TBM excavation is selected for a twin bored four-lane highway crossing of the Lower Thames to the east of London, and to provide relief to the overstretched Dartford Tunnel and Elizabeth Bridge.
Highways England, an agency of the UK Government, announces that three alternative road alignments (Routes 2, 3 and 4, plus either an east or west link south of the river, Fig 1) will be taken to a final public consultation stage. However, all of the alternatives feature a 3,100m twin-bore tunnel crossing of the Thames between Gravesend and Tilbury (Fig 1, Option C).
Immersed tube and bridge options at the same location are rejected on environmental and legal grounds, and an alternative and shorter crossing point located further upstream (Option A), and closer to the existing Dartford Tunnel, is also dismissed.
A pre-consultation report prepared by Highways England concludes: “A bored tunnel is the only viable alternative as it meets the scheme objectives and is the least [environmentally] damaging alternative. This conclusion has been supported by advice provided by Counsel.”
Illustrative design specifications completed by the UK Government’s engineering consultant Halcrow/Hyder currently specify a 12.1m diameter TBM for excavation of parallel 11.1m i.d., 3,040m highway tunnels, 300m of cut and cover ramp excavation, 27 x 4.55m o.d. passenger cross passages and two 7.22m o.d. vehicular cross passages.
At this design specification the Thames Lower Crossing will require the second largest TBM ever to be used in the UK – slightly smaller than the 12.5m diameter machine specified for the twin bore Silvertown traffic tunnel located further upstream and that will provide relief for the ageing Blackwall Tunnel. A full planning application for the Silvertown project is due to be lodged by Transport for London (TfL) in March (2016).
However, Highways England specifies in its report that during development of the final design provision will be given for an additional lane in each direction “to allow for future-proofing”. If this solution is adopted the machine required would be in the 15m mega-TBM diameter range.
Depending on which route option is finally selected, total project cost is estimated at between £6–6.4 billion for the 2x2-lane option. The bridge solution was costed slightly less and the immersed tunnel solution slightly more.
A full geological profile of the tunnel alignment is yet to be completed, but previous experience in the area with the HS1 rail tunnel, and a local cable tunnel, suggest a likely mix of chalk and alluvium deposits.