Jetting and predrilling to aid driving

How bridges get built · Driven piles · Jetting and predrilling to aid driving

Also called: water jetting, preformed pile holes, pre-augering, pile driving aids

How it is done

Agree in the design which zones may be jetted or predrilled and the deepest level allowed; capacity and lateral analyses must use reduced shaft resistance there.
Jetting: fix one or two jet pipes to the pile, or use pipes cast into it, and feed water at high pressure while the pile sinks under its own weight and light hammer blows; air may be added in sand.
Predrilling: auger or drill a hole smaller than the pile (often 75-90 % of its width or diameter) to the agreed depth, so the pile still displaces and grips the soil; holes through rock or hard layers may be drilled larger and grouted.
Stop jetting or drilling well above the design tip, as specified, and drive the remaining penetration to the required set with the hammer.
Control the water and spoil: do not jet beside completed piles or existing foundations, contain turbid water in rivers and the sea, and check that neighbouring piles have not been undermined or heaved.
Confirm capacity by dynamic testing at end of driving and on restrike, after the disturbed zone has had time to reconsolidate.

Key numbers

Stop leveljetting or predrilling commonly ends about 1.5-3 m above the design tip, and the rest is driven.
Predrill diameter for displacement pilesusually 75-90 % of the pile width or diameter.
Groundjetting suits clean sand and fine gravel; in clay and silt predrilling is used instead.
Restrikeafter one or more days, once the disturbed zone has reconsolidated.

Where it fits

Prestressed concrete piles through dense sand, cemented crusts or old fill above the bearing layer.
Coastal and marine sites in clean sand with water available for jetting.
Piles that must reach a minimum tip for scour or uplift below layers that would otherwise stop them.

Where it does not

Next to existing foundations, slopes or completed piles that jet water could undermine.
Jetting in clay and silt: little benefit and much disturbance; predrill instead.
Zones where shaft friction is needed for compression or uplift capacity and cannot be discounted.

Choosing it

Choose jetting or predrilling over a heavier hammer when hard upper layers would cause excessive driving stress or refusal above the design tip in concrete piles.
Prefer predrilling to jetting where water would disturb nearby foundations, in clay, or where turbidity limits apply.
Prefer steel H or pipe piles that can be driven through the dense layer when the lost shaft friction or lateral support near the top cannot be accepted.
Prefer SIP or bored methods if most of the pile length would need assistance: the pile no longer behaves as a driven pile.

Plant, pace and money

PlantHigh-pressure water pump with jet pipes, or an auger or drill rig for predrilling; the piling rig and hammer; settlement tanks or turbidity curtains for discharge.

ProductivityAdds a jetting or drilling step to each pile, but often removes long periods of hard driving and pile damage in dense layers.

CostIndicative only: about USD 30-120 per metre of jetted or predrilled hole on top of the driving cost, depending on depth, ground and spoil handling.

Risks and controls

What goes wrong

Loss of shaft friction and lateral support in the jetted or drilled zone if the design did not allow for it.
Undermining or heave of adjacent piles and structures.
Jetting or drilling below the permitted level, leaving the toe in loosened soil with low end bearing.
Turbid discharge breaching environmental permits.

Quality assurance

Jetting or predrilling depth recorded for every pile against the permitted level; driving record over the driven length; dynamic testing at end of driving and restrike; monitoring of nearby piles and structures; turbidity checks on discharge.

Origins

Water jetting has helped sink piles in sand since the 19th century; current US practice, such as the Florida DOT specification for preformed pile holes and jetting, limits the depth of assistance relative to the minimum tip and requires the final penetration to be driven.

Examples

Florida and Gulf Coast bridge piling in dense sand, where specifications allow preformed holes and jetting to stated limits; the San Diego-Coronado Bridge used staged jetting before final driving.

Case studies

San Diego-Coronado BridgeUSA · 1969Prestressed concrete piles were installed through dense clay and sand by pre-jetting with driving, then side jetting with driving, then hammer only for the final penetration.

Jupiter Federal Bascule Bridge (US 1 over the Loxahatchee River)USA · 2025All 760 mm square in-water piles were installed in preformed holes down to their minimum tip elevation, with variable-moment vibratory hammers used in vibration-sensitive areas next to a historic lighthouse.