Experimental Assessment on the Internal Instability of Broadly Graded Silty Sand

Abstract

Suffusion can occur when fine particles are under low effective stress and free to travel through constrictions between coarse particles, and sufficient seepage forces move the fine particles through the voids. Generally, gap-graded soils, which are a binary mixture of two distinct particle sizes, are susceptible to suffusion. Such soils show clear manifestations of suffusion, such as a significant quantity of fine particles discharged and an increase in permeability and void ratio. Therefore, in previous studies on seepage erosion problems, gap-graded soils were mostly utilized as the target soil. However, it is not clear whether the broadly graded soils, the fill dam materials in South Korea, are susceptible to suffusion according to the existing criteria and previous research. Additionally, experimental studies on broadly graded soils have rarely been conducted, especially in terms of hydraulic conditions. Therefore, a deeper understanding of the suffusion progress on broadly graded soils under various hydraulic conditions (hydraulic gradients) is required. In the present study, suffusion tests were performed on gap-graded andbroadly graded soils with different relative densities by using the newly developed suffusion test apparatus. During the test, the hydraulic gradient was increased stepwise, and the occurrence and progress of internal erosion were analyzed based on the amount of discharged soil and the coefficient of permeability. In contrast to gap-graded soils, the test results for broadly graded soils showed a continuous reduction in k before the initiation of erosion. At the onset of internal instability, sudden increases in k and soil discharge occurred under a relatively high hydraulic gradient. Additionally, a series of seepage tests on broadly graded soils showed that the higher the relative density, the higher the hydraulic gradient required to reach an unstable state.