How to determine the effects of flood waves on the temporal development of the phreatic line in earth-fill dike bodies : a method applied with stochastic numerical modeling for the assessment of dike stability

The Netherlands has always been a country threatened by floods. To ensure the flood safety of the dikes, the government prescribes that the totality of the 3700 km Dutch flood defense system is periodically assessed. In 2017 new flood safety standards were enacted in the Netherlands, including new assessment methods in alegal assessment framework called WBI2017 for the 2017-2023 assessment round. The WBI contains regulations that the administrator must use to perform a safety assessment. The manual also contains instructions for the amount of research, and the type of research that is required, in order to achieve good schematizations of the subsoil and piezometric lines (water pressure lines per soil layer) and how (field) data can be converted to correct calculation parameters. The manual must provide directions for soil calculations on how uncertainties of the soils (strenght) characteristics are represented by making conservative choices in schematization and parameter choices. At this moment, there are no unambiguous guidelines for composing a soil investigation into a representative image of the soil layer structure in the underground and the geohydrological coherence. Studying available information often results in multiple interpretations. Because of this, uncertainties play an important role when choosing the schematization of both underground and hydraulic conditions (e.g. seepage characteristics). To that end, this study considered the soil uncertainties in the analyses of transient seepage through earth dikes and investigated their effect on the temporal development of the phreatic line and determined the effect of flood waves on the pore-water pressure in the aquifer and cover layers. It was found that the development of the phreatic line can be addressed by the hydraulic conductivity parameter in seepage analyses. Therefore, a case-location at the Waal river (between Tiel and Waardenburg) is investigated considering the uncertainty of hydraulic conductivity parameters for unsaturated flow modeling. A random number generation algorithm producing random values for the hydraulic conductivity of dike core materials is coupled with a finite element software, SEEP/W, to analyze the temporal development of the phreatic line. Monte Carlo Simulation is adopted for stochastic seepage analyses at which the variability effects of the hydraulic conductivity are investigated conducting sensitivity analyses. The variation effect of hydraulic conductivity of fine-grained materials is found to have a significant effect on the location of the phreatic line, whereas the hydraulic conductivity variation of course-grained materials is found to be less significant. This may cause deviations from the position of the phreatic line when assuming homogeneous materials with average material properties. The deviations of the phreatic line become greater on the long-term. The deviations of the phreatic line show no significant differences between a short-term flood wave and a long-term flood wave. However, on the long-term, the absolute location of the phreatic line can differ significantly as the duration of the flood wave increases. The pressure head built up in the dike core is significantly higher as the duration of the flood wave increases. As soon as the flood wave is dropping, the phreatic head in the aquifer and cover layer is dropping. Also, the phreatic line follows, resulting in a relative flat phreatic line regardless of the hydraulic conductivity tested in this study. However, residual pressure in the dike core remains at the outer slope side which is caused by the height and duration of the flood wave. An underpressure occurs in the surrounding soil layers. Due to this residual pressure, the effective stress decreases, causing a decrease in shear strength, which lead to deformations and eventually sliding of the soil. Therefore, the residual pressure can cause dike instability. Considering the residual pressure, the statistical and probabilistic properties of the phreatic line are assessed for outward macro stability for different flood wave types in the software D-Geo Stability. Hereby, the schematizations of piezometric lines are compared with the conservative method of WBI. It is found that for the case dike, the schematization of the phreatic line at a more realistic level and adressing a piezometic line for the residual pressure above the phreatic line, the dike is assessed to be more instable than wihout dealing with the residual pressure. Also, this alternative schematization showed significant differences with the WBI method for safety assessment. In general, this study showed more insights which piezometric lines are influencing soil layers in dike compositions and how stochastic analysis (i.e. uncertainty quantification) of the phreatic line can be used for schematization of these piezometric lines when assessing macro stability, instead of making conservative choices in schematisation or parameter choices.