Laboratory piping tests on fine gravel

Results of an experimental study are presented in which the horizontal, critical gradient required for backward erosion piping of fine gravel was measured. A horizontal flume was constructed that allowed samples to be subjected to a vertical confining stress and unidirectional flow. The seepage gradient was increased gradually during each test, while the sample was monitored with an array of pore pressure transducers as well as visually through a clear acrylic top. The maximum, global gradient required during each test for piping to initiate and progress through the sample was recorded as the critical gradient. Sixteen tests were conducted. Values of the critical gradient were found to vary from a minimum of 0.30 in a loose state to a maximum of 0.51 in a dense state. Results of the experiments were compared with the predictive methods of Sellmeijer (original), Sellmeijer et al. (with multivariate adjustment), Schmertmann, Hoffman, Lane, and Bligh. Only the method proposed by Schmertmann compared favorably with the experimental results.