Measuring the Erodibility of Cohesive Soils Influenced by Seepage Forces Using a Laboratory Jet Erosion Test Device

Abstract

Seepage influences the erodibility of streambanks, streambeds, dams, and embankments. However, the interaction between fluvial and seepage mechanisms in cohesive soils is still poorly understood. Usually the erosion rate of cohesive soils due to fluvial forces is computed using an excess shear stress equation, dependent on two major soil parameters: the critical shear stress and the erodibility coefficient. A submerged jet test apparatus (JET Jet Erosion Test) is one method for measuring these parameters. In this study, a new miniature version of the jet test device, referred as the mini JET, and a seepage column were utilized to measure the erodibility of cohesive soils influenced by seepage. The experimental setup was intended to mimic a streambed and a streambank when the mini JET and seepage column were placed in vertical and horizontal directions, respectively. The horizontal setup involved connecting the jet to the side of the column and imposing a horizontal jet on the soil. Two different soils types, sandy clay loam and sandy loam, were used in the experiments. The soils were packed in three equal lifts in a standard mold at a uniform bulk density (1.4 to 1.7 Mg/m3) at the soils optimum water contents. To achieve this optimum water content, the soils were mixed with required quantities of water and allowed to equilibrate for at least 24 hr in a closed container. In order to investigate the influence of seepage forces on erodibility, tests were performed on paired samples prepared in the same way but tested at different times. One sample was tested without seepage using only the mini JET. The other sample was tested with seepage forces by including a constant-head in the seepage column. A number of experiments were conducted using various heads and bulk densities. Results show that the erodibility of cohesive soils increased exponentially when seepage forces increased. Even higher erosion rates were observed when inducing erosion with seepage for the horizontal setup, especially at the lower soil bulk densities. The critical shear stress and soil erodibility relationships were obtained for both soils and for all experimental setups.