Under a road or railway, a sewer is best laid in a heading. In this way, the actual road or railway remains undisturbed during the sewer construction and a large reduction in excavation is possible.

• 公路和铁路下的排水管最好铺在地道内,这样管道施工期间不影响公路和铁路的使用,也可大大减少土方工程。