Effects of 3d Morphological Characteristics of Sand on Multi-scale Intergranular and Sand-geosynthetic Interactions

Abstract

To understand the process of mobilization of shear strength in sands and sand-geosynthetic interfaces at a fundamental level, it is essential to precisely characterize the size and shape of the grains and the shear-induced surface changes in geosynthetics. Due to the difficulty in defining and measuring the size and shape descriptors for a wide range of complex particle morphologies, the majority of earlier studies either completely ignored or used visual comparison charts which are highly subjective. Those who employed digital image-based techniques also limited their assessment to morphological characteristics within a two-dimensional (2D) context. The 2D shape parameters are obtained from the images taken through random projection of particles, which results in inaccurate values. In the existing literature, only a small number of studies have employed a three-dimensional (3D) particulate morphology to investigate their effects on sand-geosynthetic interfaces. In addition, most of the research on the effect of particle morphology on the shear behaviour of granular material and their interactions with geosynthetics has not been explored at multiple scales of analysis. Also, very limited studies gave importance to the micro-structural analysis of geosynthetic surfaces after shearing to understand the micro shear mechanisms responsible for macroscopic shear behaviour. In this context, the current study aims at accurate multi-scale 3D morphological characterization of sand grains and understanding the effects of particle morphology on the shear strength of sands and interface shear strength of sand-geosynthetic interfaces. In the first part, this thesis presents a systematic and comprehensive 3D quantification of the morphology including size and shape, and 3D fractal dimension of real sand particles taken from four different types of sands with various size and shape characteristics. The first step in obtaining the 3D morphology was to perform high resolution X-ray µCT scanning on the sand particles. The individual particles were then extracted and separated using various image processing techniques. The smooth and continuous 3D particle surfaces with their overall morphology preserved were then recreated using a sophisticated approach based on spherical harmonic (SH) analysis. To obtain the geometrical parameters for calculating the size and shape of sand particles, a robust MATLAB algorithm was written and implemented in this study. For computing the particle size, the three principal dimensions of the particle were obtained through principal component analysis (PCA). The particle size distribution (PSD) of the sands obtained from µCT images and mechanical sieving of the sands were compared. The 3D shape descriptors were quantified using computational geometry and image analysis methods based on the SH reconstructed particle surface. Additionally, a fractal dimension for the 3D closed surface of the sand particle was discussed and quantified using spherical harmonic-based fractal analysis. The statistical analysis of the shape descriptors revealed that they are not independent. The correlation between any two shape descriptors relies mainly on the distance between the characteristic scales of these parameters. In the second part of the work, the direct shear apparatus (DSA) was modified to make it suitable for visualizing and studying the kinematic behaviour of particles. A series of sand alone direct shear tests were performed using modified DSA on four types of sands with different morphological characteristics. High quality videos were captured during shearing to examine the development of a localized shear zone in the tested sands by analyzing particle displacement using digital image correlation (DIC)technique. Full-field shear strains were measured and plotted in order to determine the shear band thickness. The experimental data of sand alone tests reveals that sand particles with higher irregularity exhibit higher peak shear stress and dilation due to their enhanced particle interlocking compared to similar sized particles with less irregularity. It was observed that the variations in particle size also affect the resistance to rolling and sliding, thereby influencing the shear strength behaviour of granular materials, with the peak and residual friction angles increasing with an increase in the particle size. The DIC analysis revealed that shear band thickness is smaller for irregular particles than the particles with regular morphology, which is explained in terms of the difficulty for rotation that the irregular particles encounter, because of their stronger interlocking. In the last part of the work, interface shear tests were conducted on different test sands in contact with a woven geotextile or a smooth geomembrane. Experimental data revealed that the dominant shear mechanism in dilative (sand-geotextile) interfaces is the interlocking between the sand particles and the surface asperities of the geosynthetic material, whereas in non-dilative (sand-geomembrane) interfaces, the mechanisms are sliding and plowing. When particle size effects are considered, fine particles interacting with the geotextile surfaces result in higher peak friction angles, due to their ability to interlock better with the asperities of the geotextile surfaces. However, in the case of medium and coarse sand particles, their size was larger than the concavity of the geotextile's asperities, which made it difficult for them to interlock properly during shearing, resulting in lower peak friction angles and reduced dilation. In sand-geomembrane interfaces also, fine particles led to higher peak friction angles compared to medium and coarse particles. Micro-topographical analysis of sheared geomembranes revealed that finer particles tend to make significant number of effective contacts per unit area with more grooves formed at less spacing, resulting in higher shear strength. Also, higher normal stress levels caused a decrease in the peak friction angles in sand-geotextile interfaces, whereas the opposite trend was observed in sand-geomembrane interfaces, which is attributed to the transition in the shearing mechanisms. This variation can be attributed to the particles engaging in plowing behaviour on the smoother surface of the geomembrane at higher normal stresses, resulting in an increased interface friction angle. Findings from this study help in the precise characterization of particle morphology of granular materials and quantify the effects of different morphological descriptors on the intergranular and sand-geosynthetic interactions at multiple scales.