Construction methods · Bored piles and deep foundations

Pile groups, caps and group effects

Piles under a pier act as a group: closer than about 2.5-3 diameters they share soil and lose efficiency in friction, while a group settles more than a single pile because the stress bulb is deeper. The cap must spread the pier load to every pile, and a raked outer pile row helps with lateral load.

How bridges get built · Bored piles and deep foundations · Pile groups, caps and group effects

Also called: group efficiency, pile spacing, pile group settlement

Pile groups, caps and group effects — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Space piles at 2.5-3 diameters; arrange rows to give bending resistance about both axes.
Check group efficiency (block failure in clay, settlement of the equivalent raft), lateral capacity with p-y analysis, and cap shear.
Rake outer piles 1:4 to 1:8 where large lateral loads (ships, earthquakes, arch thrust) must be resisted.
Cast the cap with pile heads embedded 100-150 mm and full anchorage of the pile reinforcement.

Where it fits

Every pier on piles; the sizing routine in the game does this with the pile calculator for the real ground under each pier.

Where it does not

Very large groups (more than 30-40 piles) where a few large shafts or a caisson would be cleaner.

Plant, pace and money

PlantNone: design and cap construction.

ProductivityCap construction 2-4 weeks per pier.

CostCap concrete USD 250-400 per m3.

Risks and controls

What goes wrong

Group settlement under-estimated on layered ground, cap punching shear, corner piles overloaded by moment.

Quality assurance

Group analysis reviewed, pile positions surveyed before the cap, cap reinforcement inspected.

Examples

River piers on 6-20 piles, pylons on 20-40 large shafts.