Construction methods · Shallow foundations, cofferdams and dewatering

Spread footing on soil or rock

A reinforced concrete pad wide enough that the bearing pressure stays below what the ground can carry, and deep enough to be below frost, scour and softened surface layers. On rock, the pad is keyed or dowelled into a cleaned, sound surface. The first foundation an engineer should try and the one most often rejected too early.

How bridges get built · Shallow foundations, cofferdams and dewatering · Spread footing on soil or rock

Also called: pad foundation, strip footing, footing on rock

Concept diagram (TheBridgeEng)

How it is done

Excavate to the founding level with battered sides or shoring; keep the base dry.
Inspect and probe the formation: remove loose or softened material, blind with 50-75 mm of lean concrete within hours of exposure.
On rock, remove weathered material, clean joints, drill dowels if sliding governs; on soil, place a compacted granular layer if the surface is disturbed.
Fix reinforcement, cast the pad and the pier starter in one visit, backfill in compacted layers.

Where it fits

Dense sand, gravel, stiff clay or rock within 3-5 m of the surface, outside the scour zone.
Valley viaducts, overpasses, abutments on cut ground, pylons on rock.

Where it does not

Any river pier where scour can reach the footing.
Soft or loose ground within 1.5 times the footing width below the base.
Fills that are not engineered.

Plant, pace and money

PlantExcavator, compactor, concrete pump, dewatering pump if needed.

ProductivityA pad of 100 m3 takes a week from excavation to backfill on dry land.

CostUSD 250-450 per m3 of concrete in place; usually half to a third of a piled foundation for the same load.

Risks and controls

What goes wrong

Softening of clay formations left open in rain; heave or piping in sand below the water table; a boulder mistaken for rock.
Settlement of one footing relative to the next producing differential movement in continuous decks.

Quality assurance

Formation inspection by the geotechnical engineer before blinding, plate load or DCP tests where doubt exists, survey of the founding level against the borehole log.

Examples

Most short-span highway overpasses in dry regions; pylon footings on granite in mountain cable-stayed bridges.