Interaction between the failure mechanisms backward erosion piping and slope instability: Identification and quantification of interactions between failure mechanisms of a levee

With the shift of the safety standards for levees from exceedance probability (standard 1996) to flooding probability (new standard), the part of the levee failure process after the initial occurrence of a failure mechanism becomes more important for the calculation of the strength and thus, reliability of the levee cross-section. Interactions between failure mechanisms can take place in this part of the levee failure process. An interaction refers to one failure mechanism influencing the probability of another failure mechanism, where positive interaction is defined as a reduction and a negative interaction as an increase of the failure probability. The main problem is a lack of knowledge about the interactions of failure mechanisms and their influence on the failure probabilities of levees. Several studies (Calle, 2002; ’t Hart et al., 2016; Kok et al., 2017) suggest that there is an interaction between different failure mechanisms. However, only the effects of overtopping on slope instability (de Visser et al., 2018) and a reduction of the shear strength in slope instability due to uplift (Kanning and van der Krogt, 2016) is currently included in the statutory assessments of levees (Rijkswaterstaat WVL, 2017a,d). This research aims to quantify the influence of the interaction between the failure mechanisms backward erosion piping (BEP) and slope instability (SI) on the safety of a levee. The parametric study of this research assesses whether the occurrence of a failure mechanism (BEP or SI) affects the parameters' present states in the limit state functions of the other failure mechanism. A model was created to calculate this interaction between SI with BEP. After a slope instability, the remaining profile is assumed to contain a berm. This change in levee profile, does not result in the immediate breaching of the levee, it can however influence the probability of BEP. The model sequentially executes a stability analysis, a displacement analysis of the sliding plane and a BEP analysis. The degree of interaction is defined as the difference between the safety factor of BEP of the original profile (prior to sliding) and the safety factor of BEP of the remaining profile (after sliding). The modelling study shows that the degree of interaction from SI to BEP is significantly affected by the presence of cracks in the berm. A consequence of these cracks is that the exit point moves and is now located in the berm. The degree of interaction also depends on the shape and size of the sliding plane, because it determines the size of the berm of the remaining profile. A vast majority (50\% to over 75\%) of the considered scenarios of the seven considered levee profile cases result in a positive interaction. However, in all the considered cases, negative interactions were found that give a reduction of 50\% to 2\% on the original safety factor, which could lead to an overestimation of the safety in the assessment of the levee. All of these negative interactions result from the assumption that large cracks can form in the soil after sliding.Civil Engineering | Hydraulic Engineering | Hydraulic Structures and Flood Ris