Constant Gradient Piping Test Apparatus for Evaluation of Critical Hydraulic Conditions for the Initiation of Piping

A laboratory apparatus has been developed for assessing the hydraulic conditions necessary for the initiation of piping erosion. The apparatus is designed to provide a constant hydraulic gradient throughout a sample while retaining the sample in a sample holder. Increasing differential head is applied across the sampler while observations of soil behavior and measurements of pore pressure throughout the sampler are made. Observations of soil behavior during the tests have identified four stages of the initiation of piping: 1) first visible movement, 2) heave progression, 3) boil formation, and 4) total heave. Tests were performed on sandy soils representing a variety of grain size, grain shape, gradation, and specific gravity. The effects these soil properties have on the critical hydraulic conditions needed to initiate piping erosion were evaluated based on observations and test results.