Dikes protect people and lands all around the globe. With rising sea levels, the importance of well-designed dikes has never been more essential. One of the main factors that can compromise the stability of dikes are macro-instabilities. Macro-instabilities can cause dikes to lose their water bearing potential, leading to floods. Methods are developed to as accurately as possible determine the probability that a macro-instability takes place in order to prevent it. Therefore, recent advancements include the remaining strength after macro-instability, which may be able to prevent flooding. The foremost methods to determine this remaining strength after macro-instability use D-Stability or the Material Point Method (MPM). Both D-Stability and MPM have advantages and disadvantages. D-Stability can quickly determine the probability that a macro-instability takes place, but cannot model the process of failure, and must therefore simplify this process to estimate the remaining strength. MPM on the other hand can accurately model what happens after a macro-instability, but has a much larger computational cost, especially for probabilistic computations. By supplementing D-Stability and MPM, this thesis proposes a method that exploits the advantages of both methods and mitigates the disadvantages. For a dike with a single clay layer, a connection was made between D-Stability and MPM, allowing dike profiles to be transferred back and forth. In order to make this connection, the SHANSEP undrained shear strength model was successfully implemented in MPM. By using the quick probabilistic D-Stability calculation and the post-failure modeling option of MPM, the probability of failure and the effect of failure can be quickly determined. By taking into account the effect of failure, the method can determine the probability of flooding, without simplifying the failure process. The method can also be used to determine if flooding via retrogressive failure or a larger single instability is more likely. The method was tested via a case study and verified via a RMPM Monte Carlo analysis. For the case study, the probability of flooding was 5.189*10-3 compared to an initial probability of failure of 7.22*10-1, a reduction in the order of 139. This probability of flooding compared well to the probability of flooding of 5.308*10-3computed using the more accurate and computationally expensive RMPM. Based on these first results, the proposed method is a viable method to assess the probability of flooding after macro-instability for clay dikes.

The construction of a dike improvement often causes a decrease in reliability during construction. Raising a dike may give rise to excess pore water pressures in the subsoil, resulting in a temporal strength decrease. Observing the survival of such a loading condition might provide information on the strength of a dike. Therefore, this research investigates the effect of incorporating construction survival in estimating the slope reliability of dikes. Assessing slope reliability can be done by means of simulation. However, due to the high reliability of dikes, many realizations and thus model evaluations may be needed to achieve sufficient accuracy, leading to infeasible computation times. In this research, a method is developed for reliability updating using metamodelling, providing reasonable computation times while not losing accuracy. The effect of incorporating construction survival is explored by updating the inward slope reliability of a case study dike improvement. The case study shows that the current semi-probabilistic approach to assessing construction stability in practice leads to situations not effective for reliability updating. Though adjusting the construction phasing may lead to a significant update. It is shown that adapting the dike improvement design after survival of the critical construction step results in a 75% reduction of the dimensions of the berm to be constructed, while achieving the same reliability as prior to the update. The main finding of this research is that incorporating construction survival in the reliability assessment can be very effective in terms of reliability, and in making dike improvements more efficient and less space consuming.

In the Netherlands the design condition for a dike is surmised to occur during winter where it is assumed that the dike is fully saturated. Preliminary results by Van Duinen (2020) indicate that suction remains present, albeit ever decreasing, during the winter. This means that it would be possible for a dike to dry out during the summer period such that it becomes unsaturated, and that any effects may not have fully disappeared during a normative event.

This problem is two-fold, as (i) the strength of an unsaturated soil is higher compared to a saturated soil, and (ii) it is unknown how the strength associated with the initially unsaturated zone can be modelled in a macro-stability calculation as it varies temporally. This report investigates the strength of the initially unsaturated zone in clay river dikes in the Eastern part of the Netherlands and it explores the opportunities which the better understanding of the strength may present for the factor of safety in a macro-stability calculation

The feasibility and applicability of the SSCC as formulated by Lu and Likos (2006) for conventional geotechnical laboratory tests is investigated. A literature review bundles international knowledge on unsaturated soil behavior which is relevant in the context of dike design. Silty clay samples from the dike trajectory of Ravenstein-Lith were investigated using the SSCC. It can be successfully applied to K0-CAU triaxial test when determining strength parameters using strain-compatibility. Compared to its conventional variant, the saturation stage must be skipped. Samples are air-dried under laboratory conditions to a prescribed volumetric water content which can then be linked to an amount of suction using the SWCC and a comparison can be made with the closed form of the SSCC (Lu et al., 2010).

The SSCC cannot be applied to DSS tests successfully. In DSS tests both slip and a diagonal failure plane was observed. This research was able to pinpoint at which shear strain slip would occur, and hence when tests are valid. DS tests were used to confirm the validity of DSS tests using CCSM.

A case study performed on a representative cross-section found a positive effect on the factor of safety due the additional strength related to the initially unsaturated zone. The FoS increased by at least 1.7% in the most conservative case and at least 5.0% in a scenario supported by field measurements. The current limited number of field measurements restricts a more precise determination. It is advised to perform more of these such that the initially unsaturated zone in a clay river dike can be modelled effectively and reliably with a high degree of certainty.

To conclude, with the application of the SSCC concept, the additional strength produced by considering the initially unsaturated zone in clay river dikes in the Netherlands leads to an opportunity where the dike can be designed more efficiently by reducing conservatism. Thus, taking the initially unsaturated zone in a dike into account can be considered as a valuable contribution to the toolbox of Dutch geotechnical engineers.

The low lying areas in the Netherlands often consist of soft and low permeable soils like clay and peat on top of a sand layer. Due to high water pressures at the interface between the permeable sand layer and the low permeable top layers, the effective stresses decrease, which can cause lifting of the low permeable top layers. As a consequence a deep seated long slip surface of the inward dike face can occur. Classical solutions to improve inner slope stability, like decreasing the inner slope or creating a stabilisation berm, are space consuming. Stabilisation columns are expected to improve the inner slope stability within the actual footprint. Various effects of stabilisation columns on inner slope stability are studied. The effects are studied by means of PLAXIS calculations using various modelling techniques.