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Experiments on the internal stability of widely graded cohesionless soils Moffat, Ricardo


Internal instability of widely-graded cohesionless soils results from an inability to prevent loss of the finer particles in the presence of forces induced by seepage flow and, on occasion, vibration. Instability is governed by a combination of geometrical and hydromechanical constraints, and therefore largely controlled by soil type and seepage regime. Performance monitoring of embankment dams suggests the phenomenon is an important factor in the overall stability of these structures. A large permeameter test device was designed and commissioned to perform unidirectional seepage flow tests on four widely-graded cohesionless soils, all of which are potentially unstable. The objective was to assess the main hydromechanical factors that influence the onset of instability. A modified slurry mixing technique, with discrete deposition, was found satisfactory for reconstitution of homogeneous saturated test specimens. Test variables examined include hydraulic gradient, rate of increase in hydraulic gradient, flow direction, and vertical effective stress. The onset of internal instability was found to be triggered either by an increase in hydraulic gradient, or by a decrease in vertical effective stress. The relation between effective stress and hydraulic gradient, where the onset of instability occurs, is used to characterise a hydromechanical constraint to internal stability. Each soil yielded a hydromechanical boundary, and the difference between them is attributed to the shape of the grain size distribution of the soil and to the variation of vertical effective stress in the direction of flow where suffosion initiated. A limit-equilibrium approach is used to explain the experimental findings on hydromechanical constraints, based on a geometrical characterization of the gradation curve and for an assumed distribution of effective stress between coarser and finer fraction particles of the soil. Analysis of the experimental data established that the hydromechanical boundary is not unique: it is governed by both the magnitude of vertical effective stress and the variation of that stress in the direction of flow.

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