The permeability of composite soils

Many natural soils (e.g. glacial tills, residual soils, and alluvial soils) and artificial soils (e.g. engineered fill, environmental barriers) are composite soils formed of a range of particle sizes and types. These soils are often difficult to sample and test when following standard site investigation practice thus it is necessary to resort to empirical correlations, most of which were developed for either coarse-grained (e.g. clean sands) or fine-grained (e.g. pure clays) soils. The hydraulic conductivity of clean sands is a function of the void ratio and particle size distribution, and for clays, the clay type and void ratio. This suggests that the hydraulic conductivity of composite soils will be a function of these properties and the clay content. Composite soils formed of four clay minerals and two sands were consolidated from slurry to determine the variation of hydraulic conductivity with clay content, clay type and void ratio. With matrix dominated soils, soils that contain with more than 35% fines content, the flow is a function of the matrix void ratio and clay type; and, for clast dominated soils, soils that contain less than 20% fines content, the intergranular void ratio and particle size distribution. The behaviour of soils with a fines content between 20% and 35% depends on the confining pressure and density.