Coordination of complex geotechnical works at a Tideway site in central London-Paper

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The Earl Pumping Station site formed part of the Thames Tideway Tunnel, London’s major sewer upgrade. On a confined 50 × 65 m site beside an operational pumping station and residential neighbourhood, teams constructed a 52 m deep drop shaft, a combined sewage overflow (CSO) chamber, and associated structures.<br /><br />Works were complicated by contaminated ground, two aquifers, limited access, uncertain details of existing structures, and the need to protect live utilities and pumping-station assets. Environmental requirements prohibited uncontrolled groundwater drawdown and demanded measures to prevent contamination entering the lower aquifer.<br /><br />The solution combined several specialist techniques. A 1 m thick diaphragm wall formed the shaft, with deep soil mixing around it to contain contaminants. The CSO excavation was retained by secant piles and supported by staged propping. Deep soil mixing and jet grouting created a basal plug and reinforced ground near existing structures; additional permeation grouting later strengthened areas affected by groundwater piping. Equipment, deliveries and work sequences were carefully planned to fit the restricted site and maintain access for asset operations.<br /><br />Extensive ground and structural investigations informed the design, which used hand calculations and 2D and 3D finite-element modelling. Models were calibrated against shaft excavation data, while monitoring of groundwater, strut loads and nearby asset movements guided construction. Quality checks also identified gaps in the ground-treatment plug, prompting additional columns. Unexpected water inflows during excavation were sealed by underwater injection.<br /><br />Close coordination among designers, contractors, reviewers, clients and asset owners was central to managing technical, environmental and programme risks. Despite unforeseen ground conditions, the main works were completed in the first half of 2024, with commissioning and remaining project activities continuing. The project demonstrates how integrated design, monitoring, communication and adaptable construction methods can enable complex geotechnical works in a highly constrained urban setting.

Keywords

Thames Tideway Tunnel

Earl Pumping Station

deep shaft excavation

diaphragm wall

sewage overflow chamber

contaminated ground

deep soil mixing

jet grouting

groundwater control

finite-element modelling