Crossrail contract C315 is very unusual for a major new transport scheme such as Crossrail. It doesn’t involve major new works, like the other parts of this project, rather it is nearly all about the refurbishment of Victorian infrastructure dating back over 130 years.
Connaught Tunnel is situated on the former North Woolwich branch of the North London railway and runs under the Royal Docks. It is to be refurbished to form an essential component of the Crossrail Abbey Wood branch, the re-use of existing infrastructure being rightly seen as a positive move. Given the complexity of the task as it is now understood, it would be interesting to see how the costs work out in the end compared with the likely cost of a new tunnel.
Back in time
Let’s get back to the history of the structure before delving deeper into the current project. The original North London line branch to North Woolwich arrived in East London in the middle of the nineteenth century and, amongst other uses, served the developing docks that became the Royal Docks. These docks became a challenge to the railway’s existence, because they were a huge success and needed ever more room to expand.
This expansion led to the creation of three huge new docks between 1855 and 1921, the Royal Albert, Royal Victoria and King George V docks, creating what was then the world’s largest enclosed dock system. The railway crossed the line of the new docks between the Victoria and Albert basins. Originally, a swing bridge was planned to allow a navigation channel under it, to enable ships to travel between the docks. This idea rapidly became unacceptable because the shipping traffic grew so much that the railway would have been continually interrupted by the need to open the bridge for ships.
In consequence, it was decided to re-route the line through a new tunnel beneath the dock channel instead, and this became Connaught Tunnel. It was constructed by cut and cover, lined in brick and it opened in 1878. It was about one kilometre long, with a central twin bore section about 550 metres in length and single, twin track bores at either end. Approach cuttings were built at each end, with brick retaining walls and brick arch struts overhead at regular intervals along them. A brick arch invert ran throughout the cuttings as well as the tunnel sections, and the whole was drained by means of six foot drains. These drains ran to the bottom of a pump shaft situated to the north of the docks and to the east of the tunnel. An unusual hexagonal pump house was built in brick at the head of the shaft to house the pumps and associated equipment.
Continuing expansion of shipping movements at the London docks caused further problems however. By 1935, the size, and more importantly the draft, of ships had increased to the extent that the extrados of the tunnel arch was being struck by them, with damage caused to both. To stop this, it was necessary to deepen the docks, and in particular to lower the tunnel profile. To this end some 100m of each of the single bore tunnel sections was reconstructed to a smaller gauge. This was still adequate to allow clearance for the trains then in use, but left the tunnel sufficiently reduced in height to permit the required deeper draft in the docks above.
Early use of steel
The reconstruction of the tunnel was achieved by quite unusual means as it involved the use of cast steel linings. Cast iron is commonplace in such tunnelling applications, but not, at that time, cast steel. Furthermore, the bolted, egg shaped linings were welded to one and other, presumably to ensure that they were watertight.
After this, the tunnel had a fairly uneventful life until World War II, when it was badly damaged by bombing in September 1940. This necessitated a repair roughly 10 metres long by 5 metres high in the upper tunnel haunch on the east side of the arch in the section north of the docks. This repair is still readily apparent today.
In the 1960s, the advent of containerisation and the poor industrial relations in the docks led to a rapid decline in trade in the Royal Docks. The waterways are now used for water sports and exhibitions rather than shipping. The tunnel finally ceased to carry rail traffic in 2006, when that section of the North London Line was taken out of use.
Crossrail contract 315 takes over all of this history with the aim of turning it into twenty-first century rail infrastructure. The site lies between the proposed Custom House Station and the North Woolwich portal of the intended Thames tunnel. About half is in tunnel and the remainder in cutting or above ground.
Led by Project Manager Linda Miller, who has over 20 years experience with Bechtel, the Crossrail contract team has a complex job to deal with. This goes beyond what will be apparent from the history already described as, in addition to all that, there is London City Airport to the south east of the site, with an apron having fuel bunkers below it over part of the tunnel. The Excel exhibition centre is close by to the west, alongside the docks, and the docks themselves play host to internationally significant water events such as the London Boat Show.
The principal contractor on the team is Vinci UK’s civil engineering division, Taylor Woodrow, whose team is led by Project Manager Richard Wall-Morris. Linda and Richard between them are managing works on the surface, in the tunnel and underwater.
Because of the WWII bombing of the area, one of the first tasks was to carry out a survey to check for unexploded ordnance (UXO). This was assisted by the existence of records from the bomb spotters who, during the war, tried to keep track of where bombs had fallen and plot them on maps of the city. These gave some clues as to where bombs were most likely to be found, but it was still necessary to survey the site thoroughly using modern techniques. An armoured truck was supplied by Fugro and operated by EOD Contracts, and it probed the site with a 35mm diameter magnetic probe to a depth of 8m. The probe has a 3m range of sensitivity and was deployed in a grid pattern to ensure full site coverage. All areas of permanent or temporary intrusive work had to be checked. Fortunately no UXOs were discovered.
Another task that started early was the archaeological exploration of those areas considered to be of interest. This investigation involved the archaeological excavation of four sites on the contract as it is now known that there was significant prehistory to the site. A Roman road ran through the area of the railway route, and that is believed to have been constructed on the line of a prehistoric pathway. Three of these digs have been completed but the fourth has been interrupted by the discovery of a 60kV buried cable.
Divers supplied by Abwood removed silt from the dock floor and carried out engineering surveys of the underwater area of the site. Given the location, it is perhaps not surprising to hear that the “silt” that was removed included a number of submerged cars!
