A new tunnel crossing of the Thames River, proposed for the Greenwich peninsula in London, will go to public consultation later this year, prior to seeking a development consent order or DCO. Assuming the go-ahead work could begin in around three years’ time on the first road tunnel crossing on this side of the UK capital for nearly 50 years, and one of the most significant, it’s enough to make it a ‘Nationally Significant Infrastructure Project’ (NSIP).
The scheme has been under development for several years and just how it will be built, or can be built, has been a key factor in selecting options and working them up. Input for design options has come from several constructability experts, looking at bored and immersed tubes.
Eastern expansion
The project is important. London’s centre of gravity has been shifting a little eastward in recent decades, first to the Isle of Dogs docklands financial district, then in redeveloped dockland downstream and around the Olympic Park. A new buzz of technology and media industry has revitalised Shoreditch and Hackney. The process is likely to continue with the Mayor of London’s office identifying east London as a growth area over the next few decades.
But there is one major constraint, the river Thames. Downstream of the famous Tower Bridge and the narrow Victorian Rotherhithe Tunnel, last of a score of crossings in the capital, the river widens out considerably into its tidal estuary. This has limited the number of downstream crossings, keeping north and south sides of the city separated. River width goes from 200m upstream at Putney to 490m at Charlton. Even at the chosen Greenwich location it is 450m across the river.
"There have been some new public transport crossings built, including two tunnels across to the Isle of Dogs for the Jubilee Line of the London Underground and one for the Docklands Light Railway extension" says Jason Saldanha at Transport for London (TfL), the body responsible for much of the city’s transport infrastructure. He is project manager for the crossing.
"But road traffic has only had the option of crossing on the M25 ring road further out at Dartford or through the Blackwall Tunnel," he says. The latter, a twin bore crossing at the end of the Greenwich Peninsula, is extremely busy and usually congested at peak hours, jammed northwards in the morning and south in the evening.
"On top of that the northbound is 120 years old, built in horse-and-cart days," says Saldanha "which means it cannot handle many modern trucks or buses." The result is frequent tunnel incidents, some 1,000 annually, involving the need to head off over-high HGVs. Although there are "escape lanes" on the tunnel approach, it takes time to divert vehicles onto other roads, snarling traffic.
Even then, enough big vehicles make it through the height gauges to get stuck and sometimes damage the tunnel cladding, with repairs adding even further closure times. "Sudden unplanned closure can cause massive congestion especially at busy times," he adds.
Aware of the need TfL has been looking at options to relieve the Blackwall Tunnel and provide better cross-river highway connections for some time. Possibilities included additional bores for Blackwall and even a ferry link as at Woolwich, but strategic studies suggest the best connection would be separate, but close, to the existing Blackwall Tunnel. It would carry a route from the A102, going across to Silvertown on the north bank to reconnect into the main road. The crossing will be just where the river bends south again around the Greenwich Peninsula, at the same point where the cable car crosses the river.
"A bridge was ruled out," says Saldanha. At this point the river can carry significant marine traffic including fairly large vessels and clearance requirements demand at least 55m over the river.
"Pylons would be even higher and climbing approaches long, with the whole structure out of scale with local development and infrastructure, especially planned residential schemes for Greenwich," he says. A lifting bridge was examined but would cause too many road closures, which rather misses the point of the project.
So a tunnel then. But again there were options, specifically an immersed tube or a pair of bored tunnels. They would need to be substantial in size, with room for most modern high trucks and with twin lanes in each direction, for both capacity and safety redundancy. The size would also allow for double decker buses and tall HGVs, to greatly expand the east London route network north to south.
Dual Descisions
Making the choice and then working up the final option has seen schedule and constructability issues come into play. Two different consultancies have been working on the project, the first Mott McDonald and finally Atkins, which is refining the concept stage bored tunnel design. They have both developed their schemes with input from construction specialists, Mott with London Bridge Associates (LBA) and Atkins with input from Balfour Beatty’s tunnel specialist, Roger Bridge, who is also chairman of the British Tunnelling Society.
The current design requirement is for as generic a scheme as possible, says Saldanha because the intention is to award the project as design, build, finance and maintain contract, probably on a private finance initiative concession basis. The project’s detailed design will be carried through by the winning contractor and the aim is to keep the reference design as open as possible to allow for innovation and contractor refinements.
