Characterization of soil grain surface roughness and its change with normal loading

Abstract

Abstract of thesis entitled Characterization of soil grain surface roughness and its change with normal loading Submitted by YAO Ting for the Degree of Doctor of Philosophy at The University of Hong Kong in March 2019 Surface roughness, as the third order of particle shape, influences mainly the coefficient of inter-particle friction at particle scale and the small strain stiffness at macro scale of soils. Comparing to the quantification of particle size and shape, the surface roughness of natural soil particles is more difficult to measure and describe. Although some studies have been conducted to quantify the surface roughness of sand particles by advanced techniques, systematic research on the quantification of surface roughness and the effect of surface roughness on the contact behaviour of sand particles is still limited. The current work aims at an integrated study of the surface roughness of natural sand particles and its evolution under compression, which was achieved by means of experimental tests using an optical interferometer and a custom-made single particle compression apparatus at The University of Hong Kong. The surface roughness of several natural sands was studied. Two methods including motif extraction and Power Spectrum Density (PSD) were adopted to remove the influence of surface curvature on the quantification of surface roughness. The influence of measurement points, size of field of view, particle size and mineralogy on the surface roughness of sand particles was comprehensively investigated. Comparing to the motif extraction method, the PSD method is more controllable and the fractal dimension could be used to reconstruct fractal surfaces. For all the sands tested, finer particles tend to have rougher surfaces. One difficulty in studying the surface roughness of natural sand particles is the variability between particles. Therefore, the cumulative distributions of the surface roughness of Leighton Buzzard sand (LBS) particles were fitted by three candidate statistical functions with the Weibull function chosen to represent the distributions of surface roughness. The number of tested particles required to obtain statistically representative results was discussed. For LBS, 30 particles are sufficient for obtaining statistical parameters within ±5% error. Single particle compression tests were carried out on LBS particles coarser than 2.36 mm. The plastic deformations of both the asperities and the bulk of the particles were studied by comparing the surfaces before and after loading from the optical interferometer. The evolution of the surface roughness of particles with increasing normal load was studied. Comparison between the experimental results of the relationship between load and displacement and the predictions by theoretical contact models was performed. An attempt was made to study the development of the plastic contact area of the particles with the normal load. The asperities on the surfaces of LBS deformed plastically even under very low normal load and the flattening of the asperities reduces the surface roughness of the particles. The yield of the bulk was found to be highly influenced by the geometry of the local surface that contacts with the loading platen. After the yield, the surface roughness tends to increase with loading. (466 words)