Measuring Critical Gradients for Soil Loosening and Initiation of Backward Erosion-Piping Mechanism

Abstract

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.