subgrade of special soil

例句：

1. 1.The engineer needs to know the subgrade conditions before he can design the road.
   工程师在设计道路之前需要了解下层土壤条件。
2. 2. The construction crew was working on the subgrade when it started raining heavily.
   施工队正在进行下层土壤作业时，突然下起了大雨。
3. 3. The subgrade is being compacted to improve its drainage capabilities.
   正在对下层土壤进行压实以提高其排水能力。