Overview

An abutment is the substructure that supports the end of a bridge span or forms the side support for a dam or arch. In civil engineering practice an abutment performs multiple roles: it carries vertical loads from the superstructure, resists lateral pressures from approach embankments, and provides the bearing and anchorage for expansion devices and bearings that accommodate movement at the superstructure interface. For dams, natural valley walls often act as dam abutments, while in some cases artificial abutments are constructed to receive arch dams.

Primary functions

Abutments typically perform several distinct functions:

  • Transfer vertical loads from the deck and superstructure into the foundation and soil.
  • Resist lateral thrust from backfill and approach embankments, acting as a retaining wall.
  • Provide anchor points for bearings and expansion joints to accommodate thermal and live-load movements.
  • Protect foundations from scour, erosion and other site hazards, and in seismic regions resist earthquake forces.

Types and common forms

Abutments vary by structural form and by the materials used. Common forms include gravity abutments, which depend on mass to resist forces; cantilever abutments, which use reinforced concrete walls with heel and toe slabs; and pile-supported abutments used where surface soils are weak. Backwall abutments with wing walls retain approach fill, while open or spill-through abutments allow water or fill to pass without acting as a solid retaining face. Multi-span bridges normally rely on piers for interior support and retain abutments at the spans' ends of the bridge.

Materials and construction

Historically, abutments were built of masonry and dressed stone; modern practice employs mass concrete, reinforced concrete, and driven or bored piles with pile caps. The choice of material depends on site conditions, availability, required durability and load demands. For arch structures and vaults, masonry abutments and buttresses provide the lateral resistance needed to balance arch thrusts and vault loads. Where natural rock is unsuitable for a dam abutment, engineered rock cuttings or artificial abutments may be formed to receive the dam face for arch dams.

Design considerations

Designers consider bearing capacity, expected settlement, lateral earth pressures behind the abutment, drainage behind the wall, and the effects of live and dead loads. Surcharge from roadways, vehicle braking forces, thermal expansion and contraction of the superstructure, seismic loading and scour potential at foundations are routinely evaluated. Detailing for waterproofing, backfill compaction and drainage is important to limit long-term deterioration and reduce the risk of differential settlement that could impair bearings or expansion joints at the interface.

Common distress and maintenance

Abutments can exhibit several forms of distress: settlement or rotation due to inadequate bearing capacity, cracking from thermal or seismic stresses, scour and undermining of foundations near waterways, and deterioration of mortar or concrete in older structures. Regular inspection of joints, bearings, wing walls and drainage outlets is a key part of maintenance. Rehabilitation techniques include underpinning, installation of piles or micropiles, reconstruction of wing walls, adding scour protection, and replacement or refurbishment of bearings and expansion devices.

Architectural and historical uses

Beyond bridges and dams, the term abutment is used in architecture and masonry to describe the support that resists the lateral thrust of an arch or vault. In historic buildings thickened masonry walls, buttresses or solid abutments served this role and can be seen in many vaulted structures where they stabilize lateral forces in masonry and provide support for arches and vaults.

Terminology and distinctions

Abutments differ from piers (which support intermediate spans), from retaining walls (which primarily retain earth), and from wing walls (which extend the abutment to retain approach fill). The word is related to the verb "abut," meaning to touch or border on; etymological references describe the notion of two elements meeting at a common boundary (to abut).

Further reading and resources