Construction methods · Caissons and marine foundations

Gravity base foundations

A large concrete base that resists sliding and overturning by its own weight, placed on a prepared gravel bed on rock or dense sand. For pylons in straits with good seabeds it is the simplest foundation there is, provided the bed can be prepared and the base delivered.

How bridges get built · Caissons and marine foundations · Gravity base foundations

Also called: gravity caisson, GBS, spread footing at sea

Gravity base foundations — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Dredge to a firm stratum and place a levelled gravel bed with underwater screeding.
Float in or lift in the base (often the lower part of a floating caisson), ballast, and grout the underside if the bed is uneven.
Place scour protection around the base.

Where it fits

Rock or dense sand seabeds, deep water where piling from platforms is slow, pylons and anchorages of suspension bridges.

Where it does not

Soft clay and silt beds, sites with heavy scour, earthquake sites without sliding checks.

Plant, pace and money

PlantDredgers, screeding frames, tugs, ballast systems, divers or ROVs.

ProductivityBed preparation weeks; seating days.

CostUSD 20-100 million per pylon foundation.

Risks and controls

What goes wrong

Bed unevenness giving point contact, sliding under wave or ship load, scour.

Quality assurance

Bed survey to tolerance, contact pressure by grout intake, post-seating survey.

Examples

Great Belt East pylons and anchor blocks, Oresund, Confederation Bridge piers.