Theoretical And Experimental Study On Size Effect Characteristic Of Strength And Deformation Of Clay Soil

Soil is a multiphase geomaterial that consists of mineral particles,pore fluid and gas.The complex microstructures and mesofabrics of soil that are formed by soil particles at different scales,and the interactions between soil components with different phase cause different physical mechanism and mechanical response of soil to appear at various scales,which presents dramatic hierarchical coupling and trans-scale evolving effect of the deformation and strength properties of soil.In the present work,both theoretical analysis and experimental study are used to investigate the the multi-scale coupling mechanical properties of soil?The theoretical analysis mainly elaborates the mechanical response of soil at different scales and establishes a hierarchical and multi-scale constituve model to reproduce and predict the multi-scale coupling mechanical properties of soil.In combination with the result of theoretical analysis,theexperimental study is used to inverstigate the influence on the macroscopic strength of soil by different partical properties when its scale changes.Besides,the experimental study is also used to solve the parameters of the mechanical model.Conbining the theoretical prediction with the experiment result to verify the correctness and applicability of hierarchical and multi-scale constitutive model.The main research contents and conclusions are as followed:(1)The interaction force between soil particles is mainly the cohesive effect or the friction effect,which is related to the ratio between micro-forces and gravity of the soil particle.When the particle size is small and the ratio between micro-forces and gravity is relatively large,force between particles is mainly for the microscopic force,reflecting the cohesive effect;with the increase of the particle size and the decrease of the ratio between microscopic force and gravity,the microscopic force between particles gradually weakened,and the macro level gravity gradually increased,reflecting the friction effect.(2)Based on MSG theory,introduced the strain gradient of equivalent effect,coordinate the crack density and the intrinsic scale of three parameters,and gives the physical significance of the three parameters,soil cell element model of"Matrix-reinforced particle" flow stress formula is derived from the energy point of view,as the soil cell element model containing the yield strength of the reinforcing particles has laid a theoretical foundation for theoretical prediction.(3)A series of soil cell element samples with different reinforcement particle combinations are prepared for direct shear tests?triaxial compression tests and unconfined compression tests.The tests result shows that the enhancement of the ratio of particle to particle and particle size have a great influence on the stress-strain relationship and yield stress of the soil cell element model.When the volume fraction of reinforcing particles is less than 0.3,the yield stress of the sample increases with the increase of the volume fraction of the reinforcing particles,but this change is more significant in the three axial compression test.When the volume fraction of the reinforcing particles is more than 0.3,the strength of the reinforcing particles is increased,but the strength of the particles remaina unchanged and is even reduced;For the unconfined compression test,the yield stress of specimens increases first and then decreases with the increase of reinforcement particle size,while the three axis of the unconsolidated undrained shear test,the yield stress of specimens significantly reduced with the increase of particle size and strengthen;the change of reinforcing particle size has little effect on the yield stress of the direct shear test specimens.(4)The yield stress of the soil cell element model sample and reinforcing particles' volume fraction and particle size is not a simple relationship between increasing or decreasing,existing a "0.275?0.3" critical volume fraction "and a"0.4mm?0.6mm "critical size".When the reinforcing particle volume fraction is less than the "critical volume fraction",the yield stress increases with the reinforcing particles volume fraction increases,when the reinforcing particle volume fraction is larger than that of the "critical volume fraction",the sample yield stress rather than increase or even decrease with the increase of the reinforcing particles volume fraction;When the reinforcement particle size is less than "critical size",specimen unconfined compressive strength increases with the increase of particle size,when reinforcing particle size is larger than that of the " critical size",specimen unconfined compressive strength decreases with the increase of the particle size,and the critical diameter decrease with the increase of the reinforcing particles volume fraction.(5)The effects of the reinforcing particle size fraction and particle size on the strain gradient of equivalent effect of the soil cell element model,the density of the micro cracks and the intrinsic scale are greatly affected.When the particle size is constant,the reinforcing particles volume fraction is increased,and the strain gradient of equivalent effect increases with the increase of the reinforcing particles volume fraction;When the reinforcing particles volume fraction is constant,the particle size is increased,and the strain gradient of equivalent effect decrease with the increase of the ratio of the particles.When the particle size is constant,the density of the micro cracks increases with the increase of the reinforcing particles volume fraction;When reinforcing particles volume fraction is constant,the density of the micro cracks of the soil cell element model increases with the decrease of the particle size.For the direct shear test,the intrinsic scale of the soil cell element model increases with the increase of the particle size,but the change of the reinforcing particles volume fraction has little effect on the intrinsic scale of the soil cell element model;As for the three axis unconsolidated undrained test,intrinsic scale of the soil cell element model decreases with the increase of particle size,when the volume fraction is less than 0.3,intrinsic scale of the soil cell element model increased with the increase of volume fraction,and has a good linear relationship.The relative error |?th-?/?te| between the predicted value of the model theory and the sample size is less than 10%,most of which is less than 5%,and the prediction results of the yield stress model of the shear test of soil cell element model are generally acceptable.(6)On the basis of the soil cell element model,introducing some scale parameter,and connecting with the triaxial undrained shear test conditions,a multi-scale Mohr-coulomb strength criterion is established under the condition of triaxial undrained test.According to the experiment results,the yield surfaces of the MSMC strength criterion and its conventional counterpart are obtained and analyzed.The results show that,the multi-scale Mohr-coulomb strength criterion Has a good applicability and effectiveness.