Much has been written about the importance and complexity of the Crossrail works in London, but it is easy to overlook the London Underground upgrade works that are going on alongside them.
A typical example is Bond Street, which opened in 1900 and today sees over 173,000 people/day travelling through. To put it in context, this comes close to the 200,000 a day using Heathrow. When Crossrail is completed in 2018, this figure is expected to rise to about 225,000/day. The station is already stretched by the current numbers, as are its regular users! As with many other busy Underground stations, the concourse and circulation areas get overcrowded and there is restricted access to the platforms, with only one way in and out for some.
London Underground intends to remedy this through a scheme which began in 2010 and is now well advanced. Rail Engineer last visited the site in July 2013 (issue 105) so it was interesting to take up an offer from Miles Ashley, London Underground’s programme director for Crossrail, to see the progress that has been made since then.
The key improvements being delivered include a new station entrance on Marylebone Lane to the north of Oxford Street, a new ticket hall accessed from this entrance, new escalators to serve the Jubilee line, new lifts providing step- free access from street level to all platforms and, finally, facilities within the station for interchange with Crossrail. With the exception of the Crossrail interchange, which will go live when the new line opens, these facilities are all due to open in 2017.
Four measures of success
London Underground and its suppliers are justifiably proud of this project. Miles Ashley described four key reasons for this. The project has an excellent safety record, and has won a RoSPA safety award four years in succession. This has not been achieved by throwing money at safety, indeed the project team would argue that conversely, the good safety achievements have led to reduced costs.
That leads to the second reason for pride, cost. The cost of this project as measured in relation to the extra capacity provided is around one third lower than the European average for similar schemes.
These good results owe a great deal to the third reason, the excellent team spirit within the project team. All of the project representatives spoken to confirmed that the level of co-operation between client and JV members was exceptional at every level. The truth of this seemed to be clear during the visit to the underground works, when it was hard to distinguish which company employed which individual. A really important element of this teamwork has been the inclusion of London Underground operations staff in the team.
This has really helped to ensure the smooth progress of the project and the minimisation of disruption to the live railway network, a feature which has been further confirmed by the fact that the team won the 2015 Team of the Year Award at the London Transport Awards, and has been shortlisted for the BCI Team of the Year Award.
The fourth success has been the relationship with the neighbours. Westminster City Council normally only issues ‘Section 61’ approvals to big projects six months at a time, but here it has been so pleased with the project’s attitude that it has granted 12 months at a time. Given the tiny site, which has only one access, abuts a foreign embassy and a five-star hotel, and is surrounded by some of the most expensive retail and commercial sites in the world, it is clear that the project team has excelled in managing the external stakeholders around them. Oh yes, and of course, it’s in a conservation area too!
An unusual project achievement has been to encourage schoolchildren to get a close-up view of the tunnelling works. This has been possible because of the construction of one of the new access connections between existing and new in such a way that school parties can be taken to this point through the operational station and shown the new tunnel via an access gate in the hoardings.
Grouting and concreting
Grouting played a key role in limiting damage to neighbouring properties on this project, as with many of the other current London Underground and Crossrail sites in central London. In this case the grouting was all carried out from one grout shaft by Bachy Soletanche using ‘tubes á manchette’ (essentially grout tubes with means to control grout distribution along their length). 93 of these were driven from the shaft approximately horizontally on three levels under the surrounding structures. They were used to pre- grout and strengthen the ground, and later to permit compensation grouting to correct any settlement after tunnelling. This element of the project has been successfully completed and demobilised.
A key innovation by the project has been the early erection of a steel and pre-cast concrete-framed structure that is destined to form the skeleton of the permanent over-site development once the station works are done. On a small site it might seem odd to erect this structure early, but it is not so daft as it might appear.
The innovation has been to use the frame to create site space above the works, so that plant such as the ventilation and concrete machinery is installed here, out of the way of the tunnelling and construction down below. The exterior of the frame is clad with panels with high sound-attenuation properties. The single street-level access already mentioned comes into this structure right next to the five-star hotel.
The access is fitted with sound insulating doors so that, at night, the hotel residents can be spared any interruption to their sleep.
There are two shafts within the site that give access to the different levels down below. Innovation has been applied here too, as one of the shafts is fitted with a heavy steel horizontal door which may be closed over all or part of the shaft to provide extra working space for activities at ground level when needed.
Further working space was released by mining below a listed eighteenth- century building adjoining the site. The cellars of the building were removed after piled supports had been installed and a heavily reinforced concrete slab had been constructed to transfer the loads from the building into these.
Monitoring apparently showed that the building moved a maximum of about 13mm out of vertical during this massive underpinning exercise, but by its completion it had returned to exactly where it was originally.
Another neighbouring building had to be treated totally differently as it is a foreign embassy. As such, it is legally foreign territory and could not be touched. Consequently, down below ground, a tunnel has had to be constructed right round three sides of this structure to gain access to the parts of the site on the far side. This tunnel is not required for the permanent works at all, but it is now intended to make use of it as a service duct once construction uses cease.
Underground, the shafts and tunnels have been constructed using a fibre-reinforced SCL (sprayed concrete lined) technique. Most of the construction has been in London Clay, which is easily dug and, provided that its moisture content is not altered, remains quite stable after excavation. The moisture issue is critical though, and so the general rule has been to excavate one metre and then spray concrete to seal the surface before advancing the excavation again. The final lining goes on later, in two layers. The first of these is structural, again steel fibre reinforced SCL, the second is a smoothing coat that prevents injury from the rough surface of the FRC.
