Study on Particle Shape, Size and Gradation Effects on the Mechanical Behavior of Coarse-Grained Soils

Inherent particle properties such as size, shape, gradation, surface roughness and constituent material (i.e. mineralogy) control the particle-scale contact response of particles which in turn determines the global mechanical behavior of coarse-grained soils observed in both laboratory tests and full-scale geosystems. This dissertation presents a series of studies as part of two separate projects aimed to characterize the effects of particle shape and gradation on the mechanical behavior of coarse-grained soils.Systematic investigation of the effects of an individual particle property on the mechanical behavior of granular soils is a pervasive challenge in experimental studies with naturally occurring soils because it requires careful control over the remaining particle properties. Due to this challenge, conflicting interpretations of the effect of different particle properties on the behavior of soils exist in literature. By taking advantage of state-of-the-art additive manufacturing (i.e. 3D printing) technology, artificial sand analog particles can be manufactured with independent control over different particle properties such as size, shape and gradation. The first part of this dissertation investigates the feasibility of 3D printing technology to model the mechanical behavior of coarse-grained soils. The results of this study show that 3D printing technology can be used successfully to create artificial sand analogs with different sizes and shapes using either X-ray CT scans of natural sands or synthetic shape generating algorithms based on spherical harmonics. Although the 3D printed analogs exhibited greater compressibility compared to that of natural sands, the shear wave transmission behavior of 3D printed sands, measured using piezoelectric bender elements, exhibited dependencies on mean effective stress, void ratio and particle shape that are quantitatively similar to those previously reported for natural sands. By analyzing the shear wave transmission data, equations to predict the shear wave velocity taking into account the particle shape and void ratio were developed. The triaxial compression behavior of the 3D printed sands was investigated in both drained and undrained conditions, which exhibited a stress-dilatancy response typical of natural sands, and the interpretation of their shear response can be captured within the critical state soil mechanics framework. The results of tests on 3D printed sands show that changes in particle shape produce changes in friction angles and critical state parameters that are similar to those observed in natural sands. However, the greater compressibility of the 3D printed material and the smaller inter-particle friction coefficient should be considered in the interpretation of results. The use of methods based on poorly-graded sand data to characterize the strength and stress-dilatancy behavior of widely-graded soils is a common practice in geotechnical engineering; however, many naturally-occurring soils encountered in the field are widely-graded as evidenced by case studies. The second part of this dissertation examines the effect of gradation and particle size on the strength and stress-dilatancy behavior of widely-graded coarse-grained soils. A well-graded natural sand sourced from the Cape May Formation near Mauricetown, New Jersey was selectively sieved to produce different sands with similar particle shape parameters but with different gradation and median particle size. The results of a series of isotropically consolidated drained and undrained triaxial tests exhibited dependency on the peak strength, dilatancy and critical state parameters of the gradation and median particle size. The analysis of the results also showed that capturing the effects of gradation and particle size depends on the definition of soil state, where the state parameter provides more robust trends than the relative density. The results indicate that for any given state parameter, more widely-graded soils exhibit greater peak friction angles, maximum dilation angles, and differences between the peak and critical state friction angles. The research efforts presented in this dissertation show that the individual effects of particle shape, size and gradation on the soil behavior can be investigated using new experimental techniques (i.e. creating artificial sand particles using 3D printing) or selective sieving of naturally occurring soils. This can bring benefits in improved understanding of soil behavior aiding increased efficiency and robustness of geotechnical site characterization and design methodologies and in the advancement of constitutive models