The value of rock mass classification systems for weak rock masses: a case example from Huntly, New Zealand

Abstract

Three published rock mass classification systems (RMR, SMR, RMS) were applied to weak Waikato Coal Measure mudrocks in order to assess their value as indicators of rock mass conditions and stable slope angles. The SMR classification gives the most sensitive measure of rock mass conditions for the slopes studied, but none of the systems adequately predicts observed slope angles. Regression analysis indicates that where conditions for failure by sliding along discontinuities exist the slopes are most sensitive to the discontinuity parameters of parallelism, dip angle and spacing. Further, when the discontinuities are oriented favourably with respect to the slope, slope angles are most sensitive to intact rock strength and groundwater. This is supported by geomorphic evidence. Low angle natural slopes developed on unfavourable discontinuity sets are well predicted by published equations associated with both the RMR and RMS classification systems. Steeper slopes developed on favourable discontinuities are at a much lower angle than the equations predict and show evidence of extensive creep. Application of the present rock mass classification systems to these weak rocks is thus only appropriate when conditions exist under which the rocks fail by sliding on unfavourably oriented discontinuities. Where this does not occur, the contribution of intact strength to the rock mass strength is greatly overestimated by all of the rock mass classification systems studied. Development of a separate rock mass classification for these conditions is not seen as appropriate. Hence, recognition of the favourability or otherwise of the discontinuity sets is crucial to dealing with these rocks. A stereonet overlay that facilitates this division is presented.