Influence of rapid draw down on dike stability

Abstract

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.