Abstract: According to the soft soil base hard shell course conditions of the subgrade project in high grade highway,the divided standards of the soft soil base hard shell course and the project properties of the hard shell course of the different forming cause are analyzed;the role mechanism of soft soil base hard shell course,its role on the settlement and stability of the subgrade,and its role appearance on the soil pressure are discussed.

Key words: subgrade project； soft soil base； hard shell course； role mechanism； subgrade settlement； subgrade stability；