O-Cell methodology supporting pile design optimisation in Angola-Paper

Paper

Pdf Summary

The Terminal Oceânico da Barra do Dande in Angola includes a 1.7 km pier and mooring facilities supported by 323 bored piles. Because laboratory testing of the site’s difficult-to-sample limestone produced highly uncertain strength estimates, designers initially adopted conservative rock-socket lengths.<br /><br />Four full-scale push-out load tests were conducted over water using the bi-directional Osterberg Cell (O-Cell) method. Each 700 mm diameter test pile contained a single O-Cell near its base, allowing resistance above and below the cell to be measured without constructing an overhead reaction frame. The tests were safely completed and exceeded the specified minimum loads: piles designed for 9 MN were loaded to approximately 14.9 MN, while those designed for 5.5 MN reached up to 8.8 MN.<br /><br />The measurements provided direct evidence of pile behaviour and mobilised skin friction in the limestone—information not reliably available from site investigations and laboratory tests alone. Although the limited pile movements did not establish ultimate capacity, the results enabled designers to reduce rock-socket lengths by more than 40% for some working piles, including reductions from 14 to 7 m and from 11 to 6 m.<br /><br />These changes reduced excavation, construction time and material use. In a hypothetical scenario where 161 piles were shortened by 5 m, the authors estimate savings of 323 m³ of concrete and approximately 157 tonnes of CO₂. The project demonstrates how bi-directional testing can support safer, efficient foundation verification and substantial design optimisation in challenging over-water conditions.

Keywords

Terminal Oceânico da Barra do Dande

Angola

O-Cell load test

push-out load test

bored piles

limestone

rock-socket length

pile foundation

construction optimisation

CO2 savings