As is well known from rock mechanics literature, the mechanical parameters of a rock mass cannot be obtained by conventional laboratory tests due to the difficulties encountered in the preparation of cores from a rock mass containing discontinuities. To overcome these difficulties, researchers have focused on developing empirical equations for predicting stress-strain behavior of a rock mass, including based on measurements of the discontinuity patterns. However, the UCS value of rock mass (UCSRM) can be predicted by decreasing UCSi based on quality of rock mass such as Rock Mass Rating (RMR), Geological Strength Index (GSI), Q value, etc. For this reason, an empirical equation in a unique reducing curve form has limited application in generalizing on the prediction of UCSRM from particularly soft rock mass to hard rock mass. In this study, a new empirical approach is developed to be used for predicting of strength of rock masses from soft to hard rock masses. In addition, a new procedure for defining of the disturbance effect on the strength of rock mass is introduced to the new empirical approach in conjunction with Hoek and Brown failure criterion.
Determination of overall strength of jointed rock masses have been attractive research topic in rock mechanics community due to the difficulties encountered during the preparation of representative undisturbed samples for laboratory studies. The sophisticated and large scaled laboratory equipments are needed to perform experiments on undisturbed jointed rock mass even if the cores can be obtained. Therefore empirical equations have been used as practical tools during pre-design stages of engineering application to be constructed in/on rock masses. To overcome this difficulty, numerous empirical equations have been proposed in the literature and as given in Figure 1 (Yudhbir et al., 1983; Ramamurthy, 1986; Kalamaris and Bieniawski, 1995; Sheorey, 1997; Aydan and Dalgic, 1998; Heok and Brown, 1997 etc.). By using most of the existing empirical approaches, the UCS value of rock mass (UCSRM) can be predicted by reducing UCSi based on quality of rock mass such as Rock Mass Rating (RMR), Geological Strength Index (GSI), Q value, etc (Bieniawski 1989, Hoek and Brown, 1997 and Barton 2002). At this point, the question of "which one is the best for prediction of the strength of a rock mass?" cannot be definitively answered, as each of them tried to represent their original database. Most of the empirical equations consider just RMR, GSI or Joint Parameter-JP as an input parameter. But these characterization schemes may not explicitly include the strength and deformability of rock material may play some important role on the strength behavior of particularly soft rock mass. As can be followed from literature, the lowest strength of rock materials used for the calibration of Hoek and Brown empirical failure criterion is about 39.9 MPa (Hoek and Brown, 1980). Therefore, it is difficult said that the Hoek-Brown failure criterion with its current form has sufficiently a capable of prediction of strength of rock masses composed of particularly softer rock materials.
