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Thames Tideway Tunnel gets planning approval 17 Sep 2014

Peter Kenyon, TunnelTalk

Planning permission is granted for the £4.2 billion 25km long Thames Tideway Tunnel in London following a six-month review process by a panel of experts who rejected a last-minute realignment proposal. The Planning Inspectorate state in their review that better and earlier collaboration between the designers of the Thames Tunnel and the National Grid Cable Tunnel might have removed the need for a controversial shaft at Carnwath Road that had threatened to delay the construction and procurement timetable.

The decision by the UK Secretary of State to accept the panel’s recommendation and give the project the go-ahead in its current design form means that underground construction can begin in 2016, as scheduled. Award of the three tunnel construction lots – valued together at between £1.4-2.25 billion – is expected next year (2015), following selection of a private sector Infrastructure Provider (IP) by project owner Thames Water.

Thames Tideway construction lots and major shaft locations
Thames Tideway construction lots and major shaft locations

The IP, which will be appointed by Summer 2015, will finance, build, manage and operate the new CSO tunnel. It will be responsible for award of three tunnel construction lots (East, West and Central) from a shortlist of six joint ventures (plus Buoygues as a single company bidder) prequalified over the last 12 months by Thames Water. It will also take over the contract that lead designer CH2M Hill currently has with Thames Water.

An examination by TunnelTalk of the full 534-page inspector’s report (the relevant section is at pp. 320-34), however, reveals that serious consideration was given by the panel to objections raised by Hammersmith and Fulham Council – which included expert-backed proposals for a tunnel realignment that would have avoided the need for an intermediate TBM shaft at Carnwath Road on the bank of the River Thames.

The council engaged design consultant CDM Smith, whose report concluded that the tunnel could be built in two rather than three construction lots, without the need for an intermediate shaft at Carnwath Road. It was argued that the design could be amended to include an 11.95km reach between the Kirtling Street shaft and the Acton shaft at the extreme western end of the main tunnel’s alignment. Had such a proposal been accepted it would have thrown Thames Water’s construction procurement process – which is already under way on the basis of three construction lots – into disarray. A substantial redesign would also have caused significant project delays and cost overruns.

Thames Water countered the proposal by CDM Smith by saying it needed a minimum clearance of 6m between the proposed Thames Tunnel and the under construction National Grid Cable Tunnel; that the revised alignment did not fulfil hydraulic requirements; that the long reach introduced “increased risk of TBM failure”; and that an extended reach beyond 9km would introduce safety risks.

534-page planning report
534-page planning report

The Inspector’s report, concluded, however: “Having reviewed all the evidence, our view is that the tunnelling direct option [removing Carnwath Road shaft] may have been a feasible alternative had it been considered at an earlier stage in the project. This is because we consider that the separation distance limitation from the National Grid Cable Tunnel may have been able to be designed out by mutual agreement at an earlier stage; however we understand that construction of the National Grid tunnel has now started. Also the 9km limitation for safety access could also have been included in the design without the Carnwath Road Riverside site by selection of a slightly amended main tunnel route, which would have allowed direct access at either the Barn Elms or Putney CSO sites.

“Whilst we acknowledge that a drive length approaching 12km would increase the risk of TBM breakdown, we do not find this argument against a direct tunnel drive from Kirtling Street to Acton Storm Tanks to be persuasive. This is because it is the Applicant's own design parameter. Notwithstanding this we do find that the long drive would increase the adverse effects at Kirtling Street and that the site is insufficiently sized unless a double drive was awarded to a single contractor/consortium.

“However, we consider that the Applicant's hydraulic assessment that the alternative solution does not meet the hydraulic considerations is of key importance. This is compounded by the difficulty of threading a tunnel through existing infrastructure to tie into the invert level of the Lee Tunnel.

“Therefore, on the information put before us, and on balance, we agree that the Applicant has justified the use of Carnwath Road Riverside as a drive site.”

Natalie Slater for Thames Water, told TunnelTalk: “We have consulted with National Grid from the very beginning and have been working collaboratively to achieve the best solution for both projects. We were very confident in our choice for Carnwath Road when we submitted our development consent application, and we are glad that the Secretary of State agreed with us.”

She said that Thames Water would not comment on the IP procurement process “until we announce the successful bidder next year”, adding that the construction lots would be awarded by Summer 2015.

           

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