The increase of dike stability due to unsaturated soil strength

Abstract

River dikes in the eastern part of the Netherlands are characterized by a relatively low daily water table. In the unsaturated zone above the water table, a negative pore pressure (matric suction) is present that binds the soil particles together. Suction can be especially large in soils with small grains, such as clay, and increases the strength of a soil due to higher effective stress. High suction in soils has been proven to significantly increase the stability of dikes. However, due to water level variations and other climatic influences such as precipitation and evapotranspiration, suction in a dike will vary over the year and throughout the cross section. The research described in this report aims at evaluating whether there are parts of a cross-section of a dike where soil suction remains throughout a high water event in a river such that an increase in the strength of the soil can be used in stability analyses. The analysis is centered around a case study of an existing dike in the east of the Netherlands and uses time-dependent unsaturated groundwater flow models to simulate the spatially and temporally varying suction for a number of different initial conditions and climatic influences defined by scenarios. The results show that it is very unlikely that suction in the dike will be lost during the summer months. Due to high evapotranspiration, the water content in the dike is low, which results in a lower water table during a high water event and less infiltration in the dike occurs during precipitation. During winter, the probability that suction is lost throughout the dike cross-section is larger and will occur after heavy precipitation with a long duration where the return period is 1 – 10 years. Although in most scenarios some suction will remain present, the amount of suction during the winter is generally low, between 0.5 and 1 meter, which results in an apparent cohesion of 2 – 4 kPa. A simple stability calculation illustrates that the increase in strength in the dike due to suction does not result in a significant increase in the stability of the dike during high water conditions.