Experimental Investigation of Critical Hydraulic Gradients for Unstable Soils

The presence of unstable soils, i.e. soils in which suffusion can arise, is a potential risk to structures under which seepage occurs. It is therefore necessary to clearly identify unstable soils and to estimate hydraulic gradients at which erosion may start. An experimental study was carried out to quantify critical hydraulic gradients of unstable soils with respect to vertical upward and horizontal flow. It was found that critical gradients for unstable soils lie in the range of 0.2 both for vertical and horizontal flow, with a small dependence on the relative density. For nearly stable soils a strong effect of the relative density on the critical hydraulic gradients was found. Also, the "more stable" a soil is, the greater the difference of critical gradients for vertical and horizontal flow. The obtained results are compared to the results of other researchers.