LONDON’S WEST End is no place to be if you have an issue with crowds. Oxford Street in particular is a place of rush and tear where crossing the pavement, let alone the road can be hazardous.
And it’s a scene that is mirrored underground, where hundreds of thousands of travellers make their way through the capital’s complex Tube system every day.
Bond Street station, currently accessed from the south side of Oxford Street, is a case in point. Built in 1900, no-one at the time could have anticipated that today it would be catering for 175,000 Central line and Jubilee line passengers a day – a _ gure that is increasing all the time and that will jump to more than 225,000 a day when Crossrail arrives at the station in 2018.
The arrival of Crossrail may have provided a catalyst for the GBP 320M (USD 490M) redevelopment of the station, but the reality is that Transport for London (TfL) and London Underground (LU) have been committed to upgrading the Tube system for many years and the Bond Street Station Upgrade (BSSU) is one of many on the agenda.
"It’s often thought that when we’re carrying out improvements on the network we’re just ‘fixing the Tube’," says Miles Ashley, LU’s Programme Director of Crossrail and Stations. "It’s actually part of the continuity we need to keep our heads above water with the enormous growth we have in London.
"The number of passengers we carry has doubled every 35 years since 1863. Now we’re facing the prospect of an increase in London’s population from 8.5 million today to 10 million by 2030. That equates to another Tube train full of people every three days.
US Tariffs are shifting - will you react or anticipate?
Don’t let policy changes catch you off guard. Stay proactive with real-time data and expert analysis.
By GlobalData"The BSSU is a result of that growth – during the last seven years there has been an almost 50 per cent increase in the number of passengers using the station. Crossrail will add another 30 per cent on top of that." Approximately 50 per cent of the passengers using Bond Street do so to interchange between lines and never reach the surface.
Critical path
He added that there’s no point in investing in more and faster trains if the passengers can’t be cleared from the platforms in time.
"The Jubilee Line is one of the most frequent train services in Europe at up to 36 trains per hour, but it’s no good if you can’t get people off the platform before the next one comes along."
In a nutshell the BSSU project, which is being delivered by the Costain-Laing O’Rourke joint venture, will increase capacity by 30 per cent; new pedestrian tunnels, a new ticket hall and escalators will offer increased access to the Jubilee and Central lines. Greater "permeability" to the Jubilee Line will create a more even distribution of passengers along the platform and, therefore, through the trains, and improve the flow of passengers in and out of the station.
The project also features an improved interchange between Central and Jubilee lines and access to the new Crossrail station (which will have its own entrances at nearby Davies Street and Hanover Square).
The BSSU also features a new station entrance and ticket hall on Marylebone Lane, on the north side of Oxford Street. In addition to the eight existing escalators in the station, two escalators within the new tunnels serving the Jubilee Line and four new lifts will deliver step-free access to all platforms for the first time – LU plans for more than half of rail and Underground stations to be step-free by 2018.
The project is due for completion and full operation in 2017 and has entered the final stages of tunnel construction. More than 500m of tunnels have now been excavated beneath Oxford Street, plus the basement-level ticket hall.
The works include two approximately 10m-diameter access shafts. The base of Shaft 1 is 28m below street level while Shaft 3 is 18m below street level.
A lift installed in Shaft One will take passengers down from the ticket hall to the interchange level where they can access the Jubilee and Central lines and Crossrail. This shaft will also have an emergency escape staircase and will house various services.
Another lift has its own sprayed concrete lined shaft connecting with the Jubilee and Crossrail platforms. There are two other lifts, one in square works giving access to the Central line and one in reinforced concrete at basement level providing access from street level.
The works also feature two construction adits, two binocular cross passage tunnels, two large concourse and connection chambers, three underpass tunnels, three over-bridge tunnels cutting through the cast iron segments of existing platform tunnels, two niches for electrical and mechanical equipment and one inclined tunnel for escalators. Tunnel widths range from 4m-10.2m (external).
Cramped Conditions
Perhaps the biggest challenge for the project is that it has been taking place within a very tight space – the only available footprint within Oxford Street is just 330m2.
One space-gaining strategy has been to construct a fivestorey concrete and steel-reinforced frame building, replacing a bank that had stood on the site of No 1 Stratford Place, until LU bought and demolished it as part of the BSSU programme.
Only the frame of this over site development (OSD) building has been constructed above ground level, together with two levels of RC basement.
The top three floors currently house site offices, workshops, welfare, showers, laundry facilities and so on. Plant is housed on the roof, including large ventilation fans that force air into the tunnel works.
"We’re bringing in the freshest air we can find, well away from the Oxford Street traffic and pumping it through big ducts down into the works," said Richard Watts, LU Tunnels project manager.
The lower two ‘basement’ storeys encase the access, via two shafts, to the works below.
Spoil is also extracted from this point, leaving via Marylebone Lane. Again, the tight access has created immense challenges, with spoil removal organised on the basis of one lorry out, one lorry in, with up to 14 lorries leaving the site each day, each carrying 18.5t of London clay.
"At one time we were running three tunnel drives and two shafts through this one entry point, effectively running all the logistics in to a cul-de-sac," said Kevin McManus, Tunnel Construction Manager.
Acoustic cladding swathing the building has ensured the residents of a neighbouring five-star hotel haven’t been disturbed by the 24/7 tunnelling operations (there is no spoil removal at night).