"But at the same time it is necessary to present the project for the planning process in sufficient detail for sensible decisions about its local effects, both permanently and during construction. Local authorities and other interested parties have to know the size and scale of the scheme, traffic changes, assessments of land take needs, and the environmental impact," he says.
A PFI basis for the project would take advantage of user charging for the crossing. Vehicle charging is to be imposed for two reasons, says Saldanha. First is to allow demand management of the route, freeing traffic flows to offer an estimated 20-minute reduction in delays at peak times. Secondly it will provide a revenue stream to repay the PFI contract. "We have money for the planning of the scheme but not enough to pay for the major construction itself," he says.
This aspect of the project may prove to be one of the contentious issues in the coming period because it will go hand in hand with a toll imposed on the existing Blackwall Tunnel, currently a free crossing of the river. "But you cannot control overall demand at the new crossing if the Blackwall alternative remains free," he says, "as vehicles would simply divert there."
User charging levels are likely to be at a similar level as the current Dartford crossing, around GBP 2.50 (USD 4) for cars, probably collected by a free flow system similar to the London Congestion Charge, using technology like number plate recognition combined with Internet payment. Users may be able to get reduced charge rates if they become account holders.
That comes later however. In the meantime the various design packages have whittled down the choices. First task was to compare the immersed and bored options, a project carried out by Mott, with LBA input.
"Our role is to provide the knowledge of practical difficulties and constraints on the actual construction of any design," says LBA managing director David Sharrocks.
The consultancy comprises a mixture of "old hands" from well-known tunnelling firms such as Nuttall and Taylor Woodrow, as well as calling on a network of particular contacts from grouting firms, pile contractors and such.
Such input, to keep designs down to earth and to eliminate "can’t be done" tooth-sucking, is increasingly part of development work for projects, using such methods as early contractor involvement, for example, or simply arising from a more realistic understanding of construction practicalities within the design community, "not least because of CDM (construction design and management) safe working requirements," says Balfour Beatty’s Bridge.
Construction issues in design typically take into account factors such as practical space constraints, mucking out routes and methods, supply lines, and a host of other factors like necessary machines, their sizes and the needs they impose for temporary works. Site working safety is also a factor. But there are limitations to designer capability and ECI does not always suffice, suggests Sharrocks.
"A designer simply does not have the detailed experience of construction on site, which allows for all the factors affecting work to be taken into consideration. But a contractor may not be suitable either, firstly because you might want the input at an earlier stage, as with Silvertown for example."
Very often a contractor will have very particular ideas about the constraints, perhaps relating to the use of a specific piece of kit he has sitting in the yard or methods he does not want to divulge for competition reasons.
"But advisers like ourselves, mostly drawn from the contracting world, can provide a more general idea of what is neededm" Sharrocks says.
Beyond the ideal design for construction there is a skill in assessing just what compromises might be possible between, say, providing a generous space for a TBM starter pit, and limiting its size perhaps to reduce excavation, necessary side wall thicknesses and propping requirements, and thereby costs. Temporary and permanent strutting might also need to be juggled to cope with the need for movement of equipment and positioning of supply lines during construction.
The aim is not simply to see the practical issues for site work, but to judge carefully how such factors and their interactions affect scheduling, project costing and the optimisation of the design.
For Silvertown these factors and many others have had a significant effect in assessing and comparing the two options for a tunnel, as designs for an immersed tube and a bored tunnel were run in parallel. While the immersed tube was potentially slightly more economic for example, the impact of its construction was greater.
Multiple factors are involved but some of the critical ones include the land availability for the work site, geology, spoil removal, possibilities for alignment modifications and fire and safety considerations.
The alignment is in fact set fairly tight, says Ken Spiby, Sharrocks’ colleague at LBA also involved with the Silvertown scheme. "The client is responsible for the overall road network and gave Mott an envelope for the tunnel and its connections into the network at the end."
The immersed tube option examined was an approximately 500m length box tunnel to run under the river bed, comprising four immersed concrete boxes with twin rectangular road sections, connected by cut and cover approaches at the ends. At a higher vertical alignment than the bored option it would have needed shorter approaches to the bored alternative.
The conventional tunnel needs to run at between 25m to 30m deep to clear the river bed with twin drives of approximately 11m internal diameter, needing a nearly 12m diameter TBM. To stay within gradient limits each is just over a kilometre long and there are cut and cover sections envisaged at either end of about 200m length, to complete an overall 1,450m-long tunnel.
