failure are identified.

INTRODUCTION

Open pit mining of black coal from the Bowen and Sydney-Gunnedah Basin regions is a major contributor to the Australian economy. Over the past 15Êyears there have been rapid developments of computer-based geological and mine planning techniques, and significant improvements in mining technology. Close to pit walls, the rock mass will be damaged by blasting effects as well. For typical seam thicknesses and economic stripping ratios, pit depths are rarely greater than 100Êmetres unless multiple seams are mined. Rock material strengths range from extremely low (UCS<2ÊMPa) to high (<60MPa) and often average 10 to 25ÊMPa. As a consequence, stress states in pits are such that failure mechanisms are controlled by defects. Because the rock mass fabric is typically blocky or columnar, the incidence of classic wedge or toppling mechanisms is low. Significant material weakening and mass relaxation deformations may occur with short-term exposure, but the nature of any intact material bridges between defects is difficult to observe let alone measure and categorise.

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