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The coupled mechanics of fluid-filled granular media controls the behavior ofmany natural systems such as saturated soils, fault gouge, and landslides. Thegrain motion and the fluid pressure influence each other: It is wellestablished that when the fluid pressure rises, the shear resistance offluid-filled granular systems decreases, and as a result catastrophic eventssuch as soil liquefaction, earthquakes, and accelerating landslides may betriggered. Alternatively, when the pore pressure drops, the shear resistance ofthese systems increases. Despite the great importance of the coupled mechanicsof grains-fluid systems, the basic physics that controls this coupling is farfrom understood. We developed a new multi-scaled model based on the discreteelement method, coupled with a continuum model of fluid pressure, to explorethis dynamical system. The model was shown recently to capture essentialfeedbacks between porosity changes arising from rearrangement of grains, andlocal pressure variations due to changing pore configurations. We report herenew results from numerical experiments of a continuously shearing layer ofcircular two-dimensional grains, trapped between two parallel rough boundaries.The experiments use a fixed confining stress on the boundary walls, and aconstant velocity applied to one of the boundaries, as if this system was theinterior of a sliding geological fault filled with 'fault gouge'. In addition,we control the layer permeability and the drainage boundary conditions. Thispaper presents modeling results showing that the localization of shear (into anarrow shear band within the shearing layer) is strongly affected by thepresence of fluids. While in dry granular layers there is no preferred positionfor the onset of localization, drained systems tend to localize shear on theirboundary. We propose a scaling argument to describe the pressure deviations ina shear band, and use that to predict the allowable positions of shearlocalizations as a function of the fault and gouge properties.

Localization of Shear in Saturated Granular Media: Insights from a Multi-Scaled Granular-Fluid Model