Experimental Investigation on the Mechanism of Piping in Different Two-Stratum Levee Foundations with Weak Spots

Abstract

Abstract Earthen levees along rivers may break down because of piping; this is especially the case for levees built on two-stratum levee foundations with an impervious stratum and an underlying pervious substratum. However, the initial stage of this piping is difficult to recognize because finding the piping outlets along the whole levee is challenging. Practical experience confirms that weak spots, such as holes or thickness reductions in the blanket stratum, usually occur at the levee toe. Some of these are harmless, whereas others are not. This study takes weak spots into consideration and provides a detailed description and evaluation of the piping features. Experiments were conducted on three models with different pervious conditions in a flume. Weak spots caused by holes, a thickness reduction, or exposure of the substratum at the levee toe were tested. The results show that the thickness reduction influences the formation of continuous sand boiling once the confined water bursts the blanket stratum. The potential for piping is reduced when the pervious substratum is composed of thin and uniform soil. In the absence of a covering stratum, piping happens with sand boiling. Three typical phenomena, namely sand boiling near the levee toe, a pressure decrease of the confined water, and a rise in the hydraulic gradient in the pervious substratum, could be used as indicators of the occurrence of piping.