Piling Adjacent to Rail Tunnels at King's Cross Central-Paper

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The paper describes the design and construction of bored piles for two office buildings, S1 and S3, at King’s Cross Central, directly above Network Rail’s Thameslink twin tunnels. The tunnels have shallow crowns, and some piles were installed within 1.35–1.5 m of their linings. Ground conditions comprised Made Ground over London Clay and the Lambeth Group, including locally water-bearing sand that could have required support fluid during piling.<br /><br />The project team refined pile layouts and construction methods to manage risk and cost. Continuous flight auger (CFA) and rotary bored piles, 600–1,200 mm in diameter and up to approximately 56 m deep, were used. Design changes, including reducing pile numbers and diameters and conducting a preliminary load test at S3, helped shorten piles and avoid support fluid for most works. Installation was sequenced to limit cumulative ground effects, with piles constructed alternately and separated by at least 5 m. PRAD sensors recorded pile verticality, while automated monitoring tracked tunnel movements.<br /><br />Measured tunnel movement was modest: settlements reached no more than 3.3 mm, transverse movement was at most 1 mm, and cross-sectional distortion was less than 1 mm. These results were broadly consistent with predictions, with greater settlement associated with the larger and more numerous piles at S1. The work was completed successfully without compromising tunnel serviceability.<br /><br />The authors conclude that comparable projects require thorough ground investigation, robust and carefully sequenced piling methods, suitable equipment, experienced contractors, and close monitoring and engineering supervision.

Keywords

bored piles

King’s Cross Central

Thameslink tunnels

CFA piles

rotary bored piles

London Clay

Lambeth Group

tunnel settlement

pile monitoring

ground investigation