subgrade in snow disaster zone

例句：

1. 1.The subgrade is being regraded to improve drainage.
   正在对下一级进行重新整平以改善排水。
2. 2. The subgrade was constructed using a mixture of crushed stone and sand.
   下一级是用碎石和沙子混合建造的。
3. 3.The subgrade must be compacted before the main road is built.
   在主要道路建设之前，下一级必须压实。