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

Three published rock mass classification systems (RMR, SMR, RMS) were appli ed to weak Waikato Coal Measure mudrocks in order to assess their value as indicators of rock mass conditions and stable slope angles. The SMR classif ication gives the most sensitive measure of rock mass conditions for the sl opes studied, but none of the systems adequately predicts observed slope an gles. Regression analysis indicates that where conditions for failure by sl iding along discontinuities exist the slopes are most sensitive to the disc ontinuity parameters of parallelism, dip angle and spacing. Further, when t he 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 un favourable discontinuity sets are well predicted by published equations ass ociated with both the RMR and RMS classification systems. Steeper slopes de veloped on favourable discontinuities are at a much lower angle than the eq uations predict and show evidence of extensive creep. Application of the pr esent rock mass classification systems to these weak rocks is thus only app ropriate when conditions exist under which the rocks fail by sliding on unf avourably oriented discontinuities. Where this does not occur, the contribu tion of intact strength to the rock mass strength is greatly overestimated by all of the ruck mass classification systems studied. Development of a se parate rock mass classification for these conditions is not seen as appropr iate. Hence, recognition of the favourability or otherwise of the discontin uity sets is crucial to dealing with these rocks. A stereonet overlay that facilitates this division is presented. (C) 2001 Elsevier Science B.V. All rights reserved.