Page 9 - TunnelTalk Annual Review 2016
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Moving Crossrail 2 towards design
With 21km of twin running tunnels for the east–west Crossrail project across central London now complete, attention moved to getting the proposed north–south Crossrail 2 scheme into design and construction with developments including a proposed alignment change to address serious geotechnical concerns about siting a deep- level station in Tooting, south of the Thames.
Delivery of the £27–32 billion (2014 prices) mega-project is headed by Daniel Moylan, a former Deputy Chairman of Transport for London (TfL), the body responsible for operating and expanding metro and bus transportation services in the UK capital and a non executive member of Crossrail Ltd, the body charged by TfL with delivering Crossrail. Construction work on Crossrail is continuing ahead of a planned opening of the central section in 2017.
Like the Crossrail delivery authority, the new Crossrail 2 company, which was established in December 2015, is charged with overseeing all future development and, subject to funding, construction of a proposed north–south Crossrail 2 rail link that could open by 2030.
TfL is currently exploring funding options for its preferred route, which includes 36km of twin running tunnels between Wimbledon in the south and Tottenham Hale in the north, with a short spur section branching off at Seven Sisters towards New Southgate (Fig 1).
The latest alignment for Crossrail 2 indicatesapreferencefortheRegionalOption, which connects at either end with existing mainline services and which will require larger tunnels than the cheaper, but considered less economically beneficial, Metro-Only Option. The underground spur section in the north is extended from Alexandra Palace to New Southgate, although two possible alignments out of Seven Sisters remain on the table.
Geological risk
One major change to the alignment proposed in 2014 is for a connection with the London Underground Northern Line at Balham, rather than at nearby Tooting. TfL considers a south-of-the-river connection between Crossrail 2 and the heavily congested Northern Line of the London
Underground critical, and had planned a connection at Tooting Broadway.
However, amid concerns about a major geological fault at the site of the proposed station in Tooting, TfL commissioned a report to examine the risks associated with constructing a 30m deep-level station and associated tunnels at this location. The findings were subjected to an independent review by the Geotechnical Consulting Group, which found that the Tooting site posed “significant ground risk”, and that it would be “sensible and prudent” to examine the possibility of a Crossrail 2 station at Balham instead. Like Tooting, Balham is also served by the Northern Line.
In addition to the risk posed by the Streatham Fault, there are concerns about the geology of the ground through which both the running tunnels and the SCL station platforms would have to be excavated.
Crossrail 2 will be significantly deeper – at 30m below the surface to the tunnel crown – than the existing Northern Line, which is 7–8m deep at Tooting Broadway Underground station and constructed through the tunnelling-friendly geology of London Clay.
Upon a more detailed review of the geological data available it was noted that deep-level tunnels and SCL platforms at either Tooting Broadway or Tooting Bec would likely move out of the London Clay and through the more permeable geology of the Thanet Sand and Lambeth Group, with the potential for sudden changes in ground conditions, groundwater levels and pressures, especially at the interfaces between strata.
The review also noted the difficulties faced during excavation of the nearby Thames Water Ring Main Tunnel, which suffered a severe inundation.
Possible mitigation measures suggested include a large-scale programme of ground freezing ahead of excavation of the SCL platform tunnels, or the possible use of large diameter TBM excavation for the Tooting section. Grouting and dewatering options are considered unlikely to succeed.
The £1 billion estimate of building an underground station at Tooting, and the risks associated, made a realignment through more stable ground via Balham preferable.
Construction of a station at Balham was estimated at up to two years less than one in Tooting, and half the cost.
Peter Kenyon, TunnelTalk Development funding
TfL has a budgetary provision of £44.2 million for the planning and development of Crossrail 2. This spending is phased up to 2017/18, and more funding is needed to finance the project through to the preliminary design phase.
More funding was expected in November 2015, but a month before it was due the UK Government announced the creation of the National Infrastructure Commission, under the chairmanship of Lord Adonis, to probe the UK’s long-term infrastructure needs and focus on “the priorities for future large-scale investment of London’s public transport infrastructure”.
Money is needed to fund further design development (£2.8 million), conduct transport planning (£850,000), carry out ground investigation at the alternative Crossrail 2 station site in Balham (£250,000), and to procure and mobilise design consultants (£200,000).
Public consultation
Public consultation completed to January 2016, detailed the sites of eight ventilation and emergency exit shafts along the proposed route, two tunnel portals – one north of Wimbledon and the other south of New Southgate mainline station – plus a 300m-long cutting outside Tottenham Hale Station to run the line underground at this location. Further details of the proposed locations of the 10–11 new Crossrail 2 stations that will be required, and their associated shafts and jobsites, were also outlined. Station depths range between 20–30m, with most stations featuring SCL platform tunnels of 250m in length.
Crossrail 2 remains extremely popular. During the 2013 consultation, some 96% of the 13,767 respondents who took part either “strong supported” or “supported” the principle of Crossrail. By 2014, when the route was better known, 83% of the 5,100 who took part were in support of the scheme.
In 2016, Lord Adonis, a former UK Minister of Transport was appointed as Chairman of Crossrail 2. n
• SCL to create Crossrail junction caverns - TunnelTalk, September 2012
• Crossrail 2 attracts mass public support - TunnelTalk, December 2013
Left: Chairman Lord Adonis; Below: Fig 2. Alternative alignment via Balham; SCL platform tunnel construction similar to Farringdon Station for Crossrail (right), is predicted to be problematic at Tooting
Fig 1. Latest proposed route of Crossrail 2 TunnelTalk AnnuAl Review 2016

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