An Alternative Approach to Rock Mass Rating, RMR Determination from Geological Strength Index, GSI for Limestone Rock Mass, Ipoh, Perak, Malaysia

Ipoh, an urban centre in northern Peninsular Malaysia is somewhat unique in that its bedrock is limestone, since in many parts of the world limestone areas are avoided as urban centres. In terms of engineering geology, this avoidance is related to the existence of subsurface cavities and the instability of limestone hills. The characterisation and classification of the limestone rock mass is further aggravated by poor accessibility to these limestone hills. Bieniawski’s rock mass rating, (RMRbasic1989) and the Geological Strength Index, GSI are two of several rock mass classification systems available for rock mass classification. While the GSI determination is largely based on field observations, the determination of RMRbasic1989 requires discontinuity surveys in the field as well as rock material strength determinations. Comparison of the GSI determined directly in the field with that derived by applying the formula GSI = RMRb89 – 5 for 20 limestone hills in the Ipoh area had a variance of 15%. Therefore, a modified correlation has been developed. This is GSI = 0.889RMRb89 – 5 and the GSI value determined using this had a difference of only 7.6% compared to the field GSI determination. Thus, this modified correlation offers an alternative approach for determining RMRb89 without the need to conduct discontinuity surveys and laboratory testing. This correlation is specific for the Kinta Limestone. For other areas, this approach can be applied to establish a specific correlation for that specific lithology.