The construction of the 21 Moorfields development has eased through its most recent milestone and, in doing so, has set the record for the most heavily loaded pile test of its type in the UK.
Robert Bird Group was commissioned by Landsec to develop the structural and geotechnical engineering design of 21 Moorfields in London’s Moorgate area.
The proposed 16-storey (60,000 sqm) future London HQ for Deutsche Bank will clear span up to 55 metres over Moorgate underground station and the new western ticket hall to Liverpool Street Elizabeth Line station to avoid disruption to the stations during construction.
The complex steel frame uses a combination of long span arches and transfer trusses to provide a coordinated temporary and permanent works solution to building over a live station.
The key to unlocking the development lies in the innovative foundation solution with the entire building only supported on fifteen new 1.8 – 2.4m diameter mega-piles that extend down to the Thanet Sand, more than 50m below the station.
The unfactored load carried by individual piles will be more than 55MN, more than any other single pile in the UK, and equivalent to the typical load seen in columns of a 30-40 storey tower. To justify such an advanced foundation design, a pile test was crucial to confirm their performance under such large loads.
Ensuring that Moorgate station remains fully operational throughout all demolition and construction activities was a non-negotiable criterion when agreeing the scheme with TfL. This included both the construction and testing of the piles, all carried out above the live station.
The roof of Moorgate station has insufficient capacity to support the major plant loads required to install the piles - a 140tonne Piling Rig, 140tonne Crawler Crane and ten wet 90tonne Bentonite Tanks.
David Seel, Managing Director West, Robert Bird Group, covering the UK, Middle East and USA said: “RBG’s Construction Engineering team developed the temporary works strategy to enable piling operations to be undertaken above the station roof using a steel grillage to transfer the plant loads to the existing station columns and their foundations. The Grillage is designed for easy assembly and removal, optimise programme and minimise environmental impacts on the surrounding neighbourhood.
“RBG, Cementation Skanska, Mace and Geotechnical Consulting Group have worked closely together throughout the development of the pile design and construction methodologies. This includes the design of the pile test which used a bespoke 200 tonne, 6m tall steel reaction frame to load a 1.2m diameter, 55m long sacrificial pile to up to 50.5MN. The test pile includes full length instrumentation that allows the team to determine how the load is shed along its shaft at various depths and at its base,” adds Seel.
The reaction frame was jacked against four of the permanent piles that are also fully instrumented. The jacks were individually controlled by ALE, the jacking specialist, throughout the 72hr test with RBG on site to monitor the response of the reaction frame.
The test was completed with the pile head having settled by only 73mm under the maximum load, a load 25% greater than the failure load predicted using conventional theory of deep piles in London. It also outperformed the settlements which were predicted to be more than 300mm. This outstanding result not only validated the pile design for the project but will also go on to provide the industry with highly valuable information on the behaviour of very highly loaded deep piles in Central London.
Ross Sayers, Development Director, London at Landsec commented:
“Breaking the UK record for the heaviest pile load of this kind highlights the huge complexity of the site at 21 Moorfields. This result is a testament to Robert Bird Group’s hard work and expertise in helping us achieve our ambitions with this scheme.”
21 Moorfields is due to be completed in 2021 and will be the London HQ of Deutsche Bank.