Pile groups, caps and group effects

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

Establish the design actions at each pier (axial, lateral, moment, ship impact, seismic) and the ground model; decide on vertical or raked piles and the cap level relative to water and scour.
Choose diameter and layout: spacing 2.5–3 diameters or more, rows arranged to resist moment about both axes, and edge cover beyond the outer piles.
Analyse the group with a 3D model and pile–soil springs: load sharing, group efficiency and block failure in clay, group settlement (equivalent raft or numerical), and lateral response with p-y curves reduced by p-multipliers for trailing rows.
Design the free length for the scoured bed, including local scour around the cap and group.
Design the cap for punching, shear and tie forces (strut-and-tie for deep caps), and plan thermal control for thick caps.
Survey as-built pile positions, levels and rakes before the cap and re-check the design for piles out of tolerance.
Build the cap with piles trimmed to sound concrete and bars anchored as designed; monitor settlement of large groups during and after loading.

Key numbers

Spacing2.5–3 D centre to centre; friction groups lose efficiency below about 3 D
Lateral p-multipliers at 3 D spacingabout 0.8, 0.4 and 0.3 for the first, second and later rows (AASHTO)
Rake1:4 to 1:8 for lateral loads
Group efficiency in frictionabout 0.7–1.0; Sutong's 131-pile group was designed with 0.82
Pile numbersriver piers 4–20 piles; pylons 20–150 large piles (Sutong 131)
Position tolerance±75–150 mm in plan before the cap

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.

Choosing it

Choose a few large shafts over many small piles when the cap is in water: fewer piles mean a smaller cap and cofferdam, and each shaft can be socketed and tested.
Choose a piled raft (cap and piles sharing load) on stiff clay where settlement rather than capacity governs, as under the Rędziński Bridge pylon.
Choose raked piles for ship, ice or arch thrust; prefer vertical large-diameter piles in high seismic zones, where raked piles attract large forces.
Compare a caisson or barrettes once a group would exceed about 30–40 piles in deep water or severe scour; Sutong kept a 131-pile group because bedrock lay about 300 m down.

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.

Origins

Group behaviour was formalised in stages: the Converse–Labarre efficiency formula in the 1940s, elastic interaction methods by Poulos and Davis (1980), and full-scale lateral group tests by Brown, Reese and others in the 1980s that produced the p-multipliers now in AASHTO.

Examples

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

Case studies

Sutong BridgeChina · 2008131 floating piles under each pylon with a 113.8 × 48.1 × 13.3 m cap; design used a group coefficient of 0.82 from theory, centrifuge tests and finite elements, and measured pile-head loads formed a W-shaped distribution across the cap.

Rędziński Bridge, WrocławPoland · 2011The 122 m pylon stands on a piled raft of 160 bored piles 1.5 m × 18 m on a 3.4 × 3.6 m grid under a 67.4 × 28 m slab; piles were kept short to stay above a confined aquifer, and measured settlement agreed with the predicted 80 mm.

Mumbai Trans Harbour LinkIndia · 2024Typical marine piers on compact groups of four 2.2 m bored piles about 36.5 m long.