Seepage forces and confining pressure effects on piping erosion

An experimental study of piping erosion is presented. Various artificial granular filter and soil combinations are tested in a permeameter under variable confining pressures to determine the critical gradient where soil erodes through the filter. Previous research concentrated on establishing a grain size ratio criteria, typically D[sub 15f]/D[sub 85s] <4, which separates stable from unstable filters. These works often ignored filtration formation phenomena and did not document the influence of variabilities such as confining pressure, filter thickness, and gradient flux. To adequately control all variables required a new permeameter and careful attention to sample preparation. Artificial glass beads were water pluviated to permit consistent samples. By monitoring head, settlement, confining pressure, amount of eroded soil, and water outflow the onset of piping can be determined. It is shown that grain size ratio is the most important parameter in piping. A soil/filter system with D[sub 15f]/D[sub 85s] 12 will not retain soil. For 8 < D[sub 15f]/D[sub 85s] < 12, piping will only occur if the critical gradient is reached. The critical gradient is lowered if the head is rapidly increased, as a filtration zone is inhibited from forming. A very thin filter has a similar effect. Stability is slightly inversely related to confining pressure for small grain size ratios.Applied Science, Faculty ofCivil Engineering, Department ofGraduat