Balancing modernisation and heritage preservation at Salisbury Square

Salisbury Square is the site of the City of London’s new "Justice Quarter" on the south side of Fleet Street. At 127m long, up to 73m wide and 18.5m deep, the construction of the deepened basement at Salisbury Square was always going to be a significant challenge for the contractor, Keltbray, and the temporary works designer, Wentworth House Partnership, working in collaboration with the main contractor Mace. The site was formerly a collection of nine different buildings, including former newspaper offices.

The superstructure had been demolished by Keltbray under the previous demolition contract, and the project scope included completing the below ground demolition, pile enabling, piling, excavation, capping beam, propping, geothermal piles, construction of the new basement structure and core construction for the new superstructures.

Given that the site comprised nine different buildings, the form of construction and existing basement level around the site was highly varied in terms of form and level.  Additionally, the proposed basement was adjacent to a retained listed building, public house to the south, and office building 8 Salisbury Square to the south east.

Coupled with this, the surrounding roads were heavily populated with services close to the project perimeter but in undefined locations. This presented significant challenges and uncertainty to the project team to probe, pile and excavate the proposed basement, while maintaining the serviceability of all the assets. The uncertainties were de-risked through a combination of investigation and collaboration.

Investigation - To de-risk the uncertainty of the existing structure and potential previous temporary works used for the construction, City of London, with Avison Young, appointed Keltbray to carry out investigations and verification of the perimeter structure while the excavations verified the surrounding services and their position. This allowed Keltbray and the team to determine what methods of piling and enabling were most appropriate, reducing the risk of unforeseen conditions and the associated cost and delay.

Collaboration - Keltbray and Wentworth House Partnership worked closely with the design team and Mace to minimise the need to pile through existing walls and structures to install the new secant pile wall. The objectives of efficient pile installation and maintaining the necessary internal functioning space were balanced through a series of workshops, with both requirements achieved.

The new sub-structure consisted of deep capping beams required to distribute the high concentrated loads from the superstructure columns. On Fleet Street, in order to move the piled wall to a more effective alignment whilst maintaining the necessary floor area, the capping beam had to be lowered below the basement slab. The result was an excavation of over 10m with multiple propping phases to construct the capping beam, and subsequent basement excavation. In other locations the step in the 6m deep capping beam necessitated a 10m deep capping beam with the need for innovative temporary works to allow for efficient propping, excavation and basement construction.

Given the range of variable conditions and constraints around the perimeter over 20 different sections were identified to be understood, designed and detailed in a site wide coordinated solution.