Internal erosion of a gap-graded soil and influences on the critical state

Abstract

Water retaining structures are critical elements of civil infrastructure. Internal erosion of soils forming the containment structures may occur progressively and lead to expensive maintenance costs or failures. The strength, stress–strain behavior and critical state of soils which have eroded, as well as the characteristics of the erosion, may be affected by hydraulic gradient, confining stress and relative density of the soil at the start of the erosion. Here, erosion and triaxial tests have been conducted on gap-graded soil samples. The tests and results are novel as the samples were prepared to be homogenous post-erosion and prior to triaxial testing by adopting a new sample formation procedure. The post-erosion homogeneity was evaluated in terms of particle size distribution and void ratio along a sample’s length. The erosion-induced mechanical property changes can then be linked to a measure of initial state, more reliably than when erosion causes samples to be heterogeneous. The results show that erosion causes the critical state line in the compression plane to move upwards. The movement is lesser than the increase in void ratio caused by erosion. The state parameter is therefore reduced, consistent with the soil’s reduced peak strength and its less dilative response. Regarding the erosion characteristics, the flow rate decreases with the increase in initial relative density or effective stress, but increases with the increase in the hydraulic gradient being applied. The cumulative eroded soil mass increases with the increase in hydraulic gradient and decreases with the increase in initial density and effective confining stress.