Bachy Soletanche was appointed by JT Mackley & Co. to provide the geotechnical package on the iconic British Airways i360 tower, located on the beachfront in the seaside town of Brighton. The tower is the world’s tallest moving observation tower (162m), taking visitors 138m above Brighton, providing stunning views of the town along with the South Downs and the Isle of Wight.
Jacobs developed a scheme whereby the tower itself is not supported directly on piles, but instead sits on a reinforced concrete raft, that is in turn supported directly on the chalk. The design required a basement to be constructed using a retaining wall method, into which the main foundation would be cast. In addition to this there was a requirement for load bearing piles on the exterior of the basement to provide support for ancillary structures such as the ticket office and maintenance facilities.
The team constructed a secant retaining wall consisting of 252 continuous flight auger (CFA) secant piles, which were 600mm in diameter and bored to a depth of 10m. This structure was then excavated to form a basement, with a retained height of 6.3m, and was supported by temporary props during the bulk dig phase.
In the final stages, over 170 tonnes of steel reinforcement was installed within the basement slab to provide solid foundations for the main tower. During the construction of the basement, the water table had to be taken into consideration due to the close proximity to the sea. In addition to the basement works, 43 CFA bearing piles, with diameters of 300mm and 600mm were installed to the exterior of the basement, and bored to a maximum depth of 20m. The ground conditions consisted of made ground to 1.5m, with sand and gravels to 7m, with Chalk below this level.
The seafront location had the potential to disrupt the piling operations but the experienced team delivered the works ahead of schedule. This was achieved through a close working relationship with the client along with detailed logistical and programme planning.