subgrade in swelling soil zone

例句：

1. 1. Thesubgrade is being prepared for the newroad construction.
   正在为新的道路建设准备下一级。
2. 2. The subgrade was designed towithstand heavy traffic.
   下一级被设计来承受重交通。
3. 3. The subgrade mustbe leveled before the asphaltis poured.
   在铺设沥青之前，下一级必须平整。