Reliability analysis of a Po River embankment affected by underseepage

Backward erosion piping (BEP) is an internal erosion process that may develop as a consequence of seepage processes through the aquifers underneath river embankment structures. This phenomenon can be identified in the field by observing the formation of sand boils on the ground surface at or near the embankment toe. Along the Po River, more than 130 sections are affected by periodical sand boils, especially located in its middle-lower stretch. The paper presents the results of reliability analyses referred to BEP phenomena in a riverbank section located in Mazzorno Sinistro, in the province of Rovigo (Italy), whose stability has been threatened by periodical reactivations of this phenomenon. Fragility curves, describing the probability of BEP initiation with hydrometric level, are elaborated based on a two-dimensional model of the groundwater flow through the river section and on the application of the First-Order-Second-Moment (FOSM) method, assuming the main parameters as probabilistic variables. The soil profile and the probability distribution of the geotechnical parameters were obtained from the interpretation of several recent in situ and laboratory tests. Model results are compared to those obtained by applying the USACE theory and to the evidence of past reactivations.