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 s ince the size of representative specimens is too large for laboratory testi ng. This difficulty can be overcome by using the Hoek-Brown failure criteri on. Since its introduction in 1980, the criterion has been refined and expa nded. over the years. particularly due to some limitations in its applicati on to poor quality rock masses. In the latest version, the geological stren gth index (GSI) was introduced into the criterion by its originators. Howev er, the GSI classification scheme, in its existing form, leads to rough est imates of the GSI values. Another particular issue is the use of undisturbe d and disturbed rock mass categories for determining the parameters in the criterion, for which clear guidelines are lacking. Furthermore, the data su pporting some of these revisions, particularly the latest one, have not bee n 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 wi th their ratings and/or intervals which define the blockiness and surface c ondition of discontinuities. In addition, a method is proposed to assess th e influence of disturbance on rock mass constants due to the method of exca vation. The modifications to the GSI and the suggested method have been app lied to slope instability case histories selected from Turkey by performing back analysis, to discuss the validity of the criterion and the methodolog y 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 mas s 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 suggest ed. The back analysis of a spoil instability indicated that spoil pile mate rials consisting of blocky and angular rock pieces could be categorized as a disintegrated rock mass in the GSI classification and the criterion seeme d to be applied to such materials. The method suggested herein must, howeve r, be verified by additional data from slope failures before more precise g uidelines can be formulated. (C) 1999 Elsevier Science Ltd. All rights rese rved.