Construction methods · Bored piles and deep foundations

Full-length casing with oscillator or rotator

A steel casing is twisted into the ground segment by segment by a hydraulic oscillator or rotator, and the soil is grabbed out from inside with a hammer grab. The hole is fully supported at all times, so boulders, running sand and old foundations are handled without slurry.

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

Mount the oscillator on the pile position; add casing segments as they are driven ahead of the excavation.
Excavate inside with a hammer grab or chisel; break boulders with the chisel.
Lower the cage, tremie the concrete, extract the casing with the oscillator as the concrete rises.

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) beyond about 40-50 m; sites where a slurry rig would be twice as fast.

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.

Examples

Japanese urban expressway foundations, Alpine gravels, Hong Kong reclamations with old seawalls.