Modelling coastal embankment instability from the effects of tides.

Rees, Richard 2025. Modelling coastal embankment instability from the effects of tides. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The effect of dynamic porewater pressure (dPWP) on slope stability from tidal action is investigated by the comparison of a transient seepage analysis and Hardening/Softening Mohr-Coulomb (HSMC) hydromechanical model. Simulation lengths are typically 6 months, are followed by a higher solution two day simulation designed to capture the hydrodynamical responses occurring in a higher resolution. The two topologies investigated are 1) single slope and 2) embankment with both being constructed of homogenous clay, silt and sand (fine to course) with special heterogenous compound embankment also investigated. To quantify the effect of the hydrodynamics a global and shallow slope stability assessment is performed using the limit equilibrium methods. The serviceability of the slope is assessed using the limit state method. Furthermore, the interactions between key parameters (porewater pressure, displacement, shear stress, shear strain and effective stress) were all examined to establish whether a relationship exists. The key findings are that transient seepage analyses propagate pore water pressure (PWP) through a medium by hydraulic action only, whereas hydromechanical models such as linear elastic and the hardening/softening Mohr-Coulomb can simulate both hydraulic and mechanical compression. This allows PWP changes to propagate deeper with the slope and induce higher pressures. This is most notable within the cohesive materials whilst for the sands, hydraulic conductivity is the dominate mechanism. The topologies dictate the size and position of the phreatic surface in the slope but also the seepage pathways and the response of the wetting front with the seepage flow occurring. The single slope cases using the cohesive materials, generate greater shear band formations than that the corresponding embankment. This is due to its ability to drain on both faces, elevating excess PWP formation but also lowering the phreatic surface within. The slope stability assessment using the traditional method of circles both for global and shallow slip surface searches did not detect any instability resulting from the interception of seepage egress and ingress into the slope. The limit state and examination of the parameters including the differential of PWP change did allude to pressure changes altering the effective stress of the material which in turn for the more cohesive materials, lead to the creation of shear bands. These were assessed using the limit state criteria and the slope face was deemed to be under a mild deformation for the silt single slope.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Embankment 2. Numerical 3. Slope stability 4. Tides 5. Transient seepage 6. Hydromechanical
Date of First Compliant Deposit:	12 May 2026
Last Modified:	12 May 2026 15:09
URI:	https://orca.cardiff.ac.uk/id/eprint/186892

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