In 2017 new guidelines concerning macro stability calculations were implemented by the Dutch Ministry of Infrastructure and Environment. These guidelines are formulated in “Wettelijk Beoordelingsinstrumentarium” (WBI). The largest difference with the previous version of the guidelines concerns the material model that is prescribed to determine shear strength parameters. In triaxial tests shear strength parameters are to be determined at ultimate state (25% axial strain), which is assumed to be a good representation of critical state. Critical state is a concept from Critical State Soil Mechanics (CSSM) and assumed to be a good representation of the state reached after large deformations induced by macro instability. Another fundamental assumption in the WBI is the use of the SHANSEP method. This method encompasses a laboratory procedure and a normalisation method. CSSM was originally defined and elaborated under isotropic stress conditions while in engineering practice anisotropic conditions are mostly used.
The goal of this thesis is to investigate the undrained soil behaviour of organic clay in triaxial tests following the SHANSEP procedure and compare the results to the CSSM framework. In order to do so a series of eight K0-consolidated triaxial tests is executed using silty organic Oostvaarders plassen clay, which is assumed to be representative for a typical Dutch soil. A large range of over consolidation ratios is applied (1-20). Both compression and extension tests are executed. The triaxial tests are complemented by two K0-CRS tests and an isotropic compression test to determine relevant soil properties. The results are compared to the CSSM. The qualitative soil behaviour is analysed as well as the actual predicted undrained shear strength Su. Parameter relationships as described by CSSM are tried to be established from the data. The undrained shear strength is predicted by using numerical and analytical formulations of the Modified Cam Clay model, which is the most basic implementation
of CSSM. From the data a clear failure line could be determined in p'-q space. In the triaxial compression tests failure at ultimate state gave very consistent result, a failure line could precisely be determined. In extension failure at peak strength showed the most consistent results. Ultimate state could not be reached under representative stresses because the formation of large shear bands and necking during shearing. A unique p'-q-e relation was much harder to establish. The general trend as described by the CSSM was clearly visible but the uncertainty was rather large. Several factors contributed to the uncertainty. Among which the void ratio determination method that was used to determine the void ratio after the triaxial test, which is prone to errors. The MCC model was not able to model the stress path correctly and p'_0 at failure was not correctly predicted resulting in incorrect Su prediction. The MCC model overestimated Su in compression tests. In extension, however, Su is well predicted by the MCC model. The SHANSEP method turned out to be a very convenient way to normalise the undrained shear strength of the triaxial tests, both in compression and extension. Only at very large OCR Su/σ'_v0 is slightly overestimated.

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.

From the 3500 kilometers of primary flood defenses in the Netherlands, a third does not comply with the current standards. As a result, over 700 kilometers will be strengthened in the upcoming years. Traditional strengthening of dikes consists of strengthening with soil and requires space that is often not available. Further strengthening of dikes, therefore, requires smart solutions, which increase the strength, without changing the dikes cross-sectional area. One of these solutions is the use of sheet pile walls as stability screens. However, in contradiction to all commonly used design codes, including the Eurocode, the current design approach for stability screens in dikes does not allow for plastic calculation. Furthermore, a model factor of 1.1 is applied to the forces and moments, when the sheet pile is applied as a panel. These two aspects make the application of sheet piles as stability screens expensive to apply. A better understanding of the failure behavior is needed to clarify whether the current design approach is conservative. Within the scope of the Flood Protection Program, a cross-project exploration was formed between water boards and the national government at the end of 2014. This collaboration is called POV and the aim is to innovate dike reinforcement which will result in a better, faster and cheaper process. A separate branch of this organization, POVM, is focusing on macro-stability. Within this scope, the use of sheet pile walls as stability screens is investigated. In this context, the Eemdijk test has been performed. This consisted of two full-scale tests up to failure. Although the test was successful and led to many new insights, the reliability of strain measurement data is questionable. The strain distribution, however, is essential in understanding the failure behavior. That is why this thesis addresses the reliability of the measurement data. This is done by means of comparing measurements of different sensor types. Each sensor is then classified using four classes that indicate the reliability. Subsequently, reliable data is used to obtain a curvature distribution. In order to convert strains into moments, a moment-curvature diagram is required, which relates curvatures to moments in the sheet pile. This relationship is obtained by means of finite element calculation of a 4-point bending test. Subsequently, the chosen model is calibrated by means of reproducing the measurement results of three real documented 4-point bending tests. Finally, the model is applied to the sheet pile profiles used in the Eemdijk test. With the obtained moment distributions, conclusions are drawn with respect to the current design approach. Furthermore, recommendations are made with regards to the applied monitoring program and the influence of soil on local buckling.