WorldWide Drilling Resource

8 OCTOBER 2021 WorldWide Drilling Resource ® UK’s 21 Moorfields Heavily Burdened Piles Set Record Compiled by the Editorial Staff of WorldWide Drilling Resource ® The construction of LandSec’s 21 Moorfields development has set the record for the highest loaded piles ever to have been sunk in the City of London. The project won the British Geotechnical Association’s Fleming Award in 2020 for excellent design and construction. Judges were particularly impressed with the amount of innovation in the project. A 17-story, almost 200,000-square-foot future London headquarters for Deutsche Bank spans up to 180 feet, the full width of Moorgate under- ground station, a distance which is equivalent in length to the wingspan of a jumbo jet. With its eye-catching, exposed steel- work design, the building is expected to enhance the urban landscape of this transportation hub. Before any steelwork could be erected, a piling foundation problem required an innovative solution. The test pile was a success and actually achieved a peak test load of 5680 tons. Final settlement was three inches, which massively exceeded predictions of greater than 11 inches at this load, giving the project team confidence in their foundation solution. Because of underlying structures, locations for any new piles were extremely limited. A 154-ton piling rig, 154-ton crawler crane, and ten wet 99-ton bentonite tanks were needed to install the piles. Another problem emerged as the existing slab over Moorgate Station did not have sufficient capacity to support a rig. A 2204-ton tem- porary steel grillage, which covered most of the existing slab and trans- ferred the plant loads to the existing station columns and their foundations, was designed to support the rig. After a complex maneuvering solution by a specialist heavy lift and haulage contractor, the piling rig and crane were successfully tracked onto the grillage. Because of limited construction space, the delivery team had only one opportunity to construct each large-diameter bored pile. If a position had to be abandoned or a pile needed to be downgraded, there were no alternative locations for piles. Due to the risk, the geotechnical and deliv- ery teams collaborated early on to optimize the construction process. As a result, each pile was constructed without incident with all piles complet- ed ahead of schedule. A total of 15 bored piles, most seven feet and a few almost six feet in diameter, extending about 197 feet into the ground, were threaded between the numerous undersite constraints in two parallel rows between the train running tunnels, allowing the building’s foundation to bridge the train lines below. The piles were base grouted six and a half feet into the underlying Thanet Sands, a 58 million-year-old geological forma- tion of fine-grained gray-brown sand, more than 164 feet below the station. The load carried by individual piles was greater than any other single pile in the UK, and equivalent to the typical load seen in columns of a 30-40-story tower. Each of the super piles was fitted with strain gauges and fiber-optic monitors to enable the piles’ settlement to be closely monitored as construction pro- gressed. A complex engineering project, the development has made good use of a diverse cross section of skills and expe- rience from the project team to push the boundaries of design and construction practices. Super piles made the devel- opment of 21 Moorfields possible, and the increased load supported by each pile has the potential to open up similar- ly challenging sites in the city, enabling developers to build higher and span wider than ever before. The construc- tion on 21 Moorfields is due to be com- pleted sometime this year and will open in 2022. Piling grillage above train lines. Photo courtesy of Robert Bird Group. C&G