Of course, the ground beneath Oxford Street is also already pretty crowded and as well as synchronising with the Central and Jubilee lines (which, critically, have remained operational) and Crossrail, the BSSU project has had to contend with ducking under the mothballed – but much revered – Royal Mail underground railway line and avoiding the equally iconic and somewhat more sensitive Bazalgette-designed Victorian sewer system and water mains. The Costain-Laing O’Rourke JV contractors protected 1.2km of the fragile cast iron pipes of the latter by installing plastic strengthening sleeves.
There have also been the sensitivities of the sovereign territory of two High Commissions to overcome – those of Botswana and Tanzania. Based at 3 Stratford Place, the latter is an immediate neighbour to the development site, which occupies 1 and 2 Stratford Place. Number 1 is now the OSD building, while the basement of number 2 (a Grade II-listed building bought by LU at the beginning of the upgrade process) has been excavated over a period of 18 months and a concrete box constructed, and is being transformed into the new ticket hall – all without disturbing the building above. This basement adjoins the High Commission’s visa application office.
As the ground beneath the High Commission is technically Tanzanian soil, and the enabling legislation of the Crossrail Act doesn’t apply to Tanzania, the BSSU team had to design a solution to go around, rather than through it. So now, an access tunnel, built to get the spoil out and the materials in to enable the escalator shaft to be driven down to the Jubilee Line, heads out parallel with Stratford Place for 35m before taking a right angle turn back under the development site.
The team is phlegmatic about the task of threading new tunnels through existing utilities and stakeholder interests.
"It’s just part of the challenge of working in these very tight urban environments," said Ashley. "On a job like this you are probably trying to maintain relationships with upwards of 300 individual stakeholders, protecting their interests as well as creating additional capacity and keeping the Tube moving."
And, he added, dialogue pays. "If we weren’t as careful with those relationships people could object and constrain us in ways that would be very sub-optimal but, generally, once we’ve worked with them there is a level of understanding that allows us to get the most efficient outcomes."
Aim small, miss small
Noise and dust pollution levels have been meticulously monitored, as has ground settlement, which has been mitigated by compensation grouting.
"Worst case scenario, if we’d just dug our tunnels and done nothing we would have had up to 70mm settlement in the middle [of the project area]," said Watts. "The fact is that the team has built the tunnels very carefully, combined with the compensation grouting has resulted in much lower figures than that.
"Ninety-three Tube a Manchette’s (TaMs) were drilled in a roughly 180 degree arc from just in front of 2 Stratford Place and picking up all the main buildings," said Watts. "There were quite a lot of areas we couldn’t get into because of existing basements, foundations, piles and so on, but the operation was very successful and was demobilised just after Easter.
"We have monitoring on the whole of Stratford Place and a number of other buildings around the area and the biggest movement we’ve had is 25mm."
Conventional 360 excavators have carried out most of the BSSU tunnelling work, including the escalator shaft – with roadheaders being used in some of the larger areas such as the construction adit.
The very close proximity to existing infrastructure and assets in certain locations, such as the connection with the Central Line, between the two Central Line tube tunnels has necessitated another conventional technique – steel framed square works.
In these areas the new tunnels are squeezed between the existing tunnels, which are exposed on both sides. "Where we went under the Central line we could hear the trains running above us so we had to keep a very close eye on the track and we virtually didn’t disturb it at all," said Watts.
"In two locations the tunnellers exposed the underside of the SGI running tunnels while the trains continued to run above."
All of the square works were dug by hand and, due to space restrictions all of the passageways between the two Central line platforms connecting to the future lift were excavated this way.
SCL vanguard
However, not all the techniques employed date back to the earliest days of tunnelling and sprayed concrete lining (SCL), brings the BSSU project bang up to date.
The SCL is reinforced with 38mm steel fibres and is sprayed on in one metre advances to a thickness of around 250mm tor form the primary lining, which provides temporary support for the tunnels. A sprayed waterproof membrane is then applied before the final structural lining, using the same fibre-reinforced concrete mix. The concrete is supplied in bulk in containers and the site has storage capacity of 250t – which sees a rapid turnover.
"We had an 80m3 pour at the weekend and by Monday we were down to 50t," said Watts.
"SCL has also proved to be a very low ground movement technique and that should give confidence not only to designers, but to stakeholders in the future," said McManus.
LU has been at the vanguard of SCL use in London. Ashley added, "These are amongst the first jobs where SCL has been used on London Underground, and they have been a forerunner for the Crossrail works.
Essentially we are building data through using this technique that will give stakeholders on future projects confidence that their buildings aren’t going to suffer."
Schedule
Primary excavation of the tunnels was completed in July and construction of the secondary lining is now proceeding. Fit out is scheduled to begin this September.
The much anticipated tunnel breakthrough connecting Bond Street and Crossrail is due to take place in late September 2015.
"Crossrail has completed the excavation of its tunnel linking their station with ours and we’re in the process of digging back the other way to join it, tunnelling over the Central line in the process," said Watts.
"The final link is S-shaped and will be built in SGI. It is a very complicated section, mainly because of the Central line overbridges and the sewer, which is 500mm above them. "The work is very slow and laborious and it will take us until September to complete it."
A Job well done
The BSSU team’s pride in this cramped and complex project is evident and its accomplishments are being recognised by the wider world. LU has become the first rail authority in Europe to achieve ISO 55001, the new international standard for asset management.
These LU schemes are providing close to the best return for peak hour capacity anywhere in the world," said Ashley. "The way we approach them, and apply sophisticated modelling, such as Legion modelling, gives us much more understanding of how to optimise these environments.
"Several other metro projects are very interested in how we employ that modelling to get the best out of the operation and the infrastructure we’re building"
