Trenching refers to digging out the soil in the middle of subgrade to form a road groove, and discarding the dug soil; Slotting is to form two shoulders with soil piles on both sides of subgrade to form a road groove. Using this method, residual soil or retained soil can be used for filling.

There was no large-scale compaction equipment in the past. In order to ensure the compactness, the subgrade is filled with dozens of centimeters more, and then the design elevation is dug, which is the so-called road groove, but this process is basically unnecessary. In case of groundwater, the groundwater should be dug out; The part below the design elevation shall be backfilled and compacted with excavated stone chips.