Blackfriars Shaft, London - Advances in Deep Jet Grouting-Paper

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The Blackfriars Bridge Foreshore site is part of London’s Thames Tideway project, which is building a 25 km tunnel to reduce sewage overflows into the River Thames. The site required a 22 m-wide, 50 m-deep shaft for tunnel access and TBM maintenance. Before excavation, the ground had to be stabilised and groundwater controlled in difficult, water-bearing soils beneath a temporary river cofferdam.<br /><br />Keller installed a deep jet-grouted curtain around the shaft and a protective canopy above the tunnel, alongside dewatering wells. Because drilling deviation at these depths could leave untreated gaps between columns, the team used Monte Carlo probability analysis to assess likely gaps and optimise column diameters and layouts. Trial columns at nearby Kirtling Street helped establish construction parameters for the site’s varied sands and cohesive layers.<br /><br />Each borehole was surveyed before grouting. As-built column positions were modelled in 3D to identify potential gaps, assess risks with the client, and determine where extra columns were needed. Keller used its advanced Double Special (SuperJet) system to construct columns up to 2.5 m in diameter, with treatment depths reaching 52 m. Work proceeded continuously for 15 weeks, with equipment and spoil transported by barge.<br /><br />The combined grouting and dewatering scheme enabled the TBM intervention to be completed without compressed air, allowed the tunnel to pass through the treated zone, and supported successful shaft excavation. The predicted gaps and column quantities proved consistent with construction results. The project demonstrates how probability-based design, field verification, and advanced jet-grouting technology can extend the feasibility of deep ground treatment.

Keywords

Blackfriars Bridge Foreshore

Thames Tideway project

jet grouting

tunnel shaft excavation

ground stabilisation

dewatering wells

Monte Carlo analysis

SuperJet system

TBM maintenance

River Thames