Construction methods · Bored piles and deep foundations

Barrettes and diaphragm-wall foundations

A rectangular pile excavated with a diaphragm-wall grab or hydromill under bentonite, typically 0.8-1.5 m by 2.8-7 m, giving very high axial and bending capacity in one element. Used under pylons and tall piers where round shafts would be too many or too large.

How bridges get built · Bored piles and deep foundations · Barrettes and diaphragm-wall foundations

Also called: barrette pile, rectangular bored pile, diaphragm wall panel foundation

Barrettes and diaphragm-wall foundations — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Build guide walls; excavate the panel under bentonite slurry with a cable-hung grab or a hydromill in rock.
Clean the trench, desand the slurry, lower the cage (often in sections with couplers), tremie the concrete.
Panels can be T-, L- or cross-shaped for bending in both directions.

Where it fits

Pylon foundations, high piers with large moments, sites with obstructions where the hydromill cuts through, deep foundations to 80 m.

Where it does not

Small loads, sites where a few round piles would do, ground with cavities that swallow slurry.

Plant, pace and money

PlantDiaphragm wall grab or hydromill on a crawler crane, slurry plant with desanding, large cranes for cages.

ProductivityOne 3 x 1.2 m panel of 40 m in three to six days.

CostUSD 500-1,000 per m2 of panel face; competitive with 2-3 m shafts for large moments.

Risks and controls

What goes wrong

Trench collapse with slurry loss, panel joint quality, cage cover in a rectangular section.

Quality assurance

Slurry testing, trench verticality by Koden sonic survey, CSL tubes, O-cell test.

Examples

Pylons of several Chinese and Middle Eastern cable-stayed bridges; tall urban viaduct piers in Hong Kong.