Technical details
The Challenge
Located next to the Grade II listed Leadenhall Market, Franki Foundations were appointed to devise a piled solution to support the redevelopment of 1 Leadenhall Street. The new 35-storey tower will provide office accommodation, a free public terrace, and ground-floor retail to an important corner in the City of London.
The project required installation of 103 no. settlement reducing piles in total, 40.5m depth, comprised of 58 no. bearing piles and 45 no. plunge column piles. The site was piled twice previously and we had to be prepared for coring through existing piles.
The innovative foundation scheme, Franki Foundations & A-Squared developed closely with the Engineers, was a settlement reducing piled raft, rather than a standard piled solution. As such, the piles were constructed in dry bores, optimising costs and programme.
The Solution
Franki employed two rotary piling rigs, 1 x BG-33 and 1 x SR-75, in very tight working conditions and managed to deliver the piling scheme ahead of the programme.
Strict planning and logistical management facilitated the safe and successful delivery of the works. We developed a well-thought-out logistics plan, that allowed the piling operations to run smoothly.
The real challenge on this project was installing the Plunge Column profiles within tight tolerances. We have developed a bespoke installation frame that allowed us to control the profile center as it gets lowered down. We pre-fit milled T-shaped profiles on the PC web which engaged tightly with the installation frame and, as a result, the minimum verticality achieved was 1:300. The specifications only required 1:200 verticality for the profiles.
The pile cut-off levels were above London Clay, and this was a challenge for the PC piles as the temporary casing had to stay in the ground to allow concrete to cure and keep the steel profile in place. Having concrete within the casing could’ve put casing extraction at risk, but we overcame this by fitting a debonding tube to the outside of the reinforcement cage which contained the concrete, preventing it from adhering to the temporary casing.
