Peter Kenyon TunnelTalk

After years in design, a number of alignment changes and two rounds of public consultation over its preferred route, the £2.3 billion Thames Tunnel – backbone of the £4.5 billion Thames Tideway project aimed at substantially reducing sewage overflows into the iconic River Thames during heavy rain – is finally in the hands of the UK Planning Inspector. In three months time a recommendation will be made, after which the Government Minister will have a further three months to make his final decision. As Peter Kenyon discovers, however, fierce opposition to the location of a critical main working shaft on the route could lead to further delays and force another redesign of a project that is already in the design-build procurement phase.

Having divided the 25km Thames Tunnel into three construction lots and started a construction procurement phase on the basis of four TBM headings for the 6.5m-7.2m i.d. main tunnel, serious objections by a local council along the proposed alignment are threatening to disrupt the schedule and force a design rethink by project owner Thames Water.

Fig 1. Thames Tideway currently divided into three construction lots

Hammersmith and Fulham Council is so strongly opposed to the siting of the critical 42m deep x 25m i.d. main working and reception shaft at Carnwath Road (Fig 1), it hired consultant CDM Smith to argue its case during the planning inquiry that ended in March. In the specialist report, made available to TunnelTalk, CDM Smith makes a case that the proposed site of the shaft at Carnwath Road is unnecessary, and that a long reach drive of 12km from the project’s central working shaft in Kirtling Street is feasible.

If the argument in favour of the so-called Alternative A is accepted by the Planning Inspector, or if he decides that further investigation of a different TBM drive strategy is merited, an expensive redesign and a new round of consultation will be required. This would result in costly delays to a project that is scheduled to enter the construction phase by 2016. It could also mean having to rethink the project’s procurement strategy, which is currently based on a Western lot comprising a single TBM drive of 6.95km long, two simultaneous TBM drives of 5km and 7.68km in the Central lot, and a 5.53km TBM drive in the Eastern lot for connection with the Lee Tunnel, which is currently under construction.

CDM Smith report cover

In its report CDM Smith, using data that it collected during its time served as geotechnical consultant for the Brightwater Conveyance Tunnels in the Seattle area of Washington, USA, conclude that a long reach of 12km is technically feasible, and that the additional ventilation requirements could be mitigated by construction of a long-term facility at about the tunnel mid-point.

It concedes that tunnel construction costs could be affected upwards by about 2% (£14-25 million), and that an extra year would have to be added to the construction schedule, but, CDM Smith argues, significant schedule savings could be made using a single-pass lining system rather than the proposed two-pass system, as is the case for many CSO tunnels of similar size recently constructed and in operation in the USA. Eliminating the need for one TBM and one working shaft would accrue further savings.

The report addresses concerns about the added risk posed by excessive wear to both the cutting tools and the main bearing of a TBM during a longer drive and explains that this would not be expected to significantly increase on account of the fact that the drive would be through the less abrasive clay soil of west London.

Project owner Thames Water, however, disputes the feasibility of Alternative A, and dismisses entirely the proposal for a long-reach drive that CDM Smith concedes would force the existing Central and Western construction packages to be merged into a single, redesigned lot. “It is completely flawed,” a spokeswoman told TunnelTalk: “We disagree with the report by CDM Smith, which they themselves admit was based on only a limited understanding of the project. There are a number of factual inaccuracies on which they base their assumptions.”

Tideway project owner Thames Water has stated that it has every confidence in its application as it stands and that it does not foresee any disruption to the procurement process. Thames Water, however, does considerable financial interest in the design as it currently stands.

River Thames

In October last year seven joint ventures and a single contractor were shortlisted for design-build construction of the main Thames Tunnel, as part of a staggered tender release. The controversial (and much amended) Western lot was released for tender in December last year (2013), followed by release of the Eastern lot in January this year (2014). The Central lot is due for release later this month (April). Final awards are expected in May next year (2015), but the final planning decision – which is due in mid-September – has the potential to undermine the whole strategy if the Inspector accepts arguments that the consultation process was somehow incomplete and that viable alternatives exist.

In reference to the disputed Carnwath shaft, and for all its confidence, Thames Water was sufficiently concerned about making the necessary representations concerning noise-related issues that it applied to extend the planning deadline by two months to mid-June. The application was refused.

The rebuttal of Alignment A by Thames Water centres mostly on the need to maintain specified minimum separation distances between the alignment of the proposed Thames Tunnel and existing underground infrastructure including, specifically, the pressurised Barnes-Kew Ring Main and the Lee Valley Raw Water Main tunnels, as well as a planned National Grid cable tunnel in the area (Fig 2).

