| Under certain conditions, clay and silt size particles in gap-graded clayey to silty sands can be transported out of the soil matrix under seepage flow. When a significant amount of these particles is removed or lost, the soil permeability can increase markedly. This process, which is termed piping or "external suffusion", is potentially dangerous when the soil is part of an earthen hydraulic structure or barrier.;Piping in these soils usually occurs in two steps: (1) particle detachment and (2) particle entrainment. The factors favoring particle detachment include a high seepage velocity and all factors known to increase electrical double layer repulsion. The capture mechanisms that can impede particle entrainment in these soils include straining, plugging, sedimentation, Brownian diffusion and hydrodynamic action.;Existing analytical procedures for predicting the internal stability of cohesionless soils are primarily based on grain size distribution. These procedures are either theoretically inadequate or are too conservative to be applied to clayey to silty sands. To evaluate the internal stability of the latter soils, an alternative analytical method is proposed based on consideration of the soil's controlling constriction size (D;To establish the limits, flow tests were conducted on various compacted mixtures of clayey to silty sands. The clay particles, which have low swelling potentials, were initially dispersed and entrained in the seepage stream using distilled water as a permeant. A flow regime consisting of flow direction reversals coupled with changes in permeant electrolyte concentration was devised to discriminate between internally stable and unstable soils. Soils were classified as internally unstable when the permeability ultimately increased above the initial value.;The analyses described herein shed useful insights into the combined influence of soil granulometry and pore water chemistry on the phenomenon of external suffusion. They also provide a means of identifying potentially unstable clayey to silty sands on the basis of soil gradation properties and related parameters. |
