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.

The United States of America have a safety standard for flood protection of 1/100 year. However the flood protections in the Washington D.C. do not comply with this requirement. During this study the levees in Washington D.C. area were analysed and it was found, that in order to comply with the 1/100 safety standard the levees in the NationalMall needs to be heighten by 0.5mand those near the Anacostia river by 1.5m. The safety standard of 1/100 imposed in the United States of America is not based on exact calculation, therefore another design method was applied, which uses an optimal safety level based on the damage cost of floods and investment cost of flood protections.

The calculation of the optimal return period was based on the ’Standaardmethode 2017’ and it was found that the optimal return period at the NationalMall levee is 1/263 years. At the southern bank of the Anacostia river this optimal return period is 1/373 year. With these return periods new levees were designed in order to ensure the safety of the area of Washington D.C. There were two new levees needed around the National mall. The first was situated north of the Lincolnmemory along the Potomac river and the second replaces the temporary flood defence by the 2nd street SW and had an L-shape facing the Potomac river. The Anacostia levee was stretched on the East side in order to comply with the new 1/373 year safety standard. The old and new National mall levees needed a height of 3.5 m and those by the Anacostia needed a height of 8.5 m. It is important to take cautionwith these new levees as some houses needed to be removed. An attempt was made at modelling the above mentioned floods in ADCIRC to improve the accuracy of the design water levels and to verify the final design. However, the model showed instabilities, preventing the results from being used.
Explanations for these instabilities are the incoming water not being properly ramped up, the large vertical gradients in shallow areas and the upstream and downstream boundaries being too close to each other.