An Analysis on Soil Properties on Predicting Critical Hydraulic Gradients for Piping Progression in Sandy Soils

An Analysis on Soil Properties on Predicting Critical Hydraulic Gradients for Piping Progression in Sandy Soils By Tammy Jacobson, Master of Science Utah State University 2013 Major Professor: Dr. John Rice Department: Civil and Environmental Engineering Piping is a form of internal erosion in which soil particles are eroded at a seepage exit location due to the forces imposed on the particles by seeping water. Laboratory testing was performed on a variety of soils in order to assess a correlation between unit weight, angle of internal friction, grain size, gradation, and void ratio and the critical hydraulic gradient at which piping initiates and progresses. A multi-variable regression analysis was used to form equations to predict critical hydraulic gradient based upon each of these soil parameters. Variations in the accuracy of these equations are thought to be due to the interlocking behavior of the angular soils tested compared to that of the more spherical soils as well as the loosening of the sample and change in void ratio as piping progresses. Introduction Internal erosion includes several different mechanisms. These include heave, piping, concentrated leak erosion, contact erosion and suffusion (ICOLD 2012). Internal erosion accounts for nearly half of all world-wide dam and levee failures, with nearly one-third of all piping failures being associated with the backwards erosion model of piping (Richards and Reddy 2007). Piping is often described using the term backward erosion. This is because piping is the erosion of soil particles at some seepage exit location due to forces imposed by water transport through a porous media. This process results in channels or “pipes” that progress in an upward gradient back towards the water source (Richards and Reddy 2007). This occurs as the result of high hydraulic gradients of velocities. Attempts have been made to quantify the hydraulic conditions needed to initiate piping failure. The theory of heave was proposed by Terzaghi (1922, 1939, 1943). His theory is consistent with Darcy Law assumptions in which flow takes place vertically through a horizontal face. His equation for the critical gradient (Equation 1) at which heave occurs under this condition is the ratio of the buoyant unit weight of the soil (’) to the unit weight of water (w).