Both options used known technology and several past projects of a similar nature have been completed in the UK; the most recent immersed tube on Tyneside and a variety of bored crossings done in the last 20 years under the Thames, for tube lines, Crossrail and high speed rail into Paddington.
For both cases much of the work is envisaged as taking place at a main construction site on the north side of the river, where there would be a casting and float out yard for the concrete segments for the immersed tube, or the main access point for the tunnel drives. "There is not sufficient room for a large worksite on the south side and no easily accessible wharf," says Saldanha.
Immersed vs. Bored
Critical factors for the immersed tube would be finding space for the casting yard and the impact of the work on the river regime, says Spiby.
"The river has to be dredged to form a flat bottomed trench for the concrete segments, and that has an impact on the environment, for example in causing silt to move downstream. The casting yard would also have to have access to concrete casting materials, probably brought in by barge on the river, the method favoured by the city mayoral office for all major projects." Truck movement is possible but more difficult.
Dredging would disrupt river traffic and so too would placing operations for the box float outs, each requiring a river possession as the boxes are moved into position and lowered to the river bed. Spiby and Sharrocks visited the Port of London Authority at Gravesend to discuss how that could be done.
The immersed tube would also require breaking the river walls and reinstating them around the new entrances, with significant environmental and river regime effects.
For the bored tunnel there is less direct river impact but factors include locating sufficient space for an operating yard and spoil handling. Again river transport is envisaged for the spoil removal and also the import of segments and other materials for the tunnel drives, which would be carried out by TBM, almost certainly starting from the north. Barge loading and unloading requires berthing and, Saldanha says, that there are options on the Silvertown side.
The bored tunnel is likely to be carried out by an EPBM since the drive runs through mainly London clay and some sands and gravels. "We do not go as deep as some of the recent high speed rail tunnels, for example," says Saldanha, "and therefore do not get near underlying chalk." According to Balfour Beatty’s Bridge, who is working as constructability adviser with Atkins on the next stage of the chosen bored tunnel option, the ground is mostly clay, Lambeth group mixed clay and sand, "and river terrace gravels and made ground at the portals."
Drive options were examined by the Mott study but the obvious method is to use a single machine, driving southwards, then turning the machine and returning. A variety of questions arise for the advisers in this, particularly on the structure of the starter pit, and especially for dimensions of the southern reception and turning box. Just how the machine is supplied and spoil removed were factors affecting sizes, possible land requirements and dimensions for cross walls and so forth.
Supplying the machine is also important, with the most likely method being delivery through the first bore for both the initial and the return drive, and with spoil conveyors passing back that way too.
"Another important issue for comparison was the fire safety and evacuation provision," says Spiby. Standard Highway Agency regulations are for cross passages to be provided every 100m. But this could be quite expensive and entails some risk, particularly as each must be hand excavated.
Sharrocks explains, "You have to break through the lining which means a weak point in both temporary and potentially the permanent tunnel and there remains uncertainty about just what the local geology will be."
Bridge also points out that a decision to impose a 30mph speed limit would de-risk tunnel operations, potentially allowing an increase in the spacing of cross passages.
For an immersed tube on the other hand it is fairly simple to provide a central escape gallery into which any number of emergency doors can be provided at relatively little cost. Examining the possibilities of reducing the cross passages, by using enhanced safety detection systems, active ventilation methods to isolate fire, and other technology is part of studies.
The result of Mott’s’ study in 2012 was selection of the bored tunnel as the preferred option, with some additional elaboration worked up in a subsequent design contract.
But the finalising of the design is being carried out by Atkins, which won a competitive tender last year. "Atkins is finessing the bored tunnel design," says Bridge. One change for example is to move the north end of the bored tunnel slightly riverwards to make sure it does not sit over a backfilled lock to the Royal Docks.
Although there are "as built" drawings, says Bridge, it is not sure how far they accurately show what obstacles might be there.
Another aspect is considering further the river transport, which would allow a TBM to be delivered in larger sections than by truck, saving time. But it would in turn mean providing for larger lifting capacity.
The alignment itself cannot be changed too much however as some recently built structures anticipate a tunnel, not least a viaduct on the DLR extensions, which sets its piers wider apart at this point, and the cable car, too, which has foundations nearby.
Next steps for the project are due this summer, he says. Under the DCO process the statutory consultation can be relatively short – though about eight weeks are allocated rather than the minimum four. There was a 10-week local public consultation last year as well. An application may be submitted to the Planning Inspectorate at the beginning of next year.
