Residual strength and profile of the Eemdijk full-scale test

Abstract

This thesis tried to quantify the strength reduction of the soft soil layers of the experiment through a back analysis of the failure of the dyke without sheet pile wall. Residual strength hypothesis were formulated based on a literature study and the current design norm of dyke design on soft soil layers in the Netherlands. A Limit Equilibrium Method analysis of the pre and post failure geometry served as a basis to determine the peak and residual strength of the soft soil layers following the residual strength hypotheses. The 3D effect was taken into account in these analyses, and determined to be around 20\%. A Material Point Method model of the experiment without sheet pile wall was then created to test the different residual strength hypotheses. Factors were applied on the strength properties calculated from the LEM model to match the MPM model. A factor of 1.23 was applied to account for 3D-effect and 1.16 to correct for water on passive side being absent from MPM discretization.

The Material Point Method model showed that during failure the behaviour of the clay layers could be expressed with an Undrained SHANSEP formulation. In this formulation the residual strength of clays was found to be independent of the Over-Consolidation Ratio. In a Mohr-Coulomb formulation this results in a complete loss of cohesion. Leaving the OCR out of the strength formulation of clayley layers resulted in horizontal displacement of 4.5 m, which is close to the 6 to 8 m found during the experiment. Further decrease in S-ratio of 30\% resulted in horizontal displacement going up to 7.5m in the MPM model. A reduction of 0 to 30\% of the S-ratio could therefore be concluded to be a range of friction softening. This was concluded to be in accordance with what was found in literature. Laboratory testing and correlations based on index properties effectuated prior to the experiment showed residual friction angle around 30\textdegree. The residual strength backcalculated are much lower, and therefore in contradiction with the laboratory testing results effectuated. The use of cyDSS and LDSS tests was therefore deemed inappropriate for the determination of residual strength.

The Limit Equilibrium Method analysis of the dyke with sheet pile wall was deemed inappropriate for the back analysis of the soft soil layers. The horizontal forces induced by the soil-structure interaction cannot be disregarded. It is recommended to back calculate the peak and residual strength of the peat layer using a Finite Element Method analysis. The displacement measurements of the failing dyke with sheetpile wall in the peat layer showed similarities with strain localisation in a DSS test.