1. Introduction
Rainfall is one of the main causes of instability and failure of roadbed and other projects. After rainfall, surface runoff is formed on the road, and some of rainwater by the drainage system flows on both sides of the road, and part of rain water through the cracks of pavement structural the roadbed. On the one hand, it leads to the soil mass increases, and the original stress state in the soil of subgrade is changed, the shear strength of the soil is weakened, and the shear stress is increased; on the other hand, the effect of seepage in the soil can also promote this phenomenon. Retaining wall is one of the commonly used supporting structures in subgrade engineering. It is very necessary to analyze the strength of the back of the subgrade retaining wall to ensure the stability and durability of the roadbed. Earth pressure is one of the main loads. Drainage system is usually arranged in the design of retaining wall, in order to reduce the water content to a certain extent. When the roadbed water content is too large, poor drainage may lead to soil strength decreased and then lead to a series of disasters. Li Liang et al. [1] simplified Bishop method being applied to unsaturated soil, and the stability formula of subgrade under the condition of rainfall seepage is derived. Li Xia [2] analyzed the effect of rainfall seepage on the moisture content of subgrade. Zhao Minghua et al. [3] discussed the mechanism of the Instantaneous Event Model and Finite Duration Event Model rainfall model, and deduced the formula of the depth of the rain water infiltration Zhang Chang-guang, Zhao Junhai [4] [5], etc. On the basis of the double stress state variable of unsaturated soil, considering the intermediate principal stress effect, the unified solution of shear strength and soil pressure of unsaturated soil is established. Wang Junjie et al. [6] analysed the earth pressure, under the action of load and seismic load, and considering the steady seepage of saturated soil. The existing theory of non-saturated soil and unsaturated soil pressure under the premise combined with the relevant research results of rainfall infiltration model. In this article, reaching and analyzing the earth pressure acting on the wall of the subgrade retaining wall, under the condition of rainfall, the infiltration of rainwater into the roadbed is a certain depth, which is influenced by the underground water level.