Tide induced piping risk assessed by transient groundwater flow

Dutch dike and levee design codes provide a solid framework for the design of dikes and levees. One of the failure modes that have to be considered by these codes is ‘piping’. In the Netherlands, piping is usually checked under the assumption of stationary flow boundary conditions. Piping risk is assessed from the critical hydraulic gradient, using empirical relationships accounting for soil type, layering and particle size. The paper presents a case history of a dike design in a tidal environment. The tidal effects on the groundwater heads are assessed by transient groundwater flow analysis. The case concerns the design against piping through an overflow dike, which is part of a flood storage area. The area is located along the River Dijle between Antwerp and Mechelen (Belgium). The critical heads for the check of the piping risk were assessed by Plaxflow, a finite element code that is capable of carrying out transient groundwater flow analyses as well as steady state ones. In the analysis presented herein, the normal ground water fluctuations, which are strongly affected by tide, have firstly been modelled with Plaxflow as a calibration exercise. Once this had been done, the critical gradients were assessed under design conditions. The benefit of using a transient flow analysis for the verification of piping will be demonstrated in the paper. Based on the calculation results it could be demonstrated that in contrary to the stationary flow analysis no mitigating piping measures were required.