surface layer of subgrade bed

例句：

1. 1. The surface layerof the soil is rich in nutrients.
   土壤的表面层富含养分。
2. 2. The engineer studied thestructure of the subgrade bed to improveits surface layer.
   工程师研究了地基床的结构，以改善其表面层。
3. 3. Theconstruction crew worked diligently toensure that the surface layer was smooth andlevel.
   施工队伍努力工作，确保表面层平整光滑。