The aim of this research is to find the influence of rapid drawdown on dike stability with the Delta21 project as case study. The Delta21 project is a proposed plan for a waterdefence system in front of the Haringvliet, The Netherlands. Its main element is a water retaining lake with an extensive pumping and turbine system. Whenever other storm surge barriers close, water from inland rivers can be pumped to the sea. The water retaining lake can act as an energy storage system as well by using the turbines to generate electricity at moments of high energy prices. The pumping system has the capacity to reduce the water level with 17.5m in 12hours. The dike surrounding the lake has a gradual slope as the height/width ratio below the water line is 1/20. It consists fully of a homogeneous and permeable fine sand. The estimation of material properties were based on limited CPT data, samples from the adjacent Maasvlakte II and empirical relationships. The rapid decrease of the water level in the lake causes rapid draw down conditions in the dike. Hereby the external hydrostatic pressure is reduced due to unloading effect of removing water, while a delay in the dissipation of pore pressures inside the dike is produced. This causes stress relaxation and slope instability. The research was performed using a numerical model verified with physical modeling in the form of centrifuge tests. For the numerical model the finite element model PLAXIS2D was used. A fully coupled flow deformation analysis was chosen, which can solve the full interaction between deformation, consolidation and groundwater flow simultanously in the same phase. It considers a reduced permeability and degree of saturation in the unsaturated zone. The Hardening Soil small strainmodel was selected as constitutive model due to the potential of capturing hysteresis in the stress-strain regime. Hysteresis could occur as a result of multiple cycles of water level fluctuations. To gain better understanding of the capabilities and weaknesses of the numerical model, it was verified with several physical tests from literature. The first test is a centrifuge test with failure due to increasing hydrostatic pressures within the dike. The moment of occurrence and shape of the failure contour were compared with the numerical model. The second test was a centrifuge test where a dike experiences rapid draw down conditions with the purpose to validate the pore pressure and saturation level. Thirdly, a full scale test of a dike with a changing water level was added, because full scale tests offer the closest resemblance to situations in the field. The field test was modeled to double check pore pressures and saturation. The research aimed to further investigate the development of a failure during rapid draw down, a centrifuge test was performed at 100g. Rapid draw down conditions were simulated on a sand dike with slope 1:1.6 to generate failure. By tracking grains with Particle Image Velocimetry (PIV), correctness of estimating displacement by the numerical model was verified. The addition of pore pressure sensors could give further information on pore pressure development. The numerical model proved to accurately predict positive pore pressures and the moment at which macroscopic failures occur due to drawdown. Negative pore pressures, saturation and development of the failure, in terms of size and deformation could not be accurately approximated. With understanding of the capabilities and weaknesses of the numerical model, the dike of the Delta21 project was modeled. The original design proved to be stable with a FoS of 6.20. Increasing slope angles were modeled resulting in a decreasing FoS. The influence of hysteresis was found to be very small after modelling multiple cycles of changing water level. The positive effect on stability the addition of a high permeable filter layer was modeled. Finally the risk of erosion due to outflow was analytically calculated and proved to be closely related to the numerical results.

The transition to renewable energy has lead to many new ways of energy production, such as wind and solar energy. For solar energy power, there is an excess of energy during summer and an deficiency of it during winter, which is reversed for the energy consumption. In order to amend this problem, there is an introduction of new ways of energy storage in the form of heat for domestic and industrial use. One of these is the use of Underground Thermal Energy Storage (UTES). These systems make use of the underground to store heat during summer and extract heat during winter by using a fluid as the heat transferring agent. There are many different ways of UTES installation, however there are two which are widely used in the Netherlands: 1) Aquifer Thermal Energy Storage (open systems) which are based on heat-transfer by convection and 2) Borehole Heat Exchangers (closed systems) which are based on heat-transfer by conduction. While the way of transferring heat differs, they both need vertically drilled boreholes to reach a certain storage depth. These boreholes are drilled close to, or underneath, pile-founded buildings. The drilling of boreholes influence in-situ soil stresses around them, which might influence the bearing capacity of the piles. The aim of this thesis is to investigate the influence of installing UTES systems on the bearing capacity of piles. This is done by: 1) investigating the stress changes in sand due to the drilling of boreholes (stress analysis) and 2) the influence of these stress changes on the bearing capacity of piles (bearing capacity analysis). All of the modelling was done in PLAXIS, by using the 2D-axisymmetric model for the first part and the 3D plane strain model for the second part. An advanced soil model for sand was used, with soil mechanical behavior according to the Hardening Soil small strain model. The stress analysis consists out of a parameter sensitivity analysis, including the key parameters: back-fill grout shrinkage and expansion, Over-Consolidation Ratio and Relative Density. After this, the influence of the borehole fluid pressure and borehole diameter on the stress states were investigated. As not all boreholes are drilled perfectly, the last part of the stress-analysis takes into account drilling complications. The bearing capacity analysis includes simulations of static pile load tests of non-displacement piles. Load-displacements curves are produced for several scenario's: 1) perfectly drilled boreholes, 2) boreholes with drilling complications, 3) influence of a soft soil layer and 4) displacement piles and the influence of a varying installation phasing. The results indicate no large effects of perfectly drilled boreholes on bearing capacity of piles located outside a zone of 1.5 times the borehole diameter. For boreholes with drilling complications, the bearing capacity is influenced for piles located in a zone of influence of only several meters (2-6m), depending on the severity of the complication. At last, loaded piles of existing buildings are extremely sensitive for stress changes due to the drilling process, which will cause severe loss of bearing capacity.