The UK road transport authority is tendering for a Technical Partner to help it develop proposals and route options for a new £2.5–£5 billion crossing of the Lower Thames to the east of London. The planned crossing, which can be completed by 2025 if public funds are used, will provide much-needed relief for the heavily congested Dartford Tunnel and Elizabeth Bridge.
Scope of the 5–10 year engineering design and management contract – expected to be valued in the region of £100–200 million – includes preparation of preliminary design, completion of environmental impact assessments, managing the necessary planning permissions, heading up construction procurement, and supervising the build.
Currently two river crossing alignment options are being considered (Option A and Option C, with Option B discarded) – plus a possible third, and additional, C-variant option that involves construction of a 2km-long unidirectional two-lane bored tunnel as part of improvements to existing M2 and M20 connecting road infrastructure to the south of the main crossing point (Fig 1).
No decision has yet been made between a bridge or tunnel solution for the Thames crossing element of the project, but the UK Government (via its transportation agency Highways England) is expected to make a decision in late 2015 or early 2016 ahead of a public consultation exercise.
At this stage both a TBM bored tunnel and an immersed tube crossing, as well as a bridge, are being considered for both the Thames crossing points at A and C. A 2013 AECOM study has already provided costings for the relevant options, as well as preliminary suggestions regarding alignments (Table 2). However, it warns that every year's delay in getting the project to bid will increase prices by up to 4.6% per annum.
|Table 1. 2013 AECOM costings for Lower Thames Crossing options|
|Option A||Option C||Option C-variant|
|Crossing (£bn)||Road conn.+ (£bn)||TOTAL (£bn)||Crossing (£bn)||Road conn.+£bn)||TOTAL (£bn)||Tunnel and road works||TOTAL (£bn)|
Notes: +Cost of road infrastructure additional to the bridge/tunnel; *Increasing from 4 to 6 lanes under Option A increases cost of bridge to £1.2 billion, immersed tunnel to £1.51 billion, and TBM tunnel to £1.81 billion; ** Increasing from 4 to 6 lanes under Option C increases cost of bridge to £2.29 billion, immersed tunnel to £2.35 billion, and TBM tunnel to £3.04 billion; *** Option C-variant alignment includes 2km x 2-lane single bore TBM tunnel
The more expensive Option C costs both the bored tunnel and immersed tunnel connection at less than a bridge; while the cheaper and shorter Option A – which would sit alongside the existing Elizabeth Bridge or Dartford Tunnel – is only about 17% cheaper as a bridge solution.
AECOM’s report highlights the fact that the original TBM-bored Dartford Tunnel encountered difficult ground conditions, high rates of water ingress and subsequent construction delays. At both potential crossing locations (A and C) the same geological conditions will be encountered, but advances in slurry and EPBM technology in the intervening period make TBM excavation a viable crossing solution. An immersed tunnel solution is another possibility – however likely extensive disruption to river traffic during construction make this a less attractive option.
For the shorter possible tunnel alignment at Crossing Point A it is envisaged that an 11m EPBM or slurry machine would be procured for excavation of parallel 1,500m long tunnels at a minimum depth from river bed to tunnel crown of 11m. Each tube would accommodate two 3.5m wide traffic lanes and a 1m verge either side. The gradient of the approach tunnels has been assumed at 4%, and shafts would be excavated at either end to accommodate ventilation. A cable-stayed bridge, estimated to cost £257 million less (17%) than a tunnel at this location, is considered to have few construction constraints. An immersed tunnel – which would be pre-fabricated to accommodate two lanes of traffic either side of a central longitudinal escape passageway – is considered the most challenging of the options on account of the need to locate a suitable fabrication yard and the risk posed by dredging to existing tunnel and bridge foundations.
For the longer crossing at Point C the bridge solution is the most expensive, but at more than double the cost of Point A for any solution, a crossing here is considered less likely. A TBM crossing under Option C requires a 2,750m long alignment but, critically, passes mostly under marshland and away from developed areas on either side of the river.
A Highways England spokesman said: “We acknowledge that a bridge could accommodate high and wide freight vehicles, whereas a bored tunnel could be more resilient in extreme weather conditions and avoid adverse impacts on protected habitats. The Government is, however, drawing no conclusions at this stage about whether the new crossing should be a bridge or a tunnel.”