Abstract: Fractal model for the pore distribution of capillary action of unsaturated soil is built; the maximum rising height of capillary water in unsaturated soil is obtained. Meanwhile, based on Richards equation of water movement, boundary and initial conditions are introduced; based on Laplace transformation, analytical solution of the moisture changes in unsaturated soil subgrade under the action of capillary is obtained. Meanwhile, a calculating example is introduced; the proposed analytical solution and the numerical calculation results without parameters simplification are compared. Finally, the moisture changes under the action of capillary considering different factors in unsaturated soil subgrade are analyzed. The results show that: The results got respectively by analytical method and numerical method without simplifying parameter are consistent. It is proved that the analytic solution is reasonable and feasible. The greater the initial moisture content of subgrade filling is, the smaller the initial suction of the filling is, the smaller the rise height of capillary water and the change amount of moisture are. The rising speed of capillary water with the better subgrade filling permeability is faster. But the rise height is smaller, capillary water can rise to the largest height in a short time. Different types of subgrade filling have different moisture change states of subgrade under the action of capillary. Corresponding waterproof and drainage measures are needed according to different types of subgrade filling.

Key words: fractal model, Richards equation, capillary action, unsaturated soil subgrade, initial water content