A wingwall is a short retaining wall that extends outward from the end of a bridge or culvert, holding back the surrounding soil embankment so it doesn’t spill into the waterway or roadway below. You’ll find wingwalls at nearly every bridge abutment and box culvert in the country. They’re one of those structures most people drive past without noticing, but they play a critical role in keeping the earth in place and guiding water where it needs to go.
How Wingwalls Fit Into a Bridge
At each end of a bridge, a large vertical structure called an abutment transfers the weight of the bridge deck and traffic down into the foundation. The abutment faces the waterway or gap the bridge spans, but on either side, the road’s embankment (the sloped mound of compacted earth that ramps up to the bridge) needs something to hold it back. That’s the wingwall’s job.
Wingwalls are designed and analyzed as cantilever retaining walls, meaning they stick out from the abutment body and resist the horizontal push of soil pressing against them. In many designs, the wingwalls and the abutment are poured as a single piece of concrete, creating one continuous structure. The wall must be long enough to fully contain the embankment based on the slope of the fill material at that location.
Three Common Wingwall Shapes
Wingwalls come in three basic configurations, each defined by the angle they form with the abutment:
- Inline (straight) wingwalls extend in a straight line from the face of the abutment, forming a 180° angle. They’re essentially a continuation of the abutment wall itself, stretching outward to hold back soil on either side.
- Flared wingwalls angle outward from the abutment, typically at about 135°. This splayed shape is common at culvert entrances because the angled walls help funnel water into the opening.
- Turn-back (U-type) wingwalls make a 90° turn and run parallel to the road, creating a U-shape when viewed from above. These are often used on hillsides or where space is limited, and they can be built in stepped elevations to follow the grade of the terrain.
The choice between these shapes depends on the site. Flared wingwalls work well for directing water flow, while turn-back wingwalls are better suited to steep or confined sites where the embankment can’t be allowed to slope freely.
Wingwalls on Culverts
Wingwalls aren’t just for bridges. They’re equally common at box culverts, the rectangular concrete tunnels that carry streams and drainage channels under roads. Here, wingwalls serve a dual purpose: they retain the embankment and they improve hydraulic performance by guiding water smoothly into the culvert barrel. Without wingwalls, water approaching a culvert entrance would contract sharply at the edges, creating turbulence and energy loss. The wingwalls conduct flow directly into the barrel, reducing those contraction losses and allowing the culvert to handle more water at a given depth.
Drainage Behind the Wall
One of the biggest threats to any retaining wall is water pressure building up in the soil behind it. Wingwalls address this with weep holes, small drain openings built into the concrete. Arkansas DOT specifications, which are representative of standard practice, call for weep holes roughly 4 inches (100 mm) in diameter spaced no more than about 10 feet (3 meters) apart horizontally, with a minimum of two weep holes per wingwall. Each opening sits about 12 inches (300 mm) above the top of the wingwall’s footing.
Behind the wall, a layer of drainage fill material (coarse aggregate wrapped in filter fabric) runs the full length of the wingwall. This granular layer lets water flow freely down to the weep holes instead of saturating the backfill and pressing against the concrete. The filter fabric keeps fine soil particles from clogging the drainage aggregate over time.
Why Wingwalls Crack, Tilt, or Fail
Wingwalls are subject to the same forces as any retaining wall, and they can develop problems over their service life. The most visible signs of distress are cracking, tilting, and lateral shifting. Understanding what causes each helps explain why inspectors pay close attention to these structures.
Tilting, or rotational movement, typically results from uneven settlement beneath the footing or from horizontal earth pressure pushing the wall outward. The most common triggers are scour (erosion of soil at the base from flowing water), undermining, saturation of the backfill behind the wall, and soil bearing failure where the ground simply can’t support the load. Erosion of backfill along the sides of the abutment can also unbalance the forces acting on a wingwall, causing it to rotate.
Cracking often traces back to differential settlement, where one part of the foundation settles more than another. This can happen when subsurface conditions vary across the width of the wall, or when scour removes support from one section. Lateral movement, where the wall slides outward, occurs when the horizontal push of soil exceeds the friction holding the wall in place. Slope failure, seepage, frost action, and gradual changes in soil characteristics over time are the usual culprits.
Material deterioration adds to the problem. Concrete wingwalls develop surface erosion, spalling, and cracks from freeze-thaw cycles and chemical exposure. Stone masonry wingwalls can lose mortar and shift. Timber wingwalls, less common today, are vulnerable to rot. Inspectors look for shoulder erosion near the top of the wall, settlement cracks where the wingwall meets the abutment body, and any signs that the drainage system has become blocked, since clogged weep holes accelerate every other failure mode by allowing water pressure to build.
Design Standards
In the United States, wingwall design follows the AASHTO LRFD Bridge Design Specifications, which dedicate a specific section (11.6.1.4) to wingwalls within the broader chapter on abutments, piers, and walls. These specifications define the load combinations engineers must check, the resistance factors applied to soil bearing and sliding, and the strength limits the wall must satisfy. State departments of transportation then publish their own bridge manuals with additional guidance. Wisconsin’s manual, for example, provides equations for computing the required wingwall length based on embankment slope and specifies that wingwalls should not rely on soil friction or passive earth pressure to resist loads, a conservative approach that builds in a margin of safety.
Because wingwalls are classified as retaining structures, they must be designed to handle earth loads and any surcharge loads from traffic or equipment operating near the top of the wall. The design treats each wingwall as a cantilever projecting from the rigid abutment body, with the soil pressure increasing with depth just as it would against any freestanding retaining wall.