The removal of silt was critical to a second form of surveying that was employed in the water. A specially equipped boat, “The Galloper”, owned and operated by the Port of London Authority (PLA), carried out underwater sonar surveying of the channel above the tunnel. The “Multi-beam” system fitted to the boat deploys an array of 512 ultrasonic beams, giving coverage of the entire underwater zone from horizontally to port through vertically below to horizontally to starboard of the boat. Scanning in this way, as the boat travels along, gives the data to build up a complete picture of the underwater zone surveyed. This method revealed that the cover to the tunnel was substantially less than previously thought, in some cases being as little as 250mm. In addition, the dock floor was shown to be far more badly damaged than expected.
These survey findings led to the conclusion that the risk of catastrophic inundation of the tunnel was high, and the method of reconstructing and enlarging the tunnel needed to be reconsidered. Crossrail had intended to strengthen the central tunnel section by removing the steel linings and backfilling the tunnel bores with foamed concrete. They would then have bored through the infilled section to create new tunnels of the required larger cross section to accommodate the Crossrail trains. That method was considered to be excessively risky given the conditions found by the survey.
After consultation and discussion, a new methodology was agreed, whereby the tunnel will be exposed from above for the first time since its original construction in the 1870s. This will be achieved by construction of two cofferdams, one each side of the tunnel, across the 30 metres wide by 9 metres deep dock channel above. These will seal out the water of the docks each side, enabling the space between to be de-watered and kept dry. The old tunnels below will be removed by excavation within the space between the cofferdams, and the required larger tunnels will be built in their place. Thus, in the end, the tunnels will be renewed by the same cut and cover method as was used in their original construction. Since there are two swing bridges over the dock channel today, one for pedestrians, the other the A112, conditions for the work will be quite challenging.
That stage of the works will not be starting for some time, however. Obviously the cofferdams will preclude any boat movements between the Victoria and Royal Albert docks. This has important implications for a number of stakeholders in the docks area, in that it would interfere with events involving boats and water sports, for example. Crossrail has consulted about this with the Royal Docks Management Authority and other stakeholders, and has agreed not to start these works until after the 2013 London Boat Show and to complete the work and remove the cofferdams within 6 months of commencing.
As well as improving the gauge of the constricted centre section of the tunnel in this way, it is equally necessary to improve the remainder of the tunnel and the cuttings on either approach. A proper repair to the bomb damaged section of tunnel is part of this work, as is the provision of the extra clearance required by the new trains.
At one time it was thought that the clearance work would include removing the brick arch struts from the cutting retaining walls. However, it was decided that it would be preferable to keep these, gaining the extra headroom needed by lowering the inverts in both the tunnel and cutting instead. The drainage needed improving for a modern, intensively used railway in any case, so it was always the intention to replace the old six foot drain with a modern alternative. Invert lowering was thus not going to add to the drainage task.
Too much water
However, improving the drainage is going to be quite a job in itself. There are two aquifers separated by a clay layer in the strata above the tunnel, and the upper one is heavily charged with water. The ground is very wet around the structure, and the head of water is quite high. Specialist sub-contractor WJ Groundwater is in the process of lowering the groundwater levels under and around the site by pumping, but, despite that, things are still quite wet in the tunnel in places. The final solution involves Bachy-Soletanche grouting the ground around the tunnel using the “tube a manchette” technique to reduce the head and cut off as much as possible of the inflow.
Any remaining water ingress will be dealt with by a new drainage system. The drainage pump shaft is to be deepened by some seven metres to a final depth of 25 metres by installing a segmental concrete extension beneath the existing structure. The brick shaft will be cleaned and repaired, and modern pumping equipment will be installed. Water will drain to the base of the shaft through a new tunnel and cutting drainage system, connected to the shaft by micro-tunnelling.
Ground improvement is required in many areas of the site as much of the area is a swamp, according to Linda. Improvement is being achieved by installing “controlled modulus columns”, a displacement technique that is an alternative to piling and which is cheaper, quicker and produces less spoil than traditional methods. This work is being undertaken by Ménard, who will be installing some 3,000 columns. The technique is particularly attractive to neighbours of the site due to its low levels of noise and vibration, and the vibration reduction makes it friendlier to nearby utility infrastructure.
All of the brick structures that will be retained are to be cleaned to remove the soot and dirt of years, and will be repaired where necessary to bring them up to prime condition for their new lease of life.
The potential for disturbing the tunnel and approach cuttings is high, given the groundwater lowering and invert removal, for example. Consequently, the project team is monitoring the structures constantly by means of automated systems that will trigger alarms if significant movements are detected. Included are automatic total stations at regular intervals through the site, each monitoring a series of targets placed around the structures at appropriate points.
The pretty little Victorian Head House is to be removed from the top of the shaft, since it is too small to accommodate the equipment required for the new system. It will not be lost to posterity though. It is to be removed carefully in a way that will allow its re-erection elsewhere, and is to be donated to Newham Borough. Linda Miller understands that they intend to pass it on to the SS Robin Trust, an organisation seeking to preserve the ship “SS Robin” in a berth in the Royal Docks. The ship is one of the oldest surviving steamships in the world, and was built in East London. The pump house may form a ticket office at the quayside beside the restored ship if the Trust succeeds with its aims.
Other works include demolitions, particularly the removal of the old Silvertown Station which has been redundant since the old rail line closed in 2006. Cast in-situ concrete parapet walls along the tops of the retaining walls to the approach cuttings have to be removed and replaced with new equivalents, as they are not considered adequate for modern needs in structural or aesthetic terms.
In all, this is a fascinating and complex project, and it is easy to understand why Linda Miller particularly asked to be appointed to run this job.