Internal Erosion of Soils

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.

Granular soils in pavement systems, insidiously experience fines migration during their expected lifespan. Experimental monitoring of particles movement in a granular matrix is extremely challenging due to its concealed nature. Numerically modelling the permeation process demands a high computational power and requires additional conditions, which are frequently difficult to be obtained. This paper proposes an integrated numerical approach to estimate the conditional probability of observing a given voids ratio of a filter subjected to different fractions of permeating fines. The discrete element method (DEM) has been used for developing numerical samples of random voxels to estimate the void ratio distribution of the fines-free filter (i.e. virgin filter). Qualitative results obtained from a pilot scale experimental program have been presented to validate the hypotheses on which the novel methodology has been developed. The results further show that the distribution of the voids ratio in the filters subjected to permeating fines follow a gamma distribution, with a 0.9 probability of observing a voids ratio lower than the average value in the virgin filter.

In this study, Biopolymers are used as an attempt to create sustainable environment by eliminating the negative environmental impacts of using traditional admixtures in soil stabilization. Xanthan Gum is used as a biopolymer to treat expansive soil. A series of tests like, Standard Proctor Test, Unconfined Compressive Strength (UCS), One-Dimensional Consolidation and Standard Direct Shear tests were conducted on virgin soil and biopolymer (0, 0.5, 1, 1.5, 2, 2.5%) treated soils. The results revels that by addition of biopolymer content Maximum Dry Density (MDD) of soil decreases and Optimum Water Content (OMC) increases. The UCS value is increased by 4 times for the addition of 1% xanthan gum to soil for 28 day curing period. Compressibility of soil is deceased by 65% for 28day curing period. Shear parameters of treated soil shows improvement with addition of xanthan gum content. For further examination, SEM analyses were conducted on the tested samples and revealed that the soil fabric had white lumps and pores in the soil structure were filled with cementitious gel. Moreover, the resistance towards shear and compressibility of treated samples increased with curing times. Therefore, use of Xanthan Gum for soil stabilization is a solution for eco-friendly soil stabilizing material.

A valuable conceptual framework for the characterisation of seepage-induced internal instability
was provided in the article under discussion, particularly for the ‘suffusion and suffosion’ phenomena.
A few modifications are suggested here to clarify the definitions in the original paper.

Suffusion is defined as the migration of fine particles caused by seepage flow through pre-existing pores of a soil structure made of coarse particles. This particle transportation changes the fine particle content and its distribution, possibly impacting the mechanical behaviour of eroded soil. Although limited research has been conducted on the post-erosion mechanical consequences under monotonic shearing, little attention has been paid to the impact of suffusion on the cyclic resistance and liquefaction potential of internally unstable soils. This paper investigates the cyclic and post-cyclic behaviour of a gap-graded cohesionless soil using combined triaxial-erosion apparatus. An internally unstable soil was chosen for the erosion test and was subjected to different seepage flow velocities and durations followed by cyclic loading and post-cyclic shearing. During cyclic loading, the eroded specimens with different residual fine contents behaved in a similar manner to a soil specimen constructed only of coarse particles. Regardless of the seepage velocity and duration, the erosion of fine particles resulted in significant increase in cyclic resistance. It is understood that eroded specimens with lower intergranular void ratios show higher resistance during cyclic loading, highlighting the importance of the intergranular void ratio in understanding the post-erosion mechanical behaviour of soils.