Research On Macro-and Meso-mechanical Properties Of Weathered Granite Sand In Three Gorges Based On Particulate Discrete Element Method

Weathered granite residual soil is widely distributed in our country.It has the advantages of convenient material collection,low cost,turning waste into treasure,saving resources and protecting the environment by using resource-rich weathered granite residual soil as constructional material.With the advancement of the One Belt One Road strategy,there will be infrastructure construction in the mountainous area with weathered granite residual soil,and its promotion and application can solve the problem of resource allocation in local engineering construction to a certain extent.However,there are few research on the classification and engineering properties of weathered granite residual soil in our country,and the research on the physical and mechanical behavior of the mesoparticle level that determines the macro-mechanical properties of residual soil is even less noticed.At present,the research on weathered granite residual soil is mainly based on experimental research.Due to the large amount of coarse and gravel particles,it exists size effect when weathered granite sand is studied by laboratory experiments.The design indicators of weathered granite sand(such as bearing capacity and in-situ shear strength)required in engineering are mostly obtained by costly in-situ tests.Particulate discrete element method(PDEM in abbreviation)is based on the movement and interaction of particles.Based on PDEM,the macro-and meso-mechanical behavior of sands can be accurately obtained by using appropriate contact constitutive models and meso parameters.Therefore,it is obviously suitable for studying the meso-mechanical behavior at the particle level that determines the macroscopic mechanical properties of weathered granite sand by using PDEM.In this thesis,the cohesionless weathered granite sand in Three Gorges Reservoir Area is taken as an example,the macro and meso behavior of this sand is studied by multiscale analysis based on the combined examination of laboratory and PDEM tests.The rolling resistance Hertz contact model and particle breaking function module ware developed for accurately simulating the behavior of weathered granite sand based on particle flow code.The mechanism of the main influencing factors such as particle shape,particle gradation and particle breakage on the macro-and meso-mechanical properties of weathered granite sand is revealed;The method for accurately obtaining the bearing capacity and in-situ shear strength of weathered granite sand based on PDEM is developed.The macro and meso constitutive relationship of sand relating to the direct shear test,the binary linear function mathematical model of the boundary particle sizes and the triaxial compression strength parameter for sand with fractal gradation,and the empirical calculation formula for bearing capacity of weathered granite sand related to the depth and width of the foundation ware proposed.The research results can provide theoretical and technical support for the preparation of relevant specifications and the scientific,reasonable and efficient use of this type of weathered sand.The main research methods and results of the thesis are as follows:(1)The mineral composition,the particle shape and the gradation of weathered granite sand in the Three Gorges Reservoir area have been systematically studied.Based on various geotechnical tests,the basic physical and mechanical indicators of them have been obtained.It provides basic support for the PDEM simulation of cohesionless weathered granite sand in the Three Gorges Reservoir area.The physical and mechanical indexes of cohesionless weathered granite sand are obtained by using relevant testing techniques and laboratory geotechnical tests.Under high magnification,weathered granite sand particles are mostly approximately spherelike,with few flakes and rod-shaped particles,all particles with sharp edges and corners,and with rough surface texture,coarse and gravel particles are often accompanied by weathering cracks,and will be broken by small bending moments or pressures.Through X-Ray diffraction,the mineral composition of sand grains is mainly composed of quartz,feldspar and amphibole.The content of clay minerals is very small.The coarser the grains,the greater the content of feldspar,and the feldspar content of gravel is 40-80%,the content of quartz and amphibole in sand grains of different particle sizes is relatively uniform.The screening test results show that the weathered sand has a wide gradation range,ranging from fine particles to gravel particles,and there exists serious size effect when carry out examinations based on conventional laboratory tests.For this reason,after removing the low content of powdery clay and medium-coarse gravel,the weathered sand was screened into four groups for classification research,namely fine sand(>0.075 mm),medium sand,coarse sand and gravel sand(< 10 mm).At the same time,in order to facilitate the production of repeatable samples,the particle gradation of each group of sand samples is uniformly distributed(the uniformly distributed particle gradation curve is a straight line in the coordinate system,actually,it has fractal characteristics and with a fractal dimension of 2.0).Various geotechnical tests have been carried out on fine sand,medium sand,coarse sand and gravel sand,and the basic physical and mechanical indicators of weathered sand have been obtained.The test results provide basic support for carrying out accurate PDEM simulation tests.From the change of particle gradation before and after consolidation and direct shear test under high stress level,it is known that weathered sand has a certain particle breakage under higher stress level.