An abutment is the structure at the end of a bridge or viaduct where the deck lands on solid ground. It performs two jobs at once: it is the support on which the deck bears, and it is a retaining wall holding back the approach embankment.
Components
- Foundation — either spread footings on competent ground or piled.
- Abutment wall — the vertical element resisting earth pressure.
- Bearing shelf — the top surface carrying the bearings on which the deck beams sit.
- Wing walls — the side walls retaining the fill either side.
- Transition structure — expansion joint and approach slab between deck and carriageway.
Loads
An abutment has an unusual combination to deal with: the vertical reaction from the deck, the horizontal earth pressure of the backfill behind it, water and possibly ice loading, braking and impact forces from traffic, and the thermal movement of a deck that expands in summer and contracts in winter.
Points to watch
- Drainage behind the abutment is critical. Without a drainage layer and a working outlet, water pressure builds up on top of the earth pressure that was designed for. That is a classic cause of tilting and cracking. Allow for a granular drainage layer or drainage mat with a perforated pipe and a free outfall.
- Compact the backfill in thin layers. Poorly compacted material behind the abutment settles afterwards. The result is the familiar bump or dip in the carriageway immediately before the bridge.
- Approach slab. A concrete slab running from the abutment down into the road construction bridges the settlement difference between the rigid structure and the settling road.
- Integral abutments. In an integral bridge the deck is cast monolithically with the abutment. That removes expansion joints and bearings — both maintenance-hungry — but the abutment must then absorb the deck’s thermal movement, cyclically loading the ground behind it.
- Keep it accessible. Bearings and expansion joints must remain inspectable and replaceable; an abutment walled in solid makes maintenance needlessly expensive.
Related terms
- Bridge
- Retaining wall
- Bearing
- Settlement
- Foundation
- Sheet piling
- Soil compaction
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