Fig 2. Thames Tunnel step design

It states it can only achieve the required distances by using the Carnwath shaft as the location for a 1.65m change in elevation, which will lower the alignment under the proposed Wimbledon-Kensal Green cable tunnel to the east and raise it above the Thames Water Lee Valley Raw Water Main to the west. Continuation of the 7.2m diameter drive west of Carnwath would eliminate the possibility of making the alignment adjustment and would therefore pass within the 3m distance that Thames Water considers to be the minimum acceptable distance from the existing pressurised water tunnels.

The CDM Smith report, however, concludes that “despite the increased tunnel diameter in Alternative A, it is our opinion that with proper precautions and good tunnelling workmanship, crossing under and over these obstacles can be achieved with little to no impact to the in-place structure. Significantly closer clearances have been performed without damage to existing infrastructure.”

It was just before the planning submission deadline of March 12 that Thames Water appointed expert, Barry New, introduced the separation issue as it related to nearby pressurised tunnels, in a document entitled A note on separation distances between TBM drives and existing infrastructure.

In response to this new objection to Alternative A, and with only a day to go before the planning submission deadline, CDM Smith responded in a memorandum dated March 11 (2014): “Different designs will lead to different characteristics and we would fully expect that Thames Water engineers are capable of developing an acceptable design based on Alternative A. Thames Water was well placed to analyse the alternative in detail, given the system information available to them and their model developed for the project, [but] no such analysis has been provided in the examination [to the Planning Inspector].”

In any case, argued CDM Smith in the memorandum, the 3m separation distance could be maintained by sacrificing half a metre of clearance under the nonpressurised cable tunnel from 5.98m in the Thames Water preferred alignment to 5.44m under Alternative A.

Fig 3. Initial west Thames Tunnel shaft locations

Fig 4. Amended west Thames Tunnel shaft locations

Thames Water originally planned a much shorter western main tunnel that would end at the old Hammersmith Pumping Station site from where a smaller diameter connection tunnel would be driven to Acton Storm Tanks for capture of the Acton CSO at the far western end of the project. In the first round of consultation a small part of a much larger site at Barn Elms (on the opposite side of the Thames to Carnwath Road, in the south London borough of Wandsworth) was earmarked as the preferred location for a main tunnelling shaft (Fig 2). This followed a process that originally identified more than 200 possible sites that were eventually narrowed down to a shortlist of nine locations, including the controversial Carnwath Road site. At that time the Western lot would have involved two TBM drives using the same machine; a short westward drive from the Barn Elms-Hammersmith shaft followed by another one driven eastwards for connection with a Central shaft at Tideway Walk. A smaller diameter TBM would have been used for the connecting drive west to Acton.

A change in the designation of land use at the Hammersmith Pumping Station site however, from mixed use to residential, made it unsuitable as a location of a main construction shaft. A remodelled engineering design by Thames Water also showed that a smaller diameter tunnel to connect the Acton Storm Tanks to the main large diameter Thames Tunnel would no longer be sufficient and forced a rethink for the second round of consultation.

At this stage, Thames Water decided to make the Hammersmith-Acton connector tunnel part of the main, large diameter Thames Tunnel and move the proposed Barns Elms shaft to Carnwath Road in a riverside industrial site for the continuing large diameter Western lot TBM drive all the way to Acton. The shaft is also designated as a reception shaft for both the large diameter TBM that is planned to complete the 5km western drive of the Central lot, as well as a smaller machine that will drive the 1.1km x 2.6m i.d. Frogmore Long connection tunnel (Fig 3).

Thames Water cited a number of reasons for the change of location to Carnwath Road, which was originally dismissed because, it stated, it fell below the size theshold used for site selection prior to phase one consultation. It claims that unlike the greenfield site at Barn Elms, Carnwath Road is brownfield (a claim contested by Hammersmith and Fulham Council); that it has the benefit of already-existing wharf facilities for barged muck removal; and that larger barges can be used at this location than would be possible at the previously preferred site.

Thames Water said: “As part of our further technical work, we reviewed whether we could reduce the size of our main tunnel drive sites. Where the ground conditions are clay [Carnwath Road marks the approximate location where sandy geology gives way to London Clay], it was concluded that it was possible to reduce the size of the main tunnel drive sites. Therefore we reassessed potential sites, including Carnwath Road Riverside, which was previously discounted as the site fell below the size threshold used for site selection prior to phase one consultation.”

A final recommendation is due to be made by the Planning Inspector in mid-June, with the final decision to be made by the Government Minister in mid-September.

References

CDM Smith Alternative Drive Strategies report, November 2013
CDM Smith Memorandum, 11 March 2014
Bidders line up to deliver Thames Tideway mega-project - TunnelTalk, October 2013
Super sewer to revitalize River Thames - TunnelTalk, March 2009
Excavation complete on Brightwater - TunnelTalk, August 2011

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