The Mechanical Coupling of Fluid-Filled Granular Material Under Shear

Abstract

Abstract. The coupled mechanics of fluid-filled granular media controls the physics of many Earth systems such as saturated soils, fault gouge, and landslide shear zones. It is well established that when the pore fluid pressure rises, the shear resistance of fluid-filled granular systems decreases, and as a result catastrophic events such as soil liquefaction, earthquakes, and accelerating landslides may be triggered. Alternatively, when the pore pressure drops, the shear resistance of these geosystems increases. Despite the great importance of the coupled mechanics of grains-fluid systems, the basic physics that controls this coupling is far from understood. Fundamental questions that need to be addressed are what are the processes that control pore fluid pressurization and depressurization in response to deformation of the granular skeleton? and how do variations of pore pressure affect the mechanical strength of the grains skeleton? To answer these questions, a formulation for the pore fluid pressure and flow is developed from mass and momentum conservation, and is coupled with a granular dynamics algorithm that solves the grain dynamics, to form a fully coupled model. The pore fluid formulation reveals that the evolution of pore pressure obeys a viscoelastic rheology in response to pore space variations. Elastic-like behavior dominates with undrained conditions and leads to a linear relation between pore pressure and overall volumetric strain. Viscous-like behavior dominates under well drained conditions and leads to a linear relation between pore pressure and volumetric strain rate. Numerical simulations reveal the possibility of liquefaction under drained and initially over-compacted conditions, which were often believed to be resistant to liquefaction. Under such conditions liquefaction occurs during short compactive 1 phases that punctuate the overall dilative trend. In addition, the more established generation of elevated pore pressure under undrained compactive conditions is observed. Simulations also show that during liquefaction events stress chains are detached, the external load becomes completely supported by the pressurized pore fluid, and shear resistance vanishes.

Towards a complete model of soil liquefaction: the importance of fluid flow and grain motion

“…Also, owing to our model of this drainage, we can identify several modes of liquefaction, from fluid-flow-induced 'fast settlement' to localized water film formation at permeability boundaries, to flow-induced liquefaction of low permeability layers that overly a high permeability layer, and even potentially thermally induced liquefaction when permeability is very low. In each of these example cases, we do not attempt to couple the behaviour of the grains to that of the water, or provide time-varying dynamic solutions, as no large-scale continuum model is available for grain-fluid interaction behaviour comparable in quality to the DEM model of [7]. Our intention in this paper has been to highlight the importance of fluid flow and spur an understanding of the need to develop soil models that can account for flow.…”

“…In [7], a two-dimensional DEM model was coupled to a continuum model for fluid flow, and the interactions of grains and fluid were calculated for a sample of a few thousand grains. Their conditions are typical of 200 m-to 2 km-deep thin deposits sheared at ord(1) to ord (10) which are relevant for fault gouge conditions.…”

Bridging aero-fracture evolution with the characteristics of the acoustic emissions in a porous medium

“…The fluid can be explicitly considered by expressing this law in terms of total stress and fluid pressure, using Equations (8,9) which leads to a formulation corresponding to Terzaghi's 1936 effective stress formulation [77,78]:…”

“…However, the idea of putting them together via a system of deformable porous medium with a fluid flow makes the phenomena even harder to understand. Rapid changes in the porosity of the medium due to fluid flow, channeling and fracturing via momentum exchange with the flow make understanding the mechanics of the system a challenge [6][7][8][9]. Hydraulic fracturing of the ground is a good example for this coupled behavior of solid and fluid phases.…”

Granular suspension avalanches. II. Plastic regime

“…In the mechanics of porous media composed of a rigid skeleton, the diffusion timescale is negligibly small for incompressible interstitial fluids, 23 but when the medium is composed of loose grains, authors 19,24,25 have suggested that bulk compressibility gives rise to pore pressure diffusion even though the interstitial fluid is incompressible, as is obvious in Fig. 2.…”