DISTINCT ELEMENT MODELING OF DEFORMATION BANDS IN SANDSTONE

We have conducted numerical experiments with the distinct element method to study factors that control the development of deformation bands in sandstone. These experiments show how sorting and initial porosity of the host rock control the development and the mode of deformation in the area of strain localization. The results of the numerical experiments are in qualitative agreement with field and microstructural observations of deformation bands at Arches National Park (Utah). In our numerical experiments sand grains are modeled as cylindrical elements that move in response to externally applied boundary conditions. Systems of elements that have a large variability in radius and/or loose packing deform at lower applied stresses than systems of elements that have a uniform radius and/or tight packing. The mode of deformation in the first kind of aggregate is particulate flow, where elements of different sizes move easily with respect to each other due to a low degree of interlocking. The mode of deformation in the second kind of aggregates is localized failure on small deformation bands. Shear bands in our numerical experiments nucleate as a zone of dilatancy and propagate via organization of dilatant zones into discrete faults. The presence of a flaw in the form of a 'weak' grain promotes the nucleation and propagation of shear bands.