FORCE CHAINS AND THE FRAGMENTATION OF GRANULAR MATERIALS
In this study, the fragmentation of granular materials is evaluated by laboratory experiments and by numerical and theoretical analyses. The laboratory experiments used were the direct shear, point load, compaction, and grain size distribution tests. The Discrete Element Method (DEM) was used for the numerical analysis. Fractal theory was used to interpret the laboratory results. The direct shear tests were carried out on wooden cylindrical rods. These tests and its DEM simulation indicated that the normal and shear loads exerted by the direct shear equipment were transmitted and resisted by the rods through force chains. Only a fraction of the rods developed these resisting force chains, the rest of the rods remained idle and could be removed without affecting the stability of the system. The effect of force chains on the simulated granular material was studied in the laboratory by the point load test. Point load tests on gravel indicated that the size of the gravel and their degree of moisture influenced their strength. Proctor tests were also used to analyze fragmentation under compressive loads. Samples of gravel were subjected to different levels of compression in a cylindrical container measuring 10 cm in diameter. DEM simulation of the test indicated that the compressive force induced by the Proctor hammer was transmitted to the gravel through force chains. These force chains caused the fragmentation of the gravel. The grain size distribution of the gravel before and after the compaction tests were evaluated and used in conjunction with fractal theory to analyze the levels of fragmentation experienced by the gravel. The fragmentation fractal dimension was used to evaluate the changes in size distribution experienced by the gravel. The fragmentation fractal dimension was found to increase with the degree of fragmentation of the gravel. Changes in the shape of the gravel particles as a result of their fragmentation were evaluated using fractal theory. Thus, the fractal dimension concept from fractal theory was found to be a simple and powerful theoretical tool to analyze the changes experienced by granular materials as a result of their fragmentation.
Advisor:Julie M. Vandenbossche; Luis E. Vallejo; Jeen-Shang Lin
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Keywords:civil and environmental engineering
Date of Publication:01/28/2005