1. The subgrade is not flat enough and needs to be leveled.

2. The roadbed width is not enough, or the original roadbed cannot meet the design alignment requirements: the roadbed needs to be widened or the alignment needs to be modified;

3. When the subgrade passes through poor soil foundation such as paddy fields and ponds, the surface silt should be removed, filled with gravel or chemically treated. (such as lime stabilization)

4. Insufficient bearing capacity of subgrade: it is necessary to replace and fill the soft foundation of subgrade to meet the design requirements;

2. Subgrade refers to the belt-shaped structure built as the foundation of pavement according to the location of the line and certain technical requirements, and is the foundation of railways and highways. Subgrade is a linear structure made of earth or stone. Build subgrade under good geological, hydrological and climatic conditions. As far as materials are concerned, subgrade can be divided into three types: soil subgrade, stone subgrade and earth-rock subgrade.