Raked (battered) pile groups

How bridges get built · Driven piles · Raked (battered) pile groups

Also called: batter piles, battered piles, inclined piles, raking piles

How it is done

Lay out the group so the horizontal resultants are resisted axially; analyse it for all load cases, including those that put raked piles in tension, and for earthquake and ground settlement.
Check driveability at the rake: hammer efficiency falls with inclination, and the pile and leads carry bending from self-weight during pitching.
Set up the rake on the rig: adjustable leaders on land, or a raking template or pile gate on a barge or jack-up that holds both angle and azimuth.
Pitch the pile, set rake and azimuth by inclinometer and survey, and start at low energy until the pile is stable in the ground.
Drive to the criterion, re-checking rake and position at intervals; raked piles drift more than vertical ones in soft soil.
Splice with alignment control so the rake continues across the joint, and brace long free-standing raked piles together until the cap is cast.
Survey as-built head positions and rakes, re-analyse the group where deviations exceed tolerance, then cut off and cast the cap.

Key numbers

Rakecommonly 1:3 to 1:8 (horizontal:vertical), about 7-18 degrees; marine piers usually 1:4 to 1:6.
Land rigs with adjustable leaders commonly handle rakes up to about 1:3-1:4.
Alignment toleranceoften about 4 % deviation for raked piles against about 2 % for plumb piles.
Position tolerance at cut-offabout 75-150 mm, larger over water.
Clear spacingat least 2.5-3 diameters at the closest point, checked along the whole length because raked piles converge or diverge.

Where it fits

Marine piers, dolphins, fender structures and jetties with berthing or ship-impact loads.
High pile caps over water where lateral load would otherwise need many more vertical piles.
Abutments and retaining structures on piles that carry earth pressure.

Where it does not

High seismic zones unless the piles and cap connections are designed for the large axial forces raked piles attract; port seismic guidelines discourage them.
Ground that will settle around the piles: soft clay under new fill, reclamation, liquefiable layers.
Congested sites where raked piles would clash with neighbouring piles, services or existing foundations.

Choosing it

Choose raked piles over vertical piles when large horizontal loads (ship impact, berthing, waves, abutment earth pressure) must be carried with small pile sections and small cap movement.
Prefer vertical large-diameter piles in high seismic zones: raked groups are stiffer, attract more inertia force and concentrate damage at the pile heads.
Prefer vertical piles where the ground will settle (new embankments, soft clay, reclamation): settlement bends raked piles and overloads their heads.
On long marine viaducts, raked steel pipe piles allow high caps with few piles; check pile-to-pile clearance along the rake and navigation clearance.

Plant, pace and money

PlantPiling rig with adjustable leaders, or a barge or jack-up with a raking template or pile gate; hydraulic hammer; inclinometer and survey equipment for setting rake and azimuth.

ProductivitySlower than vertical driving because each pile's rake and azimuth must be set and checked; a marine spread with a raking template typically drives 2-6 large pipe piles per shift.

CostIndicative only: pile unit rates similar to vertical piles of the same section plus roughly 10-25 % for raking plant and the slower cycle; savings come from fewer piles and smaller caps.

Risks and controls

What goes wrong

Bending and head damage from ground settlement or downdrag acting across the raked length.
Brittle damage at pile heads or cap connections in earthquakes, because raked piles take a large share of the inertia force axially.
Clashes between piles of different rakes, or with existing piles, if alignment drifts.
Reduced hammer efficiency at steep rakes leading to underdriving.

Quality assurance

Rake and azimuth checked by inclinometer before and during driving; as-built survey of head positions and rakes; re-analysis of the group for out-of-tolerance piles; driving records and dynamic testing as for vertical piles.

Origins

Raked piles are ancient: Caesar's Rhine bridge of 55 BC used inclined timber piles with raking props downstream. In the 20th century they became the standard way to resist lateral load on wharves and marine piers, until damage to batter-pile wharves in earthquakes such as Loma Prieta (1989) led port seismic guidelines to discourage them.

Examples

Marine piers and dolphins worldwide; long sea viaducts on raked steel pipe piles such as the Donghai Bridge and the San Francisco-Oakland Bay Bridge Skyway.

Case studies

Caesar's Rhine bridgeGermany · 55 BCA timber trestle built in about ten days; paired piles were driven inclined with and against the current, with further raking piles downstream acting as buttresses.

San Francisco-Oakland Bay Bridge East Span (Skyway)USA · 2013Skyway piers stand on steel pipe piles 2.5 m in diameter and up to about 100 m long, driven at a batter of about 1:6 through soft Bay mud in a high-seismic zone.

Donghai BridgeChina · 2005The 20 km deep-sea section rests on groups of inclined steel pipe piles driven by piling barges, with precast concrete box pile caps.