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Moving Crossrail 2 towards design 10 Feb 2016

Peter Kenyon, TunnelTalk

With completion of the 21km of twin running tunnels for the east–west Crossrail project across central London, attention now focuses on getting the proposed north–south Crossrail 2 scheme into design and construction. Peter Kenyon for TunnelTalk examines the latest developments, including a proposed alignment change to address serious geotechnical concerns about siting a deep-level station in Tooting, south of the River Thames.

Appointment of Daniel Moylan as head of the newly established Crossrail 2 delivery company marks another major boost in getting the £27–32 billion (2014 prices) mega-project into the design phase.

Fig 1. Latest proposed route of Crossrail 2
Fig 1. Latest proposed route of Crossrail 2 (click to enlarge)

Moylan is former deputy chairman of Transport for London (TfL), the body responsible for operating and expanding metro and bus transportation services in the UK capital. He already serves as a non executive member of Crossrail Ltd, the body charged by TfL with delivering Crossrail. Excavation of the 21km of twin running tunnels on that project is already complete, with construction work 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 following a special meeting of the TfL Board 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, announced by TfL in autumn last year (2015) ahead of a third round of public consultation that has just ended, indicates a preference for the so-called Regional Option, which connects at either end with existing mainline services and which will require wider 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’s 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 originally planned to make such a connection at Tooting Broadway.

Fig 2. Alternative alignment via Balham
Fig 2. Alternative alignment via Balham

However, amid concerns about a major geological fault at the site of the proposed new Crossrail 2 station in Tooting, TfL commissioned a report to examine the risks associated with constructing a deep-level (30m) station, and associated tunnels, at this location. The findings were subjected to an independent review carried out by the Geotechnical Consulting Group, of which Professor Robert Mair is a founding director. Mair is also head of Civil Engineering at Cambridge University and a recognised expert in his field. He agreed 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 only 7–8m deep at Tooting Broadway Underground station and constructed through a 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 at the known faultlines.

Fig 3. Model of the Thames Water Ring Main beneath Tooting Bec
Fig 3. Model of the Thames Water Ring Main beneath Tooting Bec

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 program of ground freezing ahead of excavation of the SCL platform tunnels, or the possible use of a large diameter TBM for the Tooting section. Grouting and dewatering options are considered unlikely to succeed, and both the slurry and EPBM options present likely impacts on the excavation of the running tunnels.

In summary it is felt that the cost of building an underground station at Tooting (estimated at £1 billion), and the risks associated with such a construction, make a realignment through more stable ground via Balham more attractive. It is also noted that the construction time for a station at Balham would be up to two years less than one in Tooting, and at half the cost.

Funding through development

TfL currently has a budgetary provision of £42.2 million for the planning and development of Crossrail 2, plus a further £2m contribution by the UK Department of Transport. This spending is phased up to 2017/18, and more funding is needed to finance the project through to the developmental and preliminary design phase.

SCL platform tunnel construction, like this at Farringdon Station for Crossrail, is predicted to be problematic at Tooting
SCL platform tunnel construction, like this at Farringdon Station for Crossrail, is predicted to be problematic at Tooting

Critically, TfL has made an application to the Government’s Comprehensive Spending Review for a further £250 million to move the project out of the planning phase and into the developmental one. A decision had been expected in November last year (2015), but a month before it was due the Government announced the creation of a new 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”.

This means a delay until at least April (2016) – when Lord Adonis’s report is due – and in the meantime TfL’s Finance and Policy Committee has brought forward £4.74 million of its £22.5 million Crossrail 2 budget for next year.

This money is needed, now, 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).

Completing these works ahead of the Government’s £250 million spending decision will mean that Crossrail 2 remains on target to move seamlessly to design and development, as planned, on 1 April – thereby minimising possible delays.

Public consultation

Three rounds of public consultation have been completed so far. The latest one, which ended only last month (January), 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 two SCL platforms of 250m in length.

One thing has remained constant throughout the consultation process – Crossrail 2 is 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 2; by 2014 – when the route was better known – 83 per cent of the 5,100 who took part in the process either “strongly supported” or “supported” the scheme.

References

           

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