Full-length casing with oscillator or rotator

How bridges get built · Bored piles and deep foundations · Full-length casing with oscillator or rotator

Also called: oscillator method, Benoto piling, segmental casing, casing rotator

How it is done

Level the platform and set a guide frame; place the oscillator or rotator on the pile position, clamp the first segment with its cutting shoe and check verticality in two directions.
Oscillate (part-turn) or fully rotate the casing down ahead of the excavation, adding bolted double-wall segments; keep the casing toe leading the dig in running ground so soil cannot flow in.
Excavate inside with a hammer grab or the rig's bucket; break boulders and old foundations with a chisel or core through them, and keep a positive water head inside the casing below the water table to prevent base heave.
Seat the casing into the bearing layer or rock, clean the base by airlift or cleaning bucket, and measure depth, verticality and debris. Hold point before the cage.
Lower the cage and tremie the concrete; extract the casing segment by segment, keeping a concrete head well above the casing toe and above the external water pressure.
Check the concrete level against the casing toe after every extraction step and record volumes; break down and test integrity (CSL) as for other bored piles.

Key numbers

Diameter0.6–3 m typical; large rotators exceed 3 m
Depth20–50 m common with oscillators; about 60 m or more with heavy rotators (Huey P. Long, about 61 m)
Casingbolted double-wall segments, typically 1–6 m long
Outputone 1.5 m × 30 m pile in 2–4 days
Verticality1:100–1:200 achievable because the casing guides the excavation

Where it fits

Boulder beds, fills with obstructions, urban sites next to old foundations, river beds with cobbles, loose sands under high water.

Where it does not

Very deep piles: casing friction grows with depth, so oscillators are usually limited to about 40–50 m; heavy rotators reach 60 m or more but at higher cost.
Sites where a slurry rig would be twice as fast.

Choosing it

Choose it over slurry drilling where boulders, cobbles, old foundations or open gravels would cause fluid loss or collapse, or where slurry disposal is restricted.
Choose it beside railways, existing piers and buildings: the hole is supported at all times, with no vibration and no tall leader (M25 New Haw viaduct, Huey P. Long widening under a live bridge).
Prefer slurry or polymer drilling for deep piles in uniform soils, where it is roughly twice as fast and casing friction is not a limit.
Prefer a rotator over an oscillator for larger and deeper piles and for cutting the casing into hard rock with a toothed shoe.

Plant, pace and money

PlantCasing oscillator or rotator, crawler crane with grab and chisel, or a rotary rig with rotator attachment.

ProductivitySlower than slurry drilling: a 1.5 m pile of 30 m in two to four days.

CostUSD 600-1,200 per metre.

Risks and controls

What goes wrong

Casing stuck below cage level, necking on extraction, chisel damage to adjacent piles.

Quality assurance

Casing verticality, concrete rise monitored against extraction, integrity tests.

Origins

The Benoto system, developed in France in the 1950s, combined an oscillated full-length casing with a hammer grab; it spread to Japan as the all-casing method and to Europe and North America, where heavy hydraulic rotators later extended it to larger diameters and greater depths.

Examples

Huey P. Long widening (New Orleans) and New Benicia–Martinez (California); the Cypress Avenue Bridge over the Sacramento River at Redding, where about 2.5 m shafts were taken through gravels with boulders up to 0.6 m with an oscillator and clamshell; Japanese urban expressway foundations; Hong Kong reclamations with old seawalls.

Case studies

Huey P. Long Bridge widening, New OrleansUSA · 2009Ten 2.8 m shafts about 61 m deep in the Mississippi, built with full-length segmental casing by a rotator under restricted headroom beneath the existing bridge; accepted as a value-engineered alternative to slurry drilling and proved by an 80 MN bi-directional test.

New Benicia–Martinez BridgeUSA · 20072.2 m rock sockets were planned with reverse circulation under slurry; caving forced a switch to full-depth casing advanced by a hydraulic rotator, and a multi-level O-cell test confirmed side resistance close to the FHWA prediction.

M25 New Haw viaduct (River Wey and railway)UK · c. 1980222 bored piles of 1,050 mm installed with an oscillator and short jointed casing, chosen because tall plant and vibration were restricted next to a four-track railway.