A bridge abutment is the end support of a bridge span. It performs two jobs at once: it carries the vertical and horizontal reactions from the deck down to the foundation, and it retains the approach embankment behind it. That dual role — substructure and retaining wall in one element — is why abutments are frequently the most expensive part of a short-span crossing and the part most often implicated in long-term maintenance liabilities.
Abutment types
A bankseat, or spill-through abutment, is a small foundation set high on the embankment slope, with the fill spilling through beneath the deck. It is economical where the embankment is stable and the headroom below is not needed. A full-height abutment retains the whole depth of the approach embankment and provides a clear opening beneath, which suits highway underbridges and constrained sites but attracts the full earth pressure over its height. An integral abutment is built monolithically with the deck so there are no bearings and no deck joints; thermal movement of the deck is accommodated by cyclic movement of the abutment into the retained soil. Integral construction is the default for new short and medium spans on the UK network precisely because joints and bearings dominate whole-life maintenance costs.
Loads and design references
An abutment is checked for vertical load from the deck and its own self-weight, horizontal load from earth pressure, surcharge from traffic on the approach, braking and traction forces, and — for integral forms — the cyclic earth pressure build-up caused by repeated thermal expansion of the deck into the backfill. Design follows the Eurocodes with the UK National Annexes, with PD 6694-1 providing the recommendations for structures retaining soil and subject to traffic loading, including the treatment of integral abutment soil-structure interaction. Assessment of existing highway structures is carried out to CS 454 within the Design Manual for Roads and Bridges.
Backfill, drainage and the approach slab
Most abutment problems are backfill problems. Poorly compacted or poorly drained fill behind the wall generates higher pressures than assumed, and settlement of the fill produces the step at the deck end that drivers feel as a bump. Selected granular backfill, positive drainage at the back of the wall taken to an outlet, and, where warranted, a run-on or approach slab spanning the transition are the standard mitigations. On integral bridges the backfill also has to tolerate the cyclic strain from thermal movement without progressive stiffening, which places further constraints on the material and the compaction specification.
Avoiding abutments with buried structures
A soil-steel composite structure — a corrugated steel arch, pipe arch or box buried in engineered fill — has no abutments in the conventional sense. The structure and the surrounding soil act together, the steel carrying the ring compression and the compacted fill providing the confinement, so the loads are shed into the embankment rather than concentrated on two end supports. For short spans this eliminates the abutment, the bearings and the deck joints in one decision, which is why buried structures are frequently the lowest whole-life option for culvert-scale and small bridge-scale crossings.
Related ViaCon solutions
ViaCon designs and supplies soil-steel composite structures that remove the need for conventional abutments. See our bridges and culverts solutions and tunnel and pipe structures. Related glossary entries: road bridge, arch bridge, headwall and retaining wall.
Frequently asked questions about bridge abutment
What is a bridge abutment?
A bridge abutment is the end support of a bridge span. It carries the vertical and horizontal reactions from the deck to the foundation and simultaneously retains the approach embankment behind it.
What is an integral abutment?
An integral abutment is built monolithically with the deck, so the bridge has no bearings and no deck joints. Thermal movement of the deck is accommodated by cyclic movement of the abutment into the retained soil. It is the standard form for new short and medium spans in the UK because it removes the components that dominate maintenance cost.
What is the difference between a bankseat and a full-height abutment?
A bankseat is a small foundation set high on the embankment with fill spilling through beneath the deck. A full-height abutment retains the entire depth of the approach embankment and gives a clear opening below, at the cost of carrying much higher earth pressures.
Do buried soil-steel structures need abutments?
No. In a soil-steel composite structure the steel shell and the compacted surrounding fill act together, so loads are transferred into the embankment rather than into end supports. This removes the abutments, bearings and deck joints entirely.