Experimental study on the effect of fine contents on internal erosion in natural soil deposits

Abstract

Seepage-induced internal erosion is observed in both artificially engineered fill structures and natural soil deposits. Fine content is of great significance for the fabric of soil. This paper aimed to determine the critical fine contents of natural soil deposits beyond which the internal stability of the mix was distinctly altered and illustrate the internal erodibility from the viewpoint of fabric. To this end, a fixed-wall permeameter capable of accurately detecting the critical hydraulic gradient of erosion initiation and collecting the cumulative eroded soil mass at a constant inflow rate was developed. Silty clay particles and sandy gravel particles extracted from original soil were employed to reconstitute specimens with fine contents ranging from 0 to 100%. Porosity measurement, seepage testing, and direct shear testing were conducted on remolded samples. Companion control specimens were tested under different flow rates to verify the applicability of the experimental device. The results indicate that according to critical fine content, the fabric of the soil samples with different fine contents could be split into coarse-particle-supported structure (CPSS), fine-particle-supported structure (FPSS), and transitional coarse–fine-particle-supported structure (TCFP). The newly developed experimental device provides a feasible methodology to investigate the internal erodibility of natural soil deposits. Different fabrics correspond to disparate shear strengths and distinct erosion characteristics, including the critical hydraulic gradient of erosion initiation, cumulative eroded soil mass, and average hydraulic conductivity. For coarse-particle-supported structure specimens, the coarse particles predominantly govern the mechanical and hydromechanical properties. An increase in fine content within pores formed by coarse particles could increase shear strength and reduce susceptibility to internal erosion. The mechanical and hydromechanical properties of FPSS specimens were basically controlled by fine particles. Coarse particles suspended in a fine matrix could somewhat increase the soil’s shear strength and reduce internal erodibility. TCFP specimens were most vulnerable to internal erosion in terms of minimum critical hydraulic shear stress and maximum cumulative eroded soil mass. It is essential to expand the scope of research to cover the transitional coarse–fine-grain-supported structure instead of remaining limited to the coarse-grain-supported structure.