Theoretical and experimental research on particle size effect of direct shear mechanical properties of soil

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, a soil cell element that can describe the internal material information and particle characteristics of soil was 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 coordinated micro-cracks was introduced, and a multi-scale and hierarchical soil cell element model was proposed to establish the conversion relationship of the displacement and stress between different scales. Therefore, the microscopic soil mechanics was promoted from qualitative analysis to quantitative calculation. Moreover, various soil samples with a variety particle combinations were prepared for a series of direct shear tests to study the particle size effect of soil. Meanwhile, microscopic parameters of the model such as the strain gradient and intrinsic length scale were quantitatively studied. The results show that: The yield stress of soil increases with an increase in the volume fraction of the reinforcement particles. When the volume fraction of the reinforcement particles is low(≤0.125), the yield stress of soil increases with a decrease in the reinforcement particle size whereas when the volume fraction of these particles is high (≥0.177), the yield stress of soil increases with an increase in the reinforcement particle size. The action of the reinforcement particles that causes the strain gradient and coordinated micro-cracks of soil to appear at the matrix joined with the reinforcement particles is the microscopic physical mechanism of the particle size effect of soil. The yield stress of soil calculated by the soil cell element model is in good agreement with that of the test result. ©, 2015, Academia Sinica. All right reserved.