Bank Station Part 4: Getting Radical

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In part 3 of our look at the plans for Bank station we described the continuing growth that is taking place there and the revised plans to handle this growth, including a work site at Arthur Street that will simplify the logistics and speed up the work.

This revised scheme was the brainchild of Dragados who were successful at securing the contract when bidding against three other applicants. It was not just the initiative of the worksite at Arthur Street that helped secure the contract. There were other pleasant surprises too. How much these depended on the Arthur Street worksite is not clear but the additional tunnelling time made available by this worksite and, we believe, the advantages of having the new running tunnel slightly further away from the line of route originally proposed probably helped.

Joining up is so very hard to do

Early on in the scheme it was recognised that connecting the new southbound running tunnel to the old one north of Bank was going to be a problem. This is due to the fact that a step-plate junction is not really possible for a number of reasons – mainly the proximity of the northbound tunnel above and the DLR below. In any case step-plate junctions tend to be a feature of tunnels with segmented-lining and the plan was to use spray concrete techniques throughout. TfL may well have been hoping that at least one of the bids would show a proposed method of connecting the tunnels with only a short interruption.

New Alignment at the northern end resized

The new connection of the southbound tunnel at the northern end. The tube in purple is the new southbound running tunnel. The northbound tunnel is directly above and the DLR almost directly below. The planned extra thickness of tunnel lining present for a short distance on the new (purple) tunnel is almost certainly to help spread the load under foundations present above that location. The short connecting passage with the original tunnel is there to assist construction whilst the diversion is implemented. Despite appearances it is expected that the new southbound tunnel will be slightly larger (4m) than the existing one.

Slightly disappointingly (but not surprisingly) no construction firm was prepared to commit to a step-plate junction at the northern end. One suspects that if there had been a recreation ground above instead of some of the most prestigious and most expensive historical buildings in the land then the options would have been different. Because a step-plate junction will not be used here there is no advantage in using this expensive method of construction at the southern end either.

The method that will now be adopted to join the new tunnel to the existing one involves filling the existing tunnel with foam concrete at the point of divergence. Then, approaching from the new tunnel, one of the remote tunnel digging vehicles will simply cut through with its sharp cutter. Apparently the cast iron segments will not be an issue as, although very strong in compression, the cast iron will offer limited resistance and will just get “chewed up” by the cutter.

Once Dragados was resigned to having to close the Bank station completely for a period of about four months they looked at how they could make the best use of this. They discovered that in fact they could get on with an awful lot of other work that involved line closure – such as building side passageways that would break out into the old southbound platform tunnel. With the advantage of the early start to tunnelling provided by the Arthur Street worksite, Dragados worked out that they could have the interchange facilities ready when the line was re-opened in August 2020. As we understand it, this includes fittings and fixtures such as new escalators. This would then just leave the Cannon Street entrance to be completed and fitted out in the following year.

The enhanced Central Line link

When TfL put the job out to tender the specification wasn’t actually for a passageway here and a passageway there, but was to achieve identified objectives such as the capability of allowing a certain number of passengers to interchange between the Central Line and the Northern Line. Dragados looked at the computer modelling of the original plan and weren’t overly confident that it would actually work. What they chose to propose instead was a completely revised route involving a long straight line passage and three new escalators to the Central Line avoiding the current passages completely. Any issues about the interchange being too long compared to the original route were dealt with by including a moving walkway within the long passage – as on the Jubilee line at Waterloo and installed in many places on the Metro in Paris.

Link to Central Line Detail

The link from the Northern Line to the Central Line by a completely new route (in purple). Note that is does not rely on existing passageways. Unfortunately it is necessary to have a change in direction of more than 90° at the northern end of the moving walkway to get to the escalators. This is positioned in the bottom centre in the 3D plan and is partially hidden by one of the Central Line tunnels. The abrupt change in direction is a small price to pay for establishing this totally new link and the extra distance involved is mitigated by having the moving walkway and the convenience of escalators.

It does at first sight seem strange that Dragados could include a moving walkway where one was never present in the invitation to tender. Indeed it is believed that London Underground originally considered it as an option but dismissed it, due to it requiring a straight, and preferably level, tunnel. It appears that moving the southbound tunnel slightly westward made this physically possible and having the Arthur Street site made the logistics of the removing the additional quantity of soil more economically and logistically feasible.

DLR access

Access to the DLR at Bank has always been less than ideal and again Dragados seem to have been very bold. Passenger modelling borne out by reality checks of seeing what actually happened showed that few DLR passengers actually used the central part of the wide passageway in between the two platforms. Because DLR passengers at Bank tend to be regular passengers, they are already in the correct part of the train to secure a suitable entry or exit at the appropriate end.

DLR escalators

Escalators from the Cannon Street entrance to the Northern Line. From there, there will be a further bank of three escalators to the DLR platforms. The route between the sets of escalators is not very direct but this is deliberate as it avoids issues of conflicting passenger flows.

Based on the above knowledge, Dragados argued that installing three escalators in the central part of the wide DLR passageway would be a much better use of the space that its current use as a wide occasionally-used passageway. Those who did currently use it as a passageway would still have an alternative route available via the platforms if the central portion of the passage was not available. These three escalators would provide easy access to the DLR from the base of the new escalators taking passengers from the Cannon Street entrance to the Northern Line level.

The icing on the cake when it comes to DLR access is direct access by lift from the Cannon Street entrance. The lift shaft involved was in fact a very tight fit. Once the outline of the scheme was decided upon, the exact position of this lift shaft was then determined and the rest of the plans were finalised so they took this into account.

