Laboratory investigation of backward erosion piping – effects of inclined exit face and constricted seepage exits

While the Backward Erosion Piping (BEP) form of internal erosion is one of the least understood mechanisms in geotechnical engineering, a precise, mechanism-based analysis method for backward erosion piping (BEP) remains elusive for geotechnical engineers. BEP is still generally analyzed with empirically-based methods that do not consider many of the complexities of the soil, the subsurface geometry, the seepage regime, and the exit face conditions. Furthermore, many of the analysis methods commonly used for BEP were actually developed for other forms of internal erosion and adapted to BEP by correlating with crude empirical data. This paper presents a laboratory testing program performed to investigate the mechanisms of BEP under two conditions often encountered in the field: 1) exiting on a sloping exit face, and 2) exiting into a constricted exit. The study builds upon previous research on the mechanisms of piping initiation performed at Utah State University using a similar apparatus. A variety of soils representing a range of grain size, grain shape, and gradations are subjected to increasing hydraulic gradients under a variety of exit face condition including sloped exit faces and a range of constricted seepage exits. The results are compared with three-dimensional finite element analyses in order to develop a better understanding of the BEP initiation process.