The possible sources of water intrusion in the subgrade sand were also studied and analyzed. From the design pavement drawings as well as the present field condition, there is high possibility for the rainwater to flow into the subgrade sand through the grassing zones adjacent to the pavement structures, especially in the taxiway areas. Shrinkage cracks were observed in the topsoil of the grassing zone, increasing the amount of infiltration from the rainwater and the surface runoff from the adjacent pavement. A few heavy rains can recharge the water in the subgrade sand if there is no means to drain out the water in the isolated subgrade sand. Therefore, there is a need to reduce the infiltration through the grassing zones and subsurface drainage should also be introduced.