Abstract:
The challenge faced by the Blackfriars’ s project team, was how to construct the online shaft over a tunnel in water bearing sand. A solution was required to avoid having to delay the Tunnel Boring Machine by up to 1 year, to accommodate prior construction of the Blackfriars shaft. Jet grouting provided a constructible solution and programme savings to the Tideway project as a whole. Working 24 hours a day over 13 weeks, Keller successful installed 140Nr, 2.6m diameter columns to 52m.b.g.l at the Blackfriars shaft. Jet Grouting targeted the Thanet Sand and granular soils within the Lambeth Group, forming interlocking columns to stabilise the ground around the shaft extrados. Additionally a canopy above the TBM crown (39m below ground level), was constructed to allow safe access to the TBM to replace seals and undertake cutter head maintenance without using compressed air. The complex ground conditions, considerable depth and limited programme period were all challenges to be overcome. Keller used it’ s advanced design methods to optimise the jet grout column diameter and spacing, accounting for borehole devotions at depth. A trial was undertaken 6 months prior to the main works at Kirtling Street launch shaft where the ground conditions were sufficiently similar. The design assumptions and construction method were tested and proved, with borehole deviations surveyed and the jet grout columns exhumed and measured. Keller used their new “ double special” jet grout system for the first time in the UK, deemed to be significantly more efficient and effective than other systems on the market, it enabled the construction of the large dimeter grout columns required. The £ 6.5million project was conducted in 15 weeks; using 11, 000T of cement, generating 35, 000T of spoil and working for 40, 000 injury-free person-hours. The design showed that borehole deviations could result in gaps between jet grout columns, it also predicted the number of such occurrences, allowing additional sealing columns to be included within the scope of works. The observational approach was used to verify the design assumptions, with each borehole surveyed and the jet grout column positions plotted in a 3 dimensional model. The “ as built” data could be reviewed throughout the project allowing subsequent borehole positions to be optimised and additional sealing columns added where required. The scope of works was successfully completed within the design predictions. Developments in borehole surveying have allowed Keller to regularly survey it’ s grout columns on projects around the world. This data has been used to develop drill rods and jet grout rigs, to minimise deviation. This has allowed Keller to increase the depth at which interlocking jet grout columns can be formed. Keller’ s work to develop a new generation of jet grout equipment has allowed larger jet grout columns to formed in more challenging soils. The combination of less deviation and larger columns allowed the Blackfriars’ s project to set new standards in deep interlocking jet grout columns. The design method built on the experience gained when forming jet grout columns around the tunnels of the Victoria Station Upgrade, 2012 to 2014. The design analysis was redeveloped bespoke to the Blackfriars project allowing different layouts of jet grout columns to be modelled. This allowed the risk of gaps to be quantified for each option and the design to be optimised. The benefit of jet grout parameter optimisation could therefore be demonstrated, enabling the client FLO (Ferrovial Laing O’ Rourke J.V.) to commit to pausing the critical path launch shaft excavation for 72 hours, whilst trail columns were constructed. The project team members were therefore able to work together to find the optimal solution and to quantify the risks and contractual responsibilities. Ground treatment around the Blackfriars shaft would not have been viable without the proactive cooperation of all those involved and use of the latest technology to open up new possibilities for jet grouting in the UK.