In engineering, abutment refers to the substructure at the ends of a bridge span or dam. This is where the structure's superstructure rests or contacts. Single-span bridges have abutments at each end. These provide vertical and lateral (side to side) support for the bridge. They also act as retaining walls to resist lateral movement of the earthen fill of the bridge approach. Multi-span bridges require piers to support ends of spans that are not supported by abutments. Dam abutments are generally the natural valley walls. They may also be artificial in order to support arch dams such as Kurobe Dam in Japan.