A PRACTICAL PROCEDURE FOR THE BACK ANALYSIS OF SLOPE FAILURES IN CLOSELY JOINTED ROCK MASSES

Where closely jointed rock masses are encountered in slopes, failure c an occur both through the rock mass, as a result of combination of mac ro and micro jointing, and through the rock substance. Determination o f the strength of the category of rock mass is extraordinarily difficu lt since the size of representative specimens is too large for laborat ory testing. This difficulty can be overcome by using a non-linear roc k mass failure criterion or by back analysis of such slopes to estimat e the rock mass strength. In this paper, a practical procedure and a c omputer program are presented for the back determination of shear stre ngth parameters mobilized in slopes cut in closely jointed rock masses which obey a non-linear failure criterion rather than a linear one. T he procedure shows that the constants to derive normal stress dependen t shear strength parameters of the failed rock masses can be determine d by utilizing a main cross-section and without a pre-determined value of rock mass rating (RMR). Trials are made for different RMRm and RMR s values corresponding to various possible combinations of the constan t m and s, which are used in the Hoek-Brown failure criterion, satisfy ing the limit equilibrium condition. It is also noted that the procedu re provides a quick check for the rock mass rating obtained from the s ite investigations. The method is used in conjunction with the Bishop' s method of analysis based on circular slip surfaces. The procedure ou tlined in this paper has also been satisfactorily applied to documente d slope failure case histories in three open pit mines in Turkey. (C) 1998 Elsevier Science Ltd.