The dry SCL mix is delivered by road tanker in a similar fashion to that commonly used for mortar on building sites. It goes into large silos within the site, from which it is delivered to the mixing and pumping plant lower down the building frame. Here water is added before the wet concrete is pumped through a network of tubes to the spraying plant at the work site.
Connecting with earlier tunnels
One of the more fascinating aspects of the works was the construction of the interfaces with the existing rail tunnels and platforms. The natural reaction, on seeing the outside of a tunnel lining, is to worry. In the past, that was not something that one would expect to see unless there had been a serious problem like a collapse. On this site, there were many places where it was possible to see, and indeed to touch, the exterior of a tunnel lining. In most instances there were trains running or people on a platform only a few feet away on the other side of the tunnel rings.
This was all a part of the plan here, of course, for the project is all about new connections to the existing railways. In some instances, the new works were to connect into existing ones to provide access directly from stairs, escalators or lifts. In other cases, the need was to cross over or under an existing tunnel or pair of tunnels.
So, what happens at these interfaces between old and new? Well, all of these places on this site involve spherical graphite cast iron tunnel (SGI) rings. Engineers will understand that one doesn’t just hack a hole in these to make an aperture! Not even when the railway inside is closed, as it may then have to remain closed much longer than planned.
Where the intention is to make a new connection into the existing tunnel, say to access a platform, then it is necessary to construct a frame around the intended aperture that will carry the loads that are currently carried by the bits of SGI that are going to be removed. This means bolting a large horizontal beam into the SGI lining above the top of the required hole, bolting a second such beam across the bottom, and then interconnecting these down each side of the planned hole with further steel frame members. The side members, of course, need to be curved to fit around the circular tunnel lining profile. Once this rectangular frame has been erected and secured in place, the disc cutters can come out and the opening can be cut in the lining. Always supposing, naturally, that the other side of the lining has been hoarded out and made secure.
When there is the need to bridge over an existing tunnel it gets more complicated still. The tunnel lining cannot be used to carry any of the bridge loadings, and so suitable bridge abutments have to be constructed on appropriate foundations parallel to the tunnel on each side. As with the side entrance aperture construction, the next stage is to reinforce the tunnel lining so that it may be safely cut open where the bridge deck is to span over the railway below.
The approach to this on the Bond Street site involves the construction of heavily-reinforced concrete walls parallel to the planned bridge, one each side of it. These sit on the tunnel lining and are tied into it and the new bridge abutments. Once again, after the SGI lining has been stabilised by these works, and with the railway below closed off, the tunnel lining can be cut out to permit the bridge deck beams to be placed across between the abutments and secured.
It is important to seal the bridge deck to prevent things from falling into the tunnel below once the bridge is brought into use but, even so, there were prominent signs beside the bridges on this site forbidding ‘washing out’ on or near the bridge deck.
It is worth mentioning here how hot it gets in these working conditions. On the day of the site visit, it was initially quite a relief to get underground as it was windy, cold and wet outside. However, it soon became apparent how warm it was in the workings.
When visiting one of the stairway tunnels, which was rectangular in section rather than a circular tunnel, one really felt the heat. The walls, floor and roof of this were all of thick, heavily reinforced concrete, and the concrete had been poured relatively recently.
The heat from the reaction of the setting concrete was strong but, according to the team members working there, it was nothing. Apparently, just after the pour, it had been so hot that the concrete was unpleasant to touch. Working there must have been very hard to bear.
One of the most noticeable aspects of the work, in common with many similar ones going on in London now, are the temporary works and services – very- prominent tubes, ducts and pipes. The big flexible ducts seen are the air supply ducts, bringing fresh air pumped into the tunnels to ensure safe breathing conditions for men and machines.
The Costain/Laing O’Rourke joint venture is using diesel plant down in the tunnels, so without this air provision the atmosphere would quickly become unpleasant and potentially unsafe. Other supplies piped in include water, concrete and compressed air. Managing all these services is a critical aspect of the job. As the tasks being undertaken change, it is necessary to move ducts and pipes around to ensure that they don’t obstruct the work.
Despite the air supply, and even though all critical plant is duplicated (or more), there is still a risk of plant failure, fire or another cause of foul air in the tunnels. For this reason, everyone on site is required to wear, and be able to use, a re-breather.
Journalists are no exception, and so I was issued with one of these devices and trained in its use before being allowed onto site. For those not familiar with these devices, they are a bit like large stainless steel lunch boxes worn on a belt round the waist. Inside this is a thing rather like a WW1 gas mask.
In an emergency, the device is worn on the chest and one breathes through a mouthpiece rather like that of a snorkel. The bag on the chest contains chemicals and has to be inflated when initially donned.
After that, one breathes out into it and breathes in from it. Chemicals within the apparatus strip the carbon off the carbon dioxide breathed out, so releasing free oxygen for one to breathe. This process generates a lot of heat, so the bag on the chest gets quite hot. I’m pleased never to have needed to use one of these in earnest so far! However, just wearing the thing in its box all day every day in confined workspaces must get pretty annoying.
However, the men and women working on the site seem to be well able to grin and bear it. Despite the heat and the gas masks, the team constructing Bond Street’s enlarged underground station seem to be dedicated to getting it ready on time for its 2017 opening. Rail Engineer will be there to see the finished result.