Suffusion and clogging by one-dimensional seepage tests on cohesive soil

Abstract

The factors of suffusion progression due to a series of one-dimensional laboratory seepage tests on cohesive soil were investigated. Suffusion is the transportation of finer fractions between larger fractions with seepage force. It has been studied for noncohesive soil because suffusion often takes place in filter zones at dam sites. However, other soil structures containing cohesive soil are also threatened by suffusion, particularly in grounds imposed by a high and concentrated seepage force. Following the seepage tests, a series of laboratory penetration tests was conducted to measure the strength of ground-induced suffusion. We proposed that the potential for the onset of suffusion was governed by the gradation curves of the materials and the pore size of the outlet due to the results of seepage tests and the synthesis of previous studies. Both suffusion and clogging take place at high hydraulic gradients. Suffusion is initiated over the critical pore velocity and depends on the material properties. Namely, if suffusion is allowed in the ground and the soil is allowed to outflow from the outlet, suffusion will be initiated. If either one of these is not allowed, clogging will occur and that will lead to a reduction in hydraulic conductivity. The penetration resistance decreased in proportion to the progression of suffusion despite the fact that the absolute amount of suffused soil was subtle. This implies risks of the promotion of vulnerability in practical grounds by continuous and invisible suffusion. In addition, the turbidity of the discharged water proved that by measuring the preferential migration of finer fractions through the soil specimen, it may be possible to monitor the onset of suffusion.