Probabilistic Characterization Of Particle Morphology And Its Effects On The Macro-micro Mechanical Behaviours Of Sands

Morphology,one of the particle basic characteristics,significantly affects the physical and mechanical properties of sands.Studies on the characterization methods of realistic particle morphology and its influence on the macro and micro mechanical behaviours of sands are of important scientific significance to understand sand behaviours.Recently,with the rapid development of X-ray CT scanning and image processing techniques,researchers are able to acquire the 3D morphology data of sand particles.At the same time,numerical methods including DEM(Discrete element method)and FEM-DEM coupling method provide effective tools to investigate the macro and micro mechanical behaviours of sands.Nevertheless,problems remain within the research areas of the characterization of the particle morphology and its effect on sand behaviours,mainly including:(1)the whole process of acquiring particle morphology data based on CT scanning is time and efforts consuming,and only a limited number of particles can be scanned,which inevitably introduces many uncertainties into the characterization results of particle morphology.(2)Most of current DEM researches adopt the regular convex geometries or simple aggregates to represent the real sand particles to investigate the effect of particle shape on sand behaviours at the sample scale,which ignores the realistic geometric features of natural particles,and their morphology are usually concave and convex,and also distinct with each other,hence,the influence of particle morphology on the macro and micro mechanical behaviours of sands is still not clarified clearly.(3)Due to the influence of particle morphology characteristics(e.g.elongation)and the sedimentation process,the spatial orientation of sand particles is inherently anisotropic at the engineering scale,and until now there is a lack of micromechanism research on the effect of initial anisotropy of sands on the macro mechanical behaviors of geotechnical engineering structures(e.g.,foundation engineering).To address the above issues,this thesis firstly proposes a novel method to probabilistically characterise the realistic particle morphology based on a limited number of X-ray CT scanned particles,and realize the random generation of virtual particles.Based on those generated virtual particles similar with the parental particles,systematic studies on the influence of particle morphology on the macro and micro mechanical behaviours of sands under conventional triaxial drained and undrained shear stress paths are conducted through the DEM simulations,and the effect of particle morphology on anisotropic critical state mechanical parameters is also discussed.At last,the FEM-DEM hierarchical coupling method is used to explore the effect of initial anisotropy of sands on the macro and micro mechanical behaviours of shallow foundation.The implementation details and major conclusions are summarized as follows:(1)This thesis first presents the background and significance of the characterization of particle morphology and the study of its influence on the macro and micro mechanical behaviours of sands.A literature review of the existing methods for characterization of particle morphology and studies about the effect of particle morphology on the behaviours of sands are given.The limitations underlying the existing approaches and the key issues needed to be addressed are highlighted.In addition,technical guidelines on the thesis are also outlined.The basic theories of the Sphere Harmonic(SH)reconstruction of realistic particle morphology based on X-ray CT scanning and image processing techniques,and numerical methods including DEM,FEM-DEM hierarchical coupling method are then systematically introduced.This work provides a theoretical basis for the following researches.(2)A novel method is proposed to probabilistically characterise the realistic morphology of sand particles and randomly generate virtual particles based on a limited number of X-ray CT scanned particles.The proposed method adopts the SH function amplitude,a rotation invariant,as the particle shape descriptor,avoiding the dimensional disaster when directly using SH coefficients to describe particle shape.The joint probabilistic distribution function of SH function amplitudes is built to characterise the uncertainties of particle morphology,which can effectively consider the correlation between particle size and morphology,and makes the randomly generated virtual particles inherit both the particle size distribution and morphology characteristics from the parental scanned particles.The proposed approach is validated and further evaluated using two types of common sands,Leighton Buzzard sand(LBS)and Highly Decomposed Granite sand(HDG).This method can be used as an effective tool to consider the realistic particle shape and its uncertainties in the particle flow numerical methods like DEM.(3)Based on the proposed random generation method of realistic particle morphology,a large number of virtual LBS particles are acquired and incorporated into the DEM models to conduct conventional triaixal drained and undrained shear test simulations through clump logic.The corresponding numerical simulations of five types of binary mixtures(mixed with different proportions of spheres and LBS clumps)with same grading but different overall particle irregularity are implemented in order to systematically explore the effect of particle morphology on the drained and undrained shear behaviours of sands.The simulation results of triaxial drained shear test show that the predicted critical state internal friction angle of LBS clump specimen is consistent with the laboratory result,confirming the accuracy of DEM model considering realistic LBS particle shape.And the results also show that at the same relative density,particle morphology affects the soil stiffness,shear strength and dilatancy significantly.The simulation results of triaxial undrained shear test show that at the same initial void ratio,the deformation characteristic of sands under undrained condition is affected by the particle morphology,ranging from strain softening to strain hardening with the increasing of the overall particle irregularity,and the corresponding liquefaction resistance is gradually enhanced.The micro information including contact network characteristics and anisotropic fabric within five assemblies under different drained conditions is extracted to give insight into the micromechanisms underlying the particle morphology effect,enriching the current knowledge about the influence of particle morphology on the micro mechanical behaviours of sands,and DEM study considering the realistic particle shape overcome the difficulties in micro mechanics analysis at the laboratory tests,and provides a theoretical reference for the study of micro-mechanical behavior of sands.(4)More DEM triaxial drained shear test simulations of the aforementioned five types of granular mixtures under different initial states are conducted to investigate the effect of particle shape on the critical state mechanical properties.The results show that,under the framework of anisotropic critical state soil mechanics,with the increasing of the overall particle irregularity,in the q-p space,the critical state deviatoric stress ratio M increases,and the slope of critical state compression line in e-p space is hardly affected,while the intercept increases.And in the K-p space,the critical state line of joint invariant of anisotropic fabric can be seen as a line passing through the origin,and the slop increases with the overall particle irregularity.These numerical observations provide a theoretical reference for the constitutive modelling of sands considering both micro particle information and anisotropic fabric.(5)The FEM-DEM hierarchical coupling method is adopted to investigate the effect of inherent anisotropy on the macro and micro mechanical behaviours of sands at the engineering scale.Taking the shallow foundation as an example,the bearing capacity and shear failure behaviours under different inherent anisotropies are systematically analyzed.The space distribution changes of the contact normal direction and particle orientation,and also the mechanical behaviours of the local gaussian points are discussed.It finds that the ultimate bearing capacity and the settlement of the shallow foundation decrease with the increasing of the bending plane angle,and ignoring the effect of initial anisotropy of sands will overestimate or underestimate the ultimate bearing capacity and the corresponding settlement.The general shear failure bands are approximately symmetric for the dense sands when the initial particle orientation configuration is also approximately symmetric,and vice versa.It is more liable for sand to resist external loads by adjusting the distribution of the internal contact normal rather than that of particle orientation.This FEM-DEM coupling research extends the study scale of particle morphology effect to the engineering scale,and has a certain reference value for the practical engineering application.