Rock mass deformation is intimately controlled by the presence of discontinuities. The most obvious discontinuities are joints and fractures which can be observed in exposures and rock cores. Just as significant are discontinuities due to changes in stiffness resulting from lithology and fracture density. Discontinuities within the rock mass are typically taken into consideration using well established procedures for rock mass classification. By extending this procedure to classify individual strata rather than the overall rock mass, and using empirical correlations, it is possible to estimate a stiffness for each stratum. Measurements have verified modeled behavior of horizontally bedded sedimentary rock overlying high extraction coal mine panels. The locations and magnitudes of simulatedisplacements are consistent with those measured using time domain reflectometry (TDR). The importance of incorporating variations in rock stiffness and plastic sliding along "stiffness discontinuities" into numerical models has been confirmed by experience with TDR monitoring on many projects.


The emphasis in this approach is the identification and explicit modeling of displacements along discontinuities. This has been found to be an integral component of rock mass response to changes in shear stress. The approach is outlined in this paper using an example in which we modeled rock mass response to high extraction underground coal mining.

Verification was done by comparison between measured and predicted surface subsidence and measured and predicted subsurface displacements. The paper closes with a brief discussion of experience with TDR measurements in other projects which have demonstrated repeatedly that plastic slip along "stiffness discontinuities" is a predominant component of rock deformation.

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