Looking to the future, large tunnels will become more commonly used and of larger diameter. This subsurface project allowed the removal of highways from the banks of the Manzanares River and their development as a public amenity. A wide range of factors determine the viability of a tunnel, including engineering feasibility, environment and economic impacts, public consultation and land acquisition.For example, the Northern Line Extension (NLE) delivers benefits of increased mass transit access to the Nine Elms and Battersea regeneration area generating a cost-benefit ratio of nearly 10:1.There is a rural-urban divide in the drivers behind major tunnelling projects. Fig 3 shows the increases in tunnel diameters that have been delivered over the last two decades alongside proposals that have been announced for the next five years (see Appendix A for a list of completed, in construction, and proposed large-bore tunnels).Over the last two decades, the ability of TBMs to construct large-diameter tunnels in weak ground and significant groundwater pressures has increased significantly.

The tunnels, the largest in the world to date at 51ft in diameter, will carry three lanes of vehicular traffic and a transit line in each direction between Pudong in Shanghai and Changxing Island, a distance of 4.7 miles (Fig 2). The machine itself cost $80 million and is owned by Seattle Tunnel Partners, the project contractors. Historically, tunnels in these ground conditions were constructed with the use of compressed air to provide support to the ground. The highlighted projects suggest a typical cost range per mile of a twin-bore project of approximately $200 million to $700 million. The packaging strategy may be structured to take account of capacity as well as the impacts of geology basis. The design and engineering of tunnels can mitigate their impact and optimise constructionThe UK has a proud tunnelling heritage, including the world’s first subaqueous tunnel, built under the river Thames in 1843.

Currently, tunnels are really expensive to dig, with many projects costing between $100 million and $1 billion per mile. Given the potential for seismic events in the Pacific Northwest, tunnels perform better in earthquakes as evidenced by the damage caused to the Bay Bridge and the Embarcadero highway in San Francisco against the performance of the BART transit tunnels below the bay, which remained largely unaffected by the Loma Prieta earthquake in 1989.The importance of whole-life costing is demonstrated by the fact that the ‘conventional’ cost of an infrastructure project could amount to only 26% of the whole-life cost (Fig 7). The first stage was an upgrade to all sewage treatment works, the second is the construction of the Lee Tunnel, and finally the third is the TTT, which will run 25km from Acton to Abbey Mills where it will connect to the Lee Tunnel.The Lee Tunnel is designed to convey and store sewage and storm water from Abbey Mills pumping station Combined Sewage Outfall in East London to the Beckton sewage treatment works. The project constructed by the MVB JV has posed unprecedented challenges in dealing with high groundwater pressures in the deepest sections, where the groundwater pressure is up to 8 bar, eight times the air pressure found at sea-level.One of the greatest innovations on the project was the construction of five shafts, the largest ever sunk in the capital, with the UK’s deepest diaphragm walls. When tunnelling is the most cost effective way to achieve desired socio-economic outcomes, projects will move forward while seeking to minimise the costs and significant impacts on the local environment. In order to make vast tunnel networks feasible, tunneling costs must be reduced by a factor of more than 10, with TBC's Loop tunnels currently priced at approximately $10 million per mile. For example, the recent Bond Street Station expansion has access through two 9m diameter shafts to the tunnelling site 20m below. Tunnel Boring Machine CTBM) A self-contained and self-propelled structure equipped