Vol. 3, No. 5, 2008

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Effective properties of shear band structures in rock at different length scales

John Napier

Vol. 3 (2008), No. 5, 953–968

Failure localization in rock is observed ubiquitously on geological scales in the form of fault or earthquake damage structures. Similar failure processes are observed in confined compression tests carried out on laboratory-scale rock samples. At an intermediate scale, seismic activity is often associated with the formation of so-called burst fractures that are intermittently formed and exposed in the vicinity of deep level mining operations. Computational modeling can assist the understanding of the complex nature of these failure processes. The present study investigates the question of how the properties of macroscopic shear band features are controlled by microscopic constitutive behavior. The computational approach that is used is to consider the formation of shear band structures by selectively mobilizing members of an assembly of randomly oriented cracks that are modeled as displacement discontinuity elements. Particular issues that are addressed are the question of whether the microscopic failure processes are self-similar to the macroscopic processes, and how the density of the discontinuity assembly affects the localization patterning. It appears that the use of slip or tension-weakening constitutive models yields equivalent “macro” results that are independent of the “micro” mesh density for a given mesh type. If the intrinsic junction coordination of the mesh is altered, it is found that the equivalent macro dilation angle is changed. This has important implications in determining whether a particular distinct element or lattice model with an intrinsic junction structure is capable of replicating the observed failure behavior of a given rock type. A dimensionless parameter group is suggested as a measure of the intrinsic coordination number for a random crack model of rock micro structure.

fracturing, numerical modeling, shear bands
Received: 24 August 2007
Accepted: 10 December 2007
Published: 1 July 2008
John Napier
School of Computational and Applied Mathematics
University of the Witwatersrand
South Africa