Construction methods · Site investigation and testing

Bi-directional (Osterberg cell) load test

A hydraulic jack cast into the pile pushes the upper part of the shaft up against the lower part and base. No kentledge, no reaction piles, and loads far beyond what a surface test can reach: the standard proof for large bored piles and barrettes.

How bridges get built · Site investigation and testing · Bi-directional (Osterberg cell) load test

Also called: O-cell test, bi-directional static load test

Concept diagram (TheBridgeEng)

How it is done

Weld one or more cells into the reinforcement cage at the level that balances expected shaft and base resistance; add strain gauges and telltales.
Concrete the pile; after curing, pressurise the cell in steps and record upward and downward movements.
Combine the two curves into an equivalent top-down load-settlement curve.

Where it fits

Bored piles and barrettes above about 1.5 m diameter, pylon foundations, marine piles where reaction systems are impractical.
Tests to 50-300 MN.

Where it does not

Driven piles (the cell cannot be cast in); small piles where a conventional test is cheaper.

Plant, pace and money

PlantThe cell, hydraulic pump, data logger; no reaction frame.

ProductivityOne to two days of testing after curing; the cell is sacrificial.

CostUSD 100,000-400,000 per test depending on cell size; cheaper than kentledge above about 20 MN.

Risks and controls

What goes wrong

Cell placed at the wrong level gives an unbalanced test that stops early.
The tested pile is usually not used as a production pile without grouting the cell.

Quality assurance

Pre-test cage inspection, calibrated cells, strain gauge levels matched to the soil profile.

Examples

Used on most long-span bridge pylon foundations since the 1990s, including sea bridges in the Gulf and Asia.