I understand its originally developed for underground tunnels in hard rocks. But I am interested in understanding the practical difficulties that are encountered while using RMR?
There are no practical difficulties when using RMR provided that one considers the limitations of the method. It is an empirical rock mass classification method used for an indicative estimation of the rock mass quality and not as the primary tool for the design of primary support. It was developed mainly to estimate adequate rock support measures and in a lesser degree for a crude estimate of the deformation modulus of rock masses. It doesn't take into account very high or very low stress levels in the rock mass or zones of weak materials (fault zones, shear zones etc) or swelling rock all of which reduce the overall rock mass quality. Therefore the method should not be applied as is in poor quality rock masses (heavily jointed, sheared or fractured etc) although some modifications for the method have been proposed by various authors.
Marcelo Eduardo Rojas Vidal
Hi,
During the course of Literature review I understood that one of the reason that GSI was developed because that time they need something which doesn't rely more on numbers as RMR does and one more reason is that it was difficult to use RMR for poor rock masses. But in the paper there wasn't any description about what made it difficult. So I posted my question here.
GSI can be used as a tool for characterization of rock masses but it is not actually a rock mass classification method. Its main purpose is to provide the mechanical properties of an isotropic rock mass ie the rock mass strength based on the geological parameters of the rock mass via application of the Hoek Brown failure criterion. Its correlation with RMR has been used in former times but it does not produce sensible results for poor quality rock masses. RMR"s application in poor quality rock masses does not provide realistic results in terms of support measures due to the origin of this empirical classification method which was developed using data mostly from tunnels in competent rocks. GSI provides a means of reduction of the intact rock strength to the actual rock mass strength under various geological conditions.
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