Particle breakage in uniform and gap-graded soils

Abstract

This research investigates the particle breakage of uniform and gap-graded soils using a special designed oedometer cell to reach a high stress level and also using a Wille Geotechnik ring shear apparatus to enable soils to be crushed significantly, combined with particle size distribution analysis. Aspects of breakage investigated are: the effect of the total and effective stress, the effect of the particle sizes, and the application of the fractal theory to the results.

The effects of pore water pressure on the breakage were examined by submitted soils to different total stresses but the same effective under one-dimensional compression. The effective stress was found to dominate the breakage. A constitutive relationship based on the concept of effective stress, was derived to predict the compression behaviour. The relationship proposed in this study incorporates the individual particle strength given from single particle crushing test, changes of grading in term of breakage, and ultimate fractal distribution, which are validated by the experimental data obtained on the uniform Leighton Buzzard sand with range of stresses, and experimental results which show that breakage is a power law function with the stress.

The one-dimensional normal compression lines (NCL) are found non-convergent for 20% and 60% small grains (SG) content specimens and convergent for 40% SG content specimens prepared at different initial void ratio, while the breakage of these samples is decreasing with increasing SG content, and thus the role of breakage in the convergence of the NCL is not clear. The specimens prepared at the same initial void ratio but different SG content and ratios were found have a similar yielding stress and convergent at the similar final void ratio although the yielding stress is highly dependent on the size of particles in the uniform specimen, which indicates that the amount of small grains has no or limited effect on the yielding stress and the convergence of the NCL. It seems the yielding stress is not dominated by the tensile strength of large particle despite the force in mixture soils transmitted though the large particles. The relative breakage was found to be dependent on the size of large particles instead of the ratio of large to small size particles.

From the ring shear tests, the significance of breakage was found in the finite shear zones, and the thickness of shear zone was not affected by the presence of small grains but by the size of large particles. The fractal dimension of the ultimate grading was computed by number of particle based method, but the density distribution shows that each soil have two or three peaks in the density distribution, leading to the grading no longer to being well graded. Multi-fractal approach was proposed to analyse the ultimate grading for uniform and gap-graded soil, by dividing into two domains and three domains fractal distribution. The results presented in this thesis being more experimental evidence on the role of large and small particles in gap-graded soils, and the evolution of their grading during compression and shearing.