土体是一种颗粒物质，其强度与变形特性具有显著的颗粒尺度效应。根据土体颗粒间的连结性状和微重比，将土颗粒划分为基体颗粒与加强颗粒。构建了反映土体内部材料信息和颗粒特征信息的土体胞元，基于应变梯度理论建立可以描述土体颗粒尺度效应的土体胞元模型。设计一系列饱和重塑土的直接快剪试验以研究土体直剪力学特性的颗粒尺度效应，并定量计算了土体胞元模型的应变梯度和內禀尺度等微细观计算参数。试验结果表明，土体的剪切屈服应力随加强颗粒体积比和平均应变梯度的增加而增加，且与加强颗粒体积比呈近似线性关系，与平均应变梯度呈抛物线关系；加强颗粒粒径对土体的剪切屈服应力影响不明显。土体剪切屈服应力的试验结果与土体胞元模型的预测结果一致。

Soil is a granular medium and its strength and deformation characteristics show a strong particle size effect. On the basis of their connection characteristics and the ratio of micro-forces and gravity, soil particles are decomposed into matrix particles and reinforcement particles. In addition, a soil cell element that can describe the internal material information and particle characteristics of soil is constructed; and a soil cell element model that can characterise the particle size effect of soil is proposed. A series of direct shear tests on saturated and remoulded soil is conducted to study the particle size effect of soil and to quantitatively determine the strain gradient and intrinsic length scale of soil. The results show that the shear yield stress of soil increases with an increase in the volume fraction of the reinforcement particles and effective strain gradient, and has an approximately linear dependence on the volume fraction of the reinforcement particles. Moreover, the relationship between the shear yield stress of soil and the effective strain gradient can be presented as a parabolic function. The size of the reinforcement particles has little influence on the shear yield stress of soil. The experimental data can be well fitted to the cell element model.