(2)For the cohesionless weathered granite sand,the rolling resistance Hertz contact model and particle breaking function module were developed based on PFC,so that the macro and micro mechanical behavior of weathered sand can be accurately simulated based on the PDEM.In order to accurately simulate the mechanical behavior of weathered sand,based on the principles of contact mechanics and tribology,an improved Hertz contact constitutive model for cohesionless weathered granite sand that can consider particle rotation resistance is developed.Aiming at the particle crushing characteristics of weathered sand,by introducing particle crushing criteria and particle crushing methods,a functional module that can consider particle crushing has been developed.On this basis,the PDEM is used to simulate the geotechnical test of weathered sand.Based on the results of the laboratory test,the meso-parameters and crushing parameters required for the simulation test are calibrated.The results show that the PDEM simulation test can accurately simulate the Macro-and meso-mechanical behavior of weathered sand.(3)Through the constructed PDEM model,mechanisms of the influence of particle shape on the macro-and micro-mechanical properties of weathered granite sand in direct shear and triaxial compression is revealed.The macro and meso constitutive relationship suitable for direct shear test is established.Aiming at the problem that the volume and shape irregularities of the particles are often ignored when comparative experiments are used to study the effect of particle shape on the macro-and meso-mechanical properties of sandy soils,which leads to the problem that the influence factors of particle shape cannot be completely separated.In this thesis,the PDEM model of weathered sand was constructed referring to the outlines of several natural sand particles,and the macro-and meso-mechanical properties of simulated sand samples during direct shear and triaxial compression were obtained,and the influence of particle shape on the macro-and meso-mechanical properties of sand samples in different types of tests was revealed.Results show that: 1)Because the edges and corners can occupy the pore space,irregularly shaped particles can accumulate a smaller pore ratio than round particles;2)The shape of the particles obviously affects the peak strength of the sand sample in the direct shear test,while the effect on the peak strength of the triaxial compression test is almost zero.2)The shape of the particles obviously affects the peak strength of the sand sample in the direct shear test,while the effect on the peak strength of the triaxial compression test is almost zero.The reason for this result is that the sand sample breaks along the only horizontal shear zone in the direct shear test,and the influence of particle shape on the peak strength is magnified.While,in the triaxial compression test,the sand sample breaks along multiple sets of shear zones,the influence on the peak intensity is weakened;3)In the direct shear test,when the normal stress is low,because the maximum dilatancy angle is smaller than the round particle sample under the same conditions,the sample composed of irregularly shaped particles has a smaller peak strength.When the stress is larger,the sample composed of irregular-shaped particles has a larger peak strength.For weathered coarse sand with fractal gradation,the critical normal stress is 400 k Pa.It should be noted that the critical normal stress corresponding to different particle gradations and types of soil should be different;4)Whether it is a direct shear test or a triaxial compression test,the particle shape has almost no effect on the critical strength,and has no effect on the slope of the critical state line,but the interception of critical state line and the Y-axis for the irregularly shaped particle sample are relatively small than the round particle sample under the same conditions.5)In the direct shear test,in addition to the macroscopic shear stress,volumetric strain and critical state line,which are affected by the particle shape,at the meso-particle level,the particle shape affects the distribution of the inter-particle contact force,contact density,and the anisotropy of contact force direction and magnitude;6)The relationship between the anisotropy of the contact force direction and magnitude in the triaxial compression and direct shear tests and the macroscopic stress ratio of the sand sample is discussed,and the macro and meso constitutive relationship suitable for the direct shear test is proposed.(4)The PDEM model was used to simulate the macro-and meso-mechanical responses of 10 groups of pure and mixed sand with fractal gradation characteristics without considering particle breakage,so that the effect of particle size distribution on the macro-and meso-physical and mechanical properties of weathered granite sand is revealed.Aiming at the problem that the average particle size d50,the non-uniformity coefficient Cu and the radius of curvature Cc cannot uniquely determine the particle gradation curve,in order to facilitate the production of repeated samples,the fractal dimension theory is used to quantify the particle gradation(named as fractal gradation),The fractal gradation contains three indicators,namely the fractal dimension ?,the maximum and minimum particle sizes dmax and dmin.According to the boundary particle sizes of fine sand,medium sand,coarse sand and gravel sand,4 groups of pure sand with fractal gradation are designed,along with 6 groups of mixed sand made by mixing pure sand according to fractal gradation.In view of the influence of fractal gradation on the macro-and meso-mechanical properties of weathered granite sand,the macro-and mesophysical and mechanical responses of 10 groups of weathered granite sand with fractal gradations in the triaxial compression test are obtained based on PDEM,and the following conclusions are drawn: 1)When the minimum particle size dmin and the maximum particle size dmax remain unchanged and the fractal dimension ? changes within the range of 2.0-2.6,the peak friction angle,critical friction angle,volume strain rate,dilatancy angle,critical state line,uniform contact density,uniform contact force distribution,contact force direction and magnitude anisotropy remain unchanging;2)There are regularities between the peak and critical strength,compressive modulus,critical state line,distribution of contact force,contact density,the contact force anisotropy and boundary particle sizes of sand with fractal gradation in triaxial compression test,and a mathematical model using binary linear functions to describe the relationship between the triaxial compressive strength parameters of fractal-graded sands and their boundary particle sizes is proposed.