The presence of moisture in the soil reduces its shear strength. After the rain or snowmelt, the high percentage of moisture in the subgrade of highway, causes the instability resulting into failure of the highway due to high water table. Therefore, it is essential to install a drainage system to remove the excess moisture from the subgrade of highways to avoid its failure. In the present study, six different soil samples have been studied to observe its failure by triaxial shear strength and corresponding moisture content was noted. The tension applied to remove moisture was converted to equivalent drain depth. The relationships were studied between shear strength versus drain depth and moisture content versus drain depth. From these relationships the design drain depth for different types of soils studied was recommended. The present study also suggest that the design drain depth for the highways can be approximated directly using water retention curve.