Theoretical And Numerical Research On Granular Mechanics Of Soils

It has been long recognized that a better understanding of complex soil behavior associates with the advancing insights of its granular nature and related microstructures. So far, the granular nature and related microstructures of soils can be thoroughly studied by the approach of granular mechanics.Granular mechanical investigations of soil behavior via particle flow method developed in 1980's were adopted in the present thesis and a commercially available software called Particle Flow Code(PFC2D) allowed by Itasca? for the research purpose was intensively used as the numerical platform of analysis in this thesis.First, five elementary soil mechanics lab tests were realized in the environment of PFC, which included (1) uniaxial compression test, (2) confined compression test, (3) confined biaxial test, (4) unconfined biaxial test and (5) simple shear test. Several numerical techniques involved were developed including the creation of granular assembly of particles with the desired porosity, the flexible boundary conditions as well as the loading servo-system. Such advanced numerical testing environments had been rarely reported in the literatures to the knowledge of the author.Second, the cohesionless granular assembly of disk particles created was calibrated by the real biaxial testing data of some fine sands and the apparent strength and dilatancy indices of the granular assembly were obtained. The correlations of these apparent indices with the granular parameters were numerically analyzed which somehow described the important up-scaling law of granular mechanics, i.e. the micro-macro parameter inherence. Subsequently the calibrated model of granular mechanics was used to analyze a classical soil mechanics problem- Prandel problem of loading capacity of weightless foundations. Comparisons of the granular mechanical solution and the Prandel solution were made followed with the discussions of their agreements and disagreements.Third, the cohesive granular assembly of platety or disk particles was investigated. Following the similar calibration procedure as mentioned before, the apparent (geotechnical) behavior of clayey soils represented by the cohesive granular assembly of platety or disk particles was analyzed in relation with the granular parameters, namely the platety particle orientation, platety particle length and curvature of platety particles and the spatial distribution of platety particles. On basis of these numerical analyses, the failure envelopes in the plan of shear stress vs. mean stress were obtained with two distinct features for clayey soils around low or medium high mean stress level, respectively. Around the low mean stress (<100 kPa) the apparent friction angle of clayey soils significantly correlated with the orientation of platety particles while the failure envelope tended to fall down in case that the mean stress was brought beyond certain level (>150 kPa).In summary, the correlations of granular parameters of soils with their apparent (geotechnical) properties were numerically established. A particularly interesting phenomenon was observed in clayey soils where the geometrical anisotropy of clayey soils due to the orientation of platety particles played the crucial role in the high apparent frictional angles observed in real tests of soft clayey soils. It was then concluded that the approach of granular mechanics via particle flow method looked promising and fundamental in gaining the micromechanical insights of fundamental soil behavior.