Soil is a granular medium

and its strength and deformation characteristic behave marked grain size effect. Based on the theoretical model of cell element and triaxial shear tests

the analysis of grain size effect is presented. The matrix-reinforcing particles cell element model of soil is proposed according to cohesion and friction effect of interaction between particles of varying size. The cell element is composed of matrix and reinforcing grain. Many tiny grains are grouped into the matrix and reinforcing grain is a sand grain

so the macroscopic soil is simplified as a medium that consists of a number of cell elements. Introduced the generalized spherical strain and equivalent strain

the stress-strain relation and yielding stress formula are derived from strain energy accounting for the grain size effects. A series of triaxial undrained shear tests are performed for test samples with reinforced particles of different sizes and volumes fractions

and test results of stress-strain curves and yielding stresses are presented on the size effect. The experimental results and theoretical analysis show that the size effect of the strength and deformation of soil is increased with volume fraction raising and decreasing of particle size

which suggests strong size effect. The theoretical prediction of size effect is in good agreement with that of the test.