(5)The PDEM model is used to simulate the macro-and meso-mechanical response of 10 groups of pure and mixed sand with fractal gradation characteristics under the condition of particle breakage,so that the influence mechanism of particle fragmentation on the macro-and meso-physical and mechanical properties of weathered granite sand is revealed.In view of the impact of particle crushing on the macro-and meso-mechanical properties of weathered sand,the macro-and meso-mechanical responses of 10 groups of pure and mixed sand with fractal gradation characteristics are obtained based on PDEM with and without regard to particle crushing.Comparative analysis is used to reveal the impact of particle breakage on the macro-and meso-physical and mechanical properties of sands.The main conclusions are as follows: 1)When the boundary particle sizes remain unchanging,even though ? changes within the range of 2.0-2.6,the particle crushing rate will not be affected;2)The boundary particle sizes and stress level are the main factors that affect particle breakage.Among them,the stress level is linearly related to the particle crushing rate,and under the same stress level,the wider the boundary particle sizes ranged,the smaller the particle crushing rate;3)Particle crushing leads to an increase in the content of fine particles below the average particle size,the number of particles in the sample becomes larger,and the average contact force becomes smaller;4)In the case of particle breakage,when the boundary particle sizes remain unchanging and? changes within the range of 2.0-2.6,the peak and critical friction angle,volumetric strain rate and dilatancy angle,critical state line,uniform contact density,uniform contact force distribution and contact force anisotropy have no effect;5)Particle crushing leads to the decrease of peak friction angle,while the increase of critical friction angle,the decrease of maximum dilatancy angle,and the increases of slope and interception with the Y axis for the e-p critical state line;6)In the case of particle crushing,the regular relationship between the strength parameters,maximum dilatancy angle,critical state line,contact density,contact force distribution,contact force anisotropy and boundary particle sizes of the weathered granite sand in triaxial compressive tests remains unchanged,and a binary linear function is proposed to describe the relationship between the strength parameter and the boundary particle sizes;7)Particle breakage will affect the formation of shear bands,weaken the stress concentration in the sample,reduce the anisotropy of the contact force direction and magnitude in the peak phase,but increase the anisotropy in the critical phase.(6)A high-efficiency simulate method based on PDEM that can accurately simulate plate load tests and in-situ direct shear tests has been developed,which can greatly reduce the cost of engineering tests.It is costly to obtain the foundation bearing capacity and in-situ shear strength of weathered granite sand through plate load test and in-situ direct shear test.Based on PDEM,a high-efficiency method that can accurately simulate plate load test and in-situ direct shear test has been developed.Based on the proposed methods,the complete H-VM foundation bearing capacity envelope and the in-situ shear strength of the foundation soil can be obtained accurately,and at the same time determine the deformation modulus of the sand foundation.The following conclusions are obtained through the simulated plate load test and the in-situ direct shear test: 1)Based on theoretical analysis,the required size of the foundation model is determined when simulating the plate load test.The half-plane method is adopted,which improves the calculation efficiency while ensuring the accuracy of the results.The width and depth of the PDEM foundation is determined according to the internal friction angle of the foundation soil and the foundation width,Servo control is adopted to apply a constant stress on the foundation surface to simulate the buried depth of the foundation.Based on this method,a calculation formula for foundation bearing capacity related to the buried depth and width of the weathered granite sand foundation is proposed.Further research can provide a complete depth and width correction formula for the foundation bearing capacity of weathered granite sand;2)By continuously increasing the size of the foundation model and comparing it with the laboratory direct shear test,the required size of the foundation model for simulating the in-situ direct shear test is determined.Which is,the depth of the foundation model should not be less than 2 times the width of the foundation,and the width of the foundation model should not be less than 3 times the width of the foundation.The in-situ direct shear test results of weathered granite sand show that when the normal stress exceeds 200 k Pa,the bearing capacity of the foundation will be destroyed during the shearing process,resulting in the shear strength being less than the true value.Therefore,in order to accurately obtain the in-situ shear strength of weathered granite sand,the normal stress level should not exceed 200 k Pa;3)The dilatancy angle of the sample in the laboratory direct shear test will be greater than the in-situ direct shear test under the same conditions,resulting in a larger peak friction angle.The main reason is that the laboratory direct shear box limits the deformation of the sample,and the foundation soil has a large deformation range in the in-situ direct shear test.