The Bayan Nur Section of the Jing-Xin Expressway is located in the Hetao irrigation area. In this area, open river flooding is employed to irrigate the farmland, which causes seasonal subgrade immersion. With the fluctuation in the water levels, the water content of the subgrade soil increases, and the soil strength decreases due to water infiltration and capillary action. Thus, a field test section is built for long-term monitoring of the earth pressure, pore pressure, and settlement, and gravel cushion and reinforcement materials are utilized to reinforce the foundation in the test section. The test results show that the tidal variation in the groundwater level is obvious during the irrigation season, with the maximum increase in the pore pressure being 147.7 %, and the highest water level being 0.9 m above the ground. The change of subgrade soil moisture content and subgrade buoyancy results in the change of subgrade base pressure direction. The pressure distribution of the subgrade base after the reinforcement of the sand-gravel cushion and geotechnical materials presents an inverted bell type, which is similar to the pressure distribution of the rigid foundation base. The restraint and tension film effect of the cushion and reinforcement materials helps in improving the rigidity of the subgrade base.