pit gravel

例句：

1. 1. The farmer used to dig up the pit gravel for his garden.
   这位农民以前经常挖出坑底的碎石来种植花园。
2. 2. The old house was built on top of a pit gravel foundation.
   这座老房子建在坑底碎石地基上。
3. 3. The road was covered in pit gravel, making it difficult to walk.
   这条路上覆盖着坑底碎石，走起来很困难。