The role of the hydraulic resistance of the river bed and the time dependent response of the foundation layers in the assessment of water defences for macrostability and piping

In the assessment of water defences for macrostability and backward erosion piping the design hydraulic load mostly refers to steady state conditions in equilibrium with the maximum river water level. In this contribution, we show selected results of coupled hydromechanical numerical analyses of a paradigmatic Dutch case, which demonstrate that this assumption leads to high overestimation of the true hydraulic loads at the toe of the water defence embankment. The hydraulic resistance of the bed of the river and the deformability of the foundation layers introduce a decay in the pore pressure time history, which largely reduces the action on the hydraulic protection structure. The finite element model was developed to assist in the assessment of an innovative solution based on passive wells as a measure to reduce the risk for macrostability and piping. It was calibrated on available pore pressure measurements in the foundation of critical sections of the dykes of the river Lek in the Netherlands under the daily tidal action. The model was used to determine the distribution of pore pressure expected in the subsoil of the dykes for the design maximum load. The calibration stage of the model is specifically interesting to the aim of evaluating the reduction of the input pore pressure due to the hydromechanical resistance of the geotechnical system.