Rock mass classification systems, with suitable modifications, serve as a simple empirical approach for the design of various underground mine structures. There is a continuous effort both towards improving the scope of some of the existing classification systems as well as to introduce new classification systems. One of the primary objectives of the classification systems for a practicing engineer has been to make it simple to use as a preliminary design tool for underground mine structures in rockmass. A detailed survey was undertaken at the underground limestone mines. From the survey data safe unsupported span was estimated using RMR, Q and Q". The modifications to the rock mass quality Q are suggested duly considering the influence of width-height factor on stress and strength conditions of rockmass surrounding underground openings, the joint orientation and the hydrological condition of the mine.
It is proposed to develop a site specific rockmass classification system to be used by the limestone mining companies as an easy to use design tool for a preliminary design of underground structures in Korea. As a first step towards this two underground limestone mines were selected in which both the joint surveying for structural geological data as well as hydraulic fracturing studies for estimating the in situ stresses required for rockmass classification were undertaken.
The evaluation of stability of openings is a fundamental step in the design of underground structures. Currently, there are no generally accepted methods of deciding which ground control measures are best suited to a particular mining situation and therefore the approaches are remaining to site specific conditions. In the absence of a single accurate method, empirical method in conjunction with numerical studies is a much opted approach for the type of uncertainties encountered in designing underground openings. In the present investigations safe unsupported spans were empirically estimated following RMR and Q classification systems. Time dependent strength deterioration and the effect of opening dimensions are of primary importance to the stability issues of mine structures. Therefore, attempts were made in the present investigation to account for these factors in the existing classification systems.
In the recent years, rockmass classification systems have been used in tandem with analytical and numerical tools. Therefore, there has been a proliferation of work linking classification indexes to rock mass properties, such as modulus of elasticity, compressive strength, rock parameters such as m and s for Hoek and Brown failure criterion, etc (Hoek & Brown, 1997). The values are then used as input parameters for the numerical models. Consequently importance of rock mass classification systems has increased over time (Milne, 1998). Unlike tunneling operations, underground mine openings are more controlled by time, in the sense that, the opening numbers as well as their dimensions change with the progress in life of a mine. Following the current approaches of Q and RMR would result in a mine with same rock mass having dozens of classification values through out the mine.