Shear localization in sands with fluids

Deformation banding is one of the most common failure modes in geomaterials such as rock, concrete, and soil. It is well known that appearance of bands of intense localized deformation significantly reduces the load-carrying capacity of any structure that develops them. Furthermore, when dealing with fluid-saturated geomaterials, the interplay between the contraction/dilation of pores and development of pore fluid pressures is expected to influence not only the strength of the solid matrix but also its ability to block or transport such fluids. Accurate and thorough simulation of these phenomena (i.e., deformation banding and fluid flow) requires numerical models capable of capturing fine-scale mechanical processes such as mineral particle rolling and sliding in granular soils and the coupling between porosity and relative permeability. Until recently, these processes could not even be observed in the laboratory. Numerical models could only interpret material behavior as a macroscopic process and were, therefore, unable to model the very complex behavior of saturated geomaterials accurately.

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The phenomenon of liquefaction occurs in saturated cohesionless soils when the rate of loading is so rapid that positive excess pore pressures develop before the water can drain away. As the pressures build up, the effective stress and shear strength may decrease to zero, causing the soil to behave like a liquid. Recently, mathematical conditions have been proposed to signal the onset of static liquefaction in constitutive models. In this movie, the difference between the hardening modulus H and a critical hardening modulus Hcrit is plotted across each Gauss point in a finite element simulation, where H is a variable that describes the evolution of the elastic region with respect to plastic strains. Liquefaction occurs in areas where H-Hcrit<0. The behavior of the loose sand specimen shown here undergoing plane strain compression trends toward a shear band. By the end of the movie, the interstitial fluid flow (denoted by white arrows) reveals that water is expelled from the shear band and that the specimen exhibits elastic unloading (denoted by dark gray) in regions outside of the shear band. Note: Deformations are magnified x10.

Static liquefaction in sands