Constriction size distributions of granular filters: a numerical study

The retention capability of granular filters is controlled by the narrow constrictions connecting the voids within the filter. The theoretical justification for empirical filter rules used in practice includes consideration of an idealised soil fabric in which constrictions form between co-planar combinations of spherical filter particles. This idealised fabric has not been confirmed by experimental or numerical observations of real constrictions. This paper reports the results of direct, particle-scale measurement of the constriction size distribution (CSD) within virtual samples of granular filters created using the discrete-element method (DEM). A previously proposed analytical method that predicts the full CSD using inscribed circles to estimate constriction sizes is found to poorly predict the CSD for widely graded filters due to an over-idealisation of the soil fabric. The DEM data generated are used to explore quantitatively the influence of the coefficient of uniformity, particle size distribution and relative density of the filter on the CSD. For a given relative density CSDs form a narrow band of similarly shaped curves when normalised by characteristic filter diameters. This lends support to the practical use of characteristic diameters to assess filter retention capability

Fabric and Effective Stress Distribution in Internally Unstable Soils

Internal instability is a form of internal erosion in broadly graded cohesionless soils in which fine particles can be eroded at lower hydraulic gradients than predicted by classical theory for piping or heave. A key mechanism enabling internal instability is the formation of a stress-transmitting matrix dominated by the coarse particles, which leaves the finer particles under lower effective stress. In this study, discrete element modeling is used to analyze the fabric and effective stress distribution within idealized gap-graded samples with varying potential for internal stability. The reduction in stress within the finer fraction of the materials is directly quantified from grain-scale data. The particle-size distribution, percentage finer fraction, and relative density are found to influence the stress distribution. In particular, effective stress transfer within a critical finer fraction between 24 and 35% is shown to be highly sensitive to relative density