土体是一种复杂的颗粒体系，其强度与变形具有显著的颗粒尺度效应。为考虑土体不同尺度土颗粒对其宏观力学特性的影响，根据土颗粒间相互作用产生的黏聚和摩擦物理效应而非纯粹几何尺寸，划分土颗粒尺度层次以构造反映土体内部材料信息和颗粒特征信息的土体胞元。基于土体不同尺度结构层次上力学响应的特征，引入微重比的概念，建立具有多尺度分层次理论框架的胞元土体理论，解释土体力学特性颗粒尺度效应的物理机制，把微细观土力学理论从定性分析推进到定量计算的水平。设计一系列饱和重塑土的三轴不固结不排水剪切试验对土体的颗粒尺度效应进行测试，并定量计算胞元土体理论的应变梯度和內禀尺度等微细观计算参数。试验和理论计算结果均表明，土体强度和变形的颗粒尺度效应随加强颗粒的体分比增加以及粒径减小而增强，反映出土体强度和变形显著的颗粒尺度效应；土体强度和变形尺度效应的理论预测与试验结果具有较好的一致性。

Soil is a complex granular medium and its strength and deformation characteristics behave strong particle size effect. On the basis of the physical effects of cohesion and friction generated by the interactions between soil particles at different scales but not just the geometric dimensioning, a soil cell element that can describe the internal material information and particle characteristics of soil is constructed by dividing particle size into different scales to investigate the influence of soil particles at different scales on the macro-scale mechanical properties of soil. According to the mechanical responses of soil at various scales, the notion of ratio between micro-forces and gravity is introduced; and a multiscale and hierarchical soil cell element model is proposed to interpret the mechanism of the particle size effect of mechanical properties of soil. Therefore, the microscopic soil mechanics is promoted from qualitative analysis to quantitative calculation. A series of unconsolidated and undrained triaxial compression tests on saturated, remoulded soil are designed to study the particle size effect of soil and to quantitatively determine the strain gradient and intrinsic length scale of soil. The experimental results and theoretical analysis show that the particle size effect of the strength and deformation of soil is increased with raising volume fraction and decreasing size of the reinforcement particles, which suggests strong particle size effect. The theoretical prediction of size effect is in good agreement with that of the test result.