Construction methods · Bored piles and deep foundations

Drilled shafts (bored piles) with casing or slurry

A hole 0.8-3 m in diameter is drilled with a rotary rig, held open by a temporary casing or by bentonite or polymer slurry, then a cage is lowered and concrete placed by tremie. The universal heavy-load bridge foundation: pylons, river piers, urban viaducts, anything into rock.

How bridges get built · Bored piles and deep foundations · Drilled shafts (bored piles) with casing or slurry

Also called: bored pile, drilled shaft, cast-in-drilled-hole pile, CIDH

rock1 drill under casing2 cage lowered in slurry3 tremie, casing outDrilled shaft (bored pile)
Concept diagram (TheBridgeEng)

How it is done

  1. Install a temporary casing through the upper soils (vibrated, oscillated or drilled in); below it, drill under slurry or, in stable clay, dry.
  2. Drill with augers and buckets to the design level; socket into rock with core barrels or rock augers; clean the base with a cleaning bucket or airlift.
  3. Lower the reinforcement cage with spacers and CSL tubes; place concrete through a tremie, keeping the pipe embedded; withdraw the casing as the concrete rises.
  4. Break down the head, test integrity (CSL), and load test a sample.

Where it fits

  • Loads of 5-50 MN per pile, rock sockets, boulders and cobbles, urban noise limits, river piers where driven piles would refuse; almost any site with a rig large enough.

Where it does not

  • Very loose sands under high water pressure without adequate slurry head (hole collapse).
  • Artesian layers (the game's blowout event) without casing through them.
  • Sites where a driven pile would simply be cheaper.

Plant, pace and money

PlantRotary drilling rig (100-400 kNm torque), casing oscillator or vibro-hammer, slurry plant and desanding, service crane, tremie equipment.
ProductivityOne 1.5 m pile of 30-40 m per rig per one to two days; deep rock sockets slow it to several days.
CostUSD 400-900 per metre for 1.5 m diameter on land; two to three times that from a jack-up at sea.

Risks and controls

What goes wrong

  • Base debris and soft toe, necking where the casing is withdrawn too fast, slurry contamination of the concrete, cage lifting during pouring, hole collapse. Every one of these is invisible until tested.

Quality assurance

Base cleanliness measured by sounding or sediment probe, slurry density and sand content before concreting, concrete volume against theoretical per pile, CSL on every pile, one static or O-cell test per pile type.

Examples

The default foundation the game's reference picks under pylons and river piers on rock; Stonecutters, Incheon and Gulf sea bridge pylons stand on 2-3 m shafts.

Related methods

Full-length casing with oscillator or rotatorReverse circulation drilling (RCD)Bentonite and polymer slurry stabilisationTremie concreting of pilesRock sockets and base treatmentPile integrity testing (sonic echo and cross-hole sonic logging)

Further reading

FHWA GEC 10 Drilled Shafts · EN 1536 (bored piles) · O'Neill and Reese

Full-length casing with oscillator or rotator →