A laboratory modeling program has been conducted to evaluate the hydraulic conditions required for backward erosion piping (BEP) to initiate and progress in sandy soils. The first stage of BEP consists of loosening the soil near the unprotected seepage exit face from which the piping initiates. As the differential head on the system increases, the loosened zone increases in size to reach equilibrium with the increasing hydraulic gradient near the seepage exit. Laboratory models were performed on sandy soils to model the development of BEP and measure the hydraulic regime surrounding the developing loosened zones. Inverse analyses were performed using three-dimensional finite-element (FE) seepage analyses to assess the critical gradients needed to initiate soil loosening and create or expand the loosened zone. By analyzing the observed behavior and data of piping development with the inverse-analysis procedure, exponential and linear fit equations were developed to define the relationship between effective stress and the critical gradient at the boundary of the loosened zone. The exponential relationship was shown to have a moderately better fit of the data than the linear relationship, indicating that internal friction resistance in addition to overburden weight contributes to the resistance to soil loosening.

Backward erosion piping accounts for one of the leading threats to dams and levees throughout the world. A laboratory modeling program has been conducted to interpret pore pressure data and observations collected during backward erosion piping (BEP) initiation and progression in sandy soils. An analytical model has been applied to assess the development of BEP mechanisms as well as calculating the critical hydraulic conditions required for various BEP stages to initiate and progress. The results with the predicted model are produced by successively matching the hydraulic head regime surrounding the developing BEP stages based on observations and pore pressure measurements obtained from the laboratory models. Interpretation based on an analytical model allows assessment of the processes governing BEP initiation and progression, including: (1) substantial concentrations of seepage flow around the edge of the exit area resulting in increased gradients and BEP initiation (i.e., sand boiling); (2) soil particles collapsing leading to BEP progression (i.e., channel development). The findings of the study identified the criterion for governing channel progression that can be applied to the assessment of BEP mechanics.