Construction methods · Shallow foundations, cofferdams and dewatering

Mat (raft) foundation

One large slab under a group of columns or a wide pier, spreading the load over ground that would fail under individual pads. Used under multi-column bents, portal frames and pylon bases where the loads are close together.

How bridges get built · Shallow foundations, cofferdams and dewatering · Mat (raft) foundation

Also called: raft foundation, combined footing

How it is done

Excavate the full footprint, blind, and lay a waterproof membrane if the water table is high.
Fix a two-way reinforcement mat, often 1-3 m thick; use thermal control (cooling pipes or low-heat cement) for thick pours.
Pour in one or two lifts with construction joints planned for shear.

Where it fits

Medium-strength ground under closely spaced columns, pylon bases on dense sand or weak rock, sites where differential settlement between columns must be avoided.

Where it does not

Highly variable ground across the footprint: the raft will tilt rather than settle evenly.

Plant, pace and money

PlantAs for spread footings plus a larger concrete supply and thermal control.

ProductivityA 1,000 m3 raft is a two- to three-week operation including curing.

CostSimilar unit rate to pads; total cost dominated by concrete volume.

Risks and controls

What goes wrong

Thermal cracking in mass concrete; uplift under high groundwater.

Quality assurance

Thermal monitoring of the pour, cube tests, membrane integrity.

Examples

Pylon bases of cable-stayed bridges on dense sand in the Gulf; portal frame bents on urban viaducts.