Modifications to the geological strength index (GSI) and their applicability to stability of slopes

Determination of the strength of closely jointed rock masses is difficult since the size of representative specimens is too large for laboratory testing. This difficulty can be overcome by using the Hoek-Brown failure criterion. Since its introduction in 1980, the criterion has been refined and expanded. over the years. particularly due to some limitations in its application to poor quality rock masses. In the latest version, the geological strength index (GSI) was introduced into the criterion by its originators. However, the GSI classification scheme, in its existing form, leads to rough estimates of the GSI values. Another particular issue is the use of undisturbed and disturbed rock mass categories for determining the parameters in the criterion, for which clear guidelines are lacking. Furthermore, the data supporting some of these revisions, particularly the latest one, have not been published, making it difficult to judge their validity. In this study, in order to provide a more quantitative basis for evaluating GSI values, some modifications are suggested by introducing easily measurable parameters with their ratings and/or intervals which define the blockiness and surface condition of discontinuities. In addition, a method is proposed to assess the influence of disturbance on rock mass constants due to the method of excavation. The modifications to the GSI and the suggested method have been applied to slope instability case histories selected from Turkey by performing back analysis, to discuss the validity of the criterion and the methodology of parameter estimation. It was shown that the failure conditions in each case were confirmed, i.e. the analysed failure surfaces satisfied factors of safety of unity, when the suggested modifications and disturbed rock mass condition are considered. On the basis of the results, a chart to assess the effect of disturbance in terms of method of excavation was also suggested. The back analysis of a spoil instability indicated that spoil pile materials consisting of blocky and angular rock pieces could be categorized as a disintegrated rock mass in the GSI classification and the criterion seemed to be applied to such materials. The method suggested herein must, however, be verified by additional data from slope failures before more precise guidelines can be formulated. (C) 1999 Elsevier Science Ltd. All rights reserved.