Progressive Internal Erosion in a Gap-Graded Internally Unstable Soil: Mechanical and Geometrical Effects

Abstract

This paper investigates the posterosion geomechanical behavior of internally unstable granular material due to removal of fines caused by erosive forces of water flow. Posterosion undrained behavior of a gap-graded internally unstable soil was investigated for a range of erosion durations and inflow velocities using a triaxial-erosion apparatus. Test results indicated that the undrained behavior of the original specimen changed from a strain hardening behavior to a flow-type behavior with limited deformation after internal erosion. The initial peak strength improved and the flow potential decreased during the initial stage of erosion. This observed increase in initial peak strength is believed to be the result of a better interlocking between the coarse particles posterosion. In contrast, the slip-down movement of the particles due to an increase in the posterosion void ratio postponed the dilation tendency. Test results also suggested that even erosion of a small percentage of fine particles improved the mechanical frictional behavior of the soil. However, there was a threshold value for the loss of fine particles at which this positive effect deteriorated. This might have been due to formation of local metastable structures and/or overcoming contractive behavior after loss of the semiactive fines and a considerable increase in the global void ratio. Shear strength results, rate of erosion, and local vertical strains together suggest that the intergranular void ratio is a powerful index in evaluating the posterosion mechanical behavior of internally unstable soils.