The Effect of a Confining Cover Layer on Backward Erosion Piping Process

Abstract

The Netherlands is prone to flooding as more than a quarter of the country lies under sea level. To combat flooding and ensure that the country remains dry structures such are levees and dikes have been installed. However, older water retaining structures are more than ever failing the stringent safety standard assessments. These older conventional reinforcement measures, including berm constructions, are not only costly but require an expanse of ground to ensure performability.

Backward erosion piping is an internal erosion mechanism during which shallow pipes are formed in the direction opposite to the flow underneath water-retain structures as a result of the gradual removal of low cohesive material by the action of water. This mechanism is an important failure mechanism in both levees and dams where a cohesive layer covers a sand layer. Although failure resulting from backward erosion piping is not common, several levee failures in the United States, China and the Netherlands have been attributed to this mechanism.

There are mitigation measures known to stop the backward erosion mechanism. One such measure is the placement of a seepage wall, to create a physical barrier directly in the flow path trying to reach the lowest region of the hydraulic head. A review of the literature showed that current design rules only consider groundwater flow calculations when determining the likelihood of hydraulic heave, one of the failure modes within the backward erosion process. Hydraulic heave in the backward erosion piping context is closely linked to the quicksand condition, essentially stating that once the effective stress is zero, the sand particles become suspended, liquifying a solid layer. The absence of an assessment of the effective stresses during the design process in conjunction with hydraulic heave has contributed to the main research question addressed by this thesis; How does a restricted exit for groundwater flow affect hydraulic heave compared to Terzaghi’s free exit situation?.