Investigation of Progressive Internal Erosion of Gap-graded Granular Soils Using X-ray Microtomography

Abstract

Internal erosion refers to the long-term, progressive process wherein seepage flow carries fine particles through the interstices between coarse particles, leaving empty voids within soil structure. Soils with a gap-graded particle size distribution (PSD) are particularly susceptible to this phenomenon, especially when fine particles do not carry the overburden load. According to the International Commission on Large Dams (ICOLD), internal erosion is responsible for 46% of embankment dam failures worldwide. The removal of fine particles can induce changes in the internal structure and hydraulic properties of the soil, resulting in increased permeability and diminished soil strength. The objective of this research is to investigate the progression of internal erosion of gap-graded materials experimentally during triaxial shearing using X-ray micro-computed tomography (μCT).

To facilitate the examination of internal erosion using X-ray μCT, a customized mini-triaxial apparatus is developed. Artificial gap-graded soil samples (20 mm in diameter and 40 mm in height) made of artificial glass beads (GB) or natural Leighton Buzzard sand (LBS) are subjected to triaxial shearing under constant hydraulic gradient, and a series of μCT images are taken at selected shear strain levels. Advanced image processing methods are adopted to obtain particle and pore-scale qualities (e.g., particle volume, particle surface area, pore volume, pore sphericity, pore aspect ratio). Image analyses are used to quantify the evolution of pore structure and internal erosion. The combination of sample-scale mechanical and hydraulic measurements, along with the statistical analysis of pore-scale measurements, offers novel insights into the progressive internal erosion and the effect of mechanical disturbance, fines content, and particle shape.

The experimental results reveal that prior to triaxial shearing, internal erosion remains limited. However, the mechanical disturbance occurring during shearing modifies the pore structure of the coarse particles, thereby inducing progressive internal erosion and enhancing permeability. The internal erosion process leads to an overall volume contraction of the sample during shearing, while individual pore may either contract or expand in the initial shearing stages. The area experiencing significant changes in pore size and fines removal extends from the top loading plate for GB samples, while it is distributed differently in LBS samples due to higher resistance to internal erosion. Furthermore, the fines content exerts a substantial influence on the transitional behavior of internal erosion, transitioning from suffusion to channel-based erosion and ultimately to backward erosion as the fabric state progresses from underfilled to filled and overfilled conditions. The experimental results reveal the evident impact of particle shape on internal erosion at both sample and pore scales. In contrast to spherical GB particles, the sample formed by irregular LBS particles exhibits higher shear strength, lower rate of fines erosion and different formation mechanisms of seepage channel.

Date of Award	9 Apr 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jianfeng Jeff WANG (Supervisor)