Rock mass rating and slope stability analysis of quarry faces within the dywka [i.e. dwyka] tillite of KwaZulu-Natal.

Abstract

Dwyka tillite quarries in the Province of KwaZulu-Natal have shown remarkable stable slopes faces even though some of them were quarried over 30 years ago. This can be attributed to their resistance to weathering, the high degree of joint surface roughness, the general lack of any joint infill and the limonitic staining found on most weathered joint surfaces. The latter appears to increase joint roughness. The high percentages of joints terminating within the rock mass or against other discontinuities as well as their low persistence results in a high degree of joint interlocking. These are shown to be very important factors contributing to the overall slope stability. Detailed discontinuity surveys were carried out at five different quarries located throughout the KwaZulu-Natal region. Only three of these quarries are presently being quarried. This allowed the study and comparison of joint and slope stability characteristics for both the older, more weathered rock faces and those of the recently quarried, and thus fairly unweathered rock faces. Joint orientation data from the various sites show that two to three sets of high angle joints and one low angle joint set are common. The potential of wedge and planar failure is therefore very high. The steeply dipping discontinuities also promote the potential for flexural toppling failure and this was noted in several of the quarry faces. Recognised geotechnical techniques and computer models were used to establish potential modes of failure and to estimate factors of safety. Wedge failure, at partially saturated and saturated conditions, was identified as being the main source of potential slope instability on the quarry rock faces. The quality of the rock mass of each slope was also classified according to various rock mass classification systems. The rock mass quality generally was rated as being 'fair' to 'good', meaning that slopes are partially stable to stable. The results of each rating system were also compared.