The application of rock mass classification principles to coal mine design
This thesis aims to develop a rock mass classification system for UK Coal Measure strata such that the output from the classification system may provide a means by which the strength and stiffness properties of Coal Measure strata encountered within UK coal mines may be predicted. The development of the Coal Mine Classification system is described within this thesis. A structured methodology utilising a database of information obtained from 118 different rock mass classifications, together with consideration of the typical mechanisms of strata deformation within coal mines, was employed to determine the parameters of the Coal Measure strata that have the greatest influence on the engineering properties of the strata. These identified parameters have formed the basis of the Coal Mine Classification system. By comparison to a series of conceptual models of strata deformation that occur within the roof ,floor, ribs of roadways and within the region of the coal face, relative importance weightings and rating scales for the identified classification parameters have been proposed. The anisotropic nature of the UK Coal Measures is characterised within the Coal Mine Classification by the calculation of separate ratings for directions parallel to and perpendicular to bedding. An appraisal of the optimum method of using the classification ratings, determined by the Coal Mine Classification, to predict the strength properties of individual strata units was undertaken. Rock mass failure criteria that utilise outputs from existing rock mass classification systems to determine the rock mass strength, have been reviewed. Utilising published triaxial data the rock mass failure criterion that best predicts the failure characteristics of UK Coal Measure strata was identified. From this study the Hoek-Brown rock mass failure criterion was identified as the optimum existing criterion for predicting the intact strength and rock mass strength of Coal Measure strata. However this criterion was still found not to produce a close fit in many cases to the intact failure strength of the strata. A modified Coal Measure Failure criterion has been developed, which for a wide range of Coal Measure rock types was found to produce a better prediction of the intact strength of Coal Measure strata than any of the existing rock mass failure criteria. To determine the efficacy of the Coal Mine Classification system as a means of predicting the strength and stiffness properties of the rock mass the Coal Mine Classification was applied to the strata at case study localities within rock bolted roadways within three UK mine sites. Numerical models of the case study localities were developed using the FLAC finite difference code utilising a ubiquitous jointed elastic-perfectly plastic material model to simulate strata behaviour. The output from the modelling included predicted roof and rib side displacements, and these displacements were compared to the actual monitoring data for the case study localities. The results of the numerical modelling indicate that the predictions produced by the numerical models reflected the pattern and scale of deformations actually measured in-situ within the coal mine roadways, thus indicating that the Coal Mine Classification system provides a means of predictively determining the engineering properties of the in-situ Coal Measure strata. The modelling also indicated that time delays related to the installation of the roof extensometers may under predict that actual roof deformation that occurs within the roadway roof.