The experiences in evaluating the rock mass quality with the help of Endoscopic Rock Mass Factor (ERMF) are described. ERMF was worked out by the authors and research was carried out in the Polish underground coal mines. ERMF enables the quantitative and qualitative estimation of fracture number and size occurring around underground roadways. The new method of rock mass quality estimation is correlated with RQD index, but the investigations are done directly in the bore-holes. This is an "in situ" method, which takes into account all natural factors influencing the mine working stability. As the range of the crack zone and joint intensity affect the type of working support used, the ERMF factor was presented in view of support and the stand-up time of an excavation. Evaluations of roof separation and support loading carried out in underground mine roadways in relation to ERMF show that the new method of rock mass quality evaluation is effective in support design for Polish underground hard coal mines.
1. INTRODUCTION
The main task for mining engineers is to ensure the stability of a working after drifting. It concerns both chambers and roadways. The appearance of fracture zones in the rocks around the working are caused by the undesirable effects of induced stresses that exceed rock strength. The range of the fracture zone and joint intensity depends on several technological and geological factors, among them: primary stress (the depth of exploitation), type of rock, rock mass physical properties, rock layer inclination, the way of drifting, and previous exploitation influence [1, 2]. The stability of deep underground excavations depends upon the strength of the surrounding rock mass and upon the stresses induced in this rock. After mining in jointed rock, the rock mass is in fact made up of an interlocking mesh of discrete blocks. The strength of the jointed rock mass depends on the type and roughness of the joint surfaces between the blocks [3,4]. The scheme of support for the given roadway is selected for the type of crack zone [1,2,4]. On account of the importance of parameters relating to discontinuities (i.e. number of joints, their distribution, the character of joints), they are applied in a series of geotechnical classifications and they describe rock mass quality. Tunnelling quality index Q, distinguishes the number of fracture systems, roughness of joints and weathering of joint walls. Grimstad and Barton suggest a relationship between the value of Q and the permanent roof support pressure using joint set number and joint roughness. Palmström describes RQD by the number of joints per cubic meter. Parameters relating to joints are also included in the most well-known geomechanical classifications based on the RMR system (joint spacing, character of joint surfaces and orientation of joints in relation to the load direction) [4,5,6,7]. The authors of this paper present an evaluation method for rock mass quality on the basis of endoscopic observations. Such tests were carried out for several coal mines in Poland using a borehole endoscope and its realistic pictures of discontinuities inside the rock mass [2,8,9].