It is also notable that Dragados have slightly relocated the Cannon Street entrance within the available working space (the worksite) so it is slightly east of where it was intended to be at the time that the tender was issued. This is partly down to the DLR lift shaft which, once fixed, meant that the escalators had to go one side or the other. The advantage of going slightly east is that the alignment of the escalators is much better and fits better into the scheme as a whole and helps make the additional escalators to the DLR a practical proposition. The disadvantage is that one loses the entrance from King William Street but, as stated in Part 3, that is no great loss.

A Bijou Entrance to Bank

Cannon Street Entrance

A very small entrance. Most passengers will be regular users who will pass through the station entrance quickly so a larger entrance space is not necessary. For what one expects to be a busy entrance though, it will be small by modern standards.

One thing that is very noticeable from the supplied computer generated image of the Cannon St entrance is just how small it will be. Compare it for example with the future Walbrook entrance. Although the Cannon Street entrance will be large enough for its needs, it really is a compact minimalist site and there would not be enough space for a ticket office if one wanted to put one there – which obviously London Underground don’t. One forced concession that goes against modern best practice is that the entrance is located on the main road (Cannon Street) rather than a side road off it, but the alternative would be a much more unsatisfactory 90° turn at the top of the escalator.

A Temporary Site Accommodation Free Zone?

Another really neat idea from Dragados, which is so obvious when you think about it, is to not bother with demolishing office space only to the replace it with temporary accommodation. Instead some buildings on a part of the Cannon St entrance worksite will be retained as site offices. This will have the additional advantage of retaining a facade that has been identified as being of architectural value. Of course the offices may well be demolished at the end of the scheme before the land is handed back to a developer but that can be done when, as far as the public is concerned, the scheme is complete and so it would not be delay the opening of the upgraded station.

Taking the Heat Out

There are still exact details to be finalised and additions to be made to the scheme if a case for these can be made. One major area for more investigation is the possibility of heat extraction. This could be any combination of three methods.

The first method of removing heat, which is thought to be highly likely to be implemented, is heat extraction of a conventional nature to heat the replacement office developments that would be built above the new Cannon Street entrance. With the price of energy so high this would seem easy to justify on straightforward commercial grounds and would probably use conventional well-established technology. Unfortunately it may not do much for the Central Line platforms where the need is probably greatest.

The second method is groundwater extraction as done at Green Park station using two boreholes within Green Park itself. The work tunnels at Bank station will be large enough for it to have its own “source” and “sink” sufficiently far enough apart. What could scupper this idea is the fear of doing anything to affect the foundations of the surrounding buildings.

The third method is probably a long shot but Crossrail have developed some very innovative ideas to extract heat from the tunnel lining which the Bank project team are aware of. Unfortunately, as both time was against them and it was untested in the real world, it was thought too risky to include on Crossrail itself. The original method does presume concrete segments in the tunnel so it may not be suitable at this location and so, if it ever debuts at all, it will probably not make an appearance until Crossrail 2 is built.

You can never have too much space

A much less exciting and less dramatic further enhancement to the scheme is the possibility of inclusion of extra storage or other space within the final plans. Underground stations never seem to have adequate space for ancillary functions and Bank has been particularly problematic. With spray concrete lining it will be as easy as it can be to create additional rooms or other spaces should a suitable case be able to be made for it.

An Air of Optimism

There are other reasons for excitement and a positive feel from the project team. The first is the perceived lack of objections. Of course there are concerns that need to be addressed but the feeling in the city and from the public is that this project cannot come soon enough.

A second reason for optimism is seeing how well Crossrail is progressing, construction-wise, and Dragados is one of the main contractors involved with the project. The project team hope by getting started early with the contractors they can do better still. As a result of lessons learnt on Crossrail they are proposing to “embed” National Grid Gas and Thames Water teams into the joint construction team so there aren’t delays as utility pipes are encountered or supplies are required – not only does concrete requires a supply of clean water to mix it, there is also bound to be unwanted dirty contaminated water that needs getting rid of.

Onward to tackle Camden Town?

It is still early days but the project team feel that TfL are getting much better at putting forward their case for major schemes involving station rebuilds. Certainly if they had their time again they believe TfL would have presented their case for Camden Town differently. They would have also spent much longer anticipating and understanding the objections in advance of the public inquiry with a view to resolving as many as possible prior to it – anyone familiar with the original proposals for Camden would be aware of the sheer diversity of objections made to the scheme.

As has been indicated before, TfL are now in the process of the early stages of preparing a revised case for Camden Town. If a revised scheme there is approved and the money can be found for it then there should be nothing standing in the way of the long-desired complete split on the Northern Line. One has the feeling that TfL will prepare the Camden Town proposal very slowly, methodically and with determination. Bank will be good preparation for that. One is lucky when one gets two bites of the cherry – it would be a very optimistic person who thought there could be a third chance if the second one failed.

Next Steps

The next stage for the Bank Upgrade is for the project team to arrange test borings and prepare a draft Transport and Works Act Order. The latter should be sent for approval in July 2014 according to the planned timetable. It is almost inevitable that, even if there are no objections to the scheme on principle, a scheme of this nature will have objections on the finer detail, lack of compensation or proposed extent of disruption during the construction phase. With this in mind the need for a public inquiry is seen as almost unavoidable. Assuming a favourable outcome, work will not start until April 2016 and four years later a closure of around sixteen weeks will take place to bring the new southbound tunnel and platform into use.

Looking at the proposals for Bank station and the final outcome, however, it does all seem to be good news. There is a significant downside though and that is the need to close the station completely for around four months. When we eventually get around to part five we will look at the issues involved in closing Bank station in 2020.

Written by Pedantic of Purley