Numerical Investigation into Dynamic Porewater Pressure with Coastal Sea Defences from Tidal Cycles

Abstract

Recent winters in the UK have demonstrated the susceptibility of the sea defences to increasingly powerful and more prevalent storm systems, resulting in major damage to defence themselves and the infrastructure it is designed to protect. This trend is expected to continue with climate change and exasperated by an increase in sea level. In the future, to ensure continued confidence that these defences are able to perform their task, modifications to old, and the construction of new defences will have to occur. Essential to the design resilience, would be how to mitigate the effect of previously unknown or under considered instability factors.

Pore water pressure (PWP) within a soil plays a vital role determining its stability, for a coastal structure, seepage can cause geo-hydraulic failure if not properly accounted for in the design though internal erosional processes (Vorogushyn, Merz and Apel, 2009). Several different researchers (Vorogushyn, Merz and Apel, 2009; Chen et al., 2013) have been investigating the hydraulic action of cyclical events i.e. tides and wave impacts on stability of earthworks. A particular focus has been the fabric of the soil within the earthwork and the dynamic effect of varying seepage gradients and velocities. The hydraulic forcing in respect to tides has been initially investigated through the numerical modelling and physical experimentation by the previous authors.

This paper is to present the results of the numerically investigation of the effects of tidal action on two differing homogeneous embankments on the PWP, using both seepage only analysis and fully coupled stress-seepage analysis using an elasto-plastic soil model. A base line was established per model, with the main simulations employing transient seepage analysis. Within these simulations, the water level was varied using S2 and M2 tides over a 42-day period. The phase and PWP difference between the tidal cycle and migrating pressure pulse was established within the earthwork, by comparing the values of different vertical sections.

Initial results demonstrated that PWP were higher within the embankment for the flood tide but significantly, lower for the ebb, this is due to the ability of the later to simulate deformation of the embankment due to loading and unloading cycles. Further investigation revealed that a pressure pulse does exist between vertical sections and were found to occur at the higher reaches of the earthwork, which experience the greatest periods of saturation and unsaturation.