Mat (raft) foundation

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

Also called: raft foundation, combined footing

How it is done

Confirm by settlement analysis that the raft keeps differential settlement between columns within tolerance; check punching shear at each column and fix the pour sequence and joint positions.
Excavate the full footprint with support and groundwater control; inspect the formation as for spread footings and blind it the same day.
Where groundwater is high, lay a waterproofing membrane or specify watertight concrete, and check flotation for the construction stage when the raft is still light and the water table recovers.
Fix two-way top and bottom mats with shear links at the column zones; mats 1-3 m thick need steel chairs or trestles strong enough to carry the top layer, workers and pump lines.
Prepare the thermal control plan: low-heat binder (slag or fly ash), a placing-temperature limit, insulation, and cooling pipes and thermocouples for pours thicker than about 1.5-2 m.
Pour in one continuous operation, or in planned bays or layers with keyed and roughened construction joints; arrange two batching sources and a standby pump for large rafts.
Cure under insulation until the core-to-surface temperature difference has fallen within the limit, then backfill and set settlement points at the column positions.

Key numbers

Thicknesstypically 1-3 m under multi-column bents and pylon bases; anchorage blocks are far thicker.
Net bearing pressure on soilcommonly 100-300 kPa; a raft becomes economic when pads would cover more than about half the plan.
Pour size500-5,000 m3 placed continuously is common, at 50-150 m3/h per pump.
Thermal limitspeak core temperature 70 C; core-to-surface difference about 20 C.
Construction-stage flotationfactor of safety of at least about 1.1 against uplift before the superstructure weight arrives.

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.

Choosing it

Choose a raft over separate pads when column loads are close together, pads would overlap or cover more than about half the plan, or differential settlement between columns must be limited.
Prefer a piled raft or a pile group when the ground under the raft is compressible enough that settlement, not bearing, governs.
Prefer separate pads on uniform ground with widely spaced columns: less concrete and less thermal risk.
For suspension bridge anchorages, a gravity block on a raft is chosen over a tunnel anchorage when the rock is not strong or massive enough to carry the cable pull in shear.

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.

Origins

Combined footings and rafts became routine with reinforced concrete in the early 20th century; in bridge work their largest applications are the gravity anchorages of suspension bridges and the bases of pylons and multi-column bents.

Examples

Gravity anchorages of suspension bridges on rock or dense ground; pylon bases and multi-column bents where pads would merge.

Case studies

Akashi Kaikyo Bridge (Awaji-side anchorage 4A)Japan · 1998The Awaji anchorage is a gravity block on a spread (direct) foundation, while the Kobe-side anchorage needed an 85 m diameter diaphragm-wall shaft: the two ends show where a direct raft is and is not possible.

Yangluo Yangtze River Bridge (north anchorage)China · 2007Gravity anchorage of a 1,280 m suspension span set on an enlarged spread foundation in a deep, battered open excavation; the south anchorage on poorer ground needed a 73 m diameter diaphragm-wall shaft.