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23.03.17

Breathing life back into the Connaught Tunnel

Source: RTM Feb/Mar 17

Linda Miller, former Crossrail project manager, Connaught Tunnel, reflects on the challenges of widening and deepening the vital Victorian tunnel for the new Elizabeth Line.

Built in 1878, the Connaught Tunnel allowed the railway to be diverted under the Connaught passage, a water link which connected the Victoria and Albert Docks. 

Originally, the railway served the working docks in the area before it was used for passenger services. Disused since 2006, the tunnel will play a vital role in the operation of London’s newest railway, the Elizabeth Line, when it opens in December 2018. 

However, the original tunnel was too small for the new Elizabeth Line trains and so needed a significant amount of work to widen and deepen the structure. 

The original plan was to strengthen the central section of the tunnel that runs beneath the dock water. We were planning to remove the existing steel lining of the tunnel, backfill the entire section with concrete foam and then enlarge the tunnel using a standard tunnel boring machine (TBM). 

However, underwater surveys revealed that sections of the structure were in a very poor condition. In the 1930s, as ever-larger ships came into service, they began to scrape the bottom of the dock which sits above the top of the tunnel. As part of work to deepen the dock, the central section of the tunnel was narrowed, brickwork was removed and steel segments were installed to strengthen the tunnel. 

To our alarm, we realised that using a TBM could bring about a complete collapse of the tunnel and bring the dock water flooding in. This revelation about the weakness of the tunnel where it runs beneath the dock meant that our ‘Plan A’ was no longer feasible. We had to go back to the drawing board to find a different solution to the problem. 

After months of detailed work, our ‘Plan B’ was to place cofferdams in the Connaught Passage between the Victoria and Royal Albert Docks, pump out the water and create a dry construction site allowing workers to dig down and enlarge the tunnel in the open air. 

In theory, it sounds like a simple solution, but in practice there were a whole range of factors that made our job more difficult. 

Firstly, during World War II, more than 40,000 explosive devices were dropped on London, with the docks and rail lines particularly targeted due to their crucial role in delivering supplies to the British war effort. 

We had to conduct an extensive search of the wider construction area to identify any remaining undiscovered devices that failed to detonate on landing. The geology of the Royal Docks area meant that some devices that didn’t explode on landing sunk into the first few metres of soil. 

A team of highly-trained specialists used armoured vehicles with magnetic equipment to investigate the ground around the tunnel. Their work involved sending probes into the ground in three-metre intervals and analysing the results. 

Secondly, our worksite was a matter of metres away from the London City Airport runway. This caused significant restrictions on the work that could be carried out as we had to ensure that air safety and operations were maintained. 

Finally, we were very much working against the clock. The work to the central section of the tunnel had to be carried out between January and September 2013, between two national showcase events at the nearby ExCeL Centre, that needed the dock passage open for huge ships to pass through. Given that we had eight months to complete the work, it’s no exaggeration to say that every second really did count. 

A cofferdam the size of a football pitch was successfully installed, and 13 million litres of water drained from the dock passage. Once the dry worksite had been created, our team worked 24/7 to widen, deepen and strengthen the tunnel. Once the newly enlarged tunnel was watertight, the water was pumped back into the dock and the cofferdams finally removed. 

The feeling of relief, but also pride, was tangible once the final sheet pile of the cofferdam was removed, and a matter of days later, the first of the huge ships for the ExCeL trade fair passed through the dock. 

It’s fantastic to know that by breathing life back into this beautiful Victorian rail tunnel we’ve played our part in bringing it back into use more than 150 years since it was first built.

Tell us what you think – have your say below or email opinion@railtechnologymagazine.com 

Comments

Andrew Gwilt   23/03/2017 at 09:41

I know that the Elizabeth Line is to only go as far as Abbey Wood but there could be future extensions for the Elizabeth Line to extend to Kent such as Maidstone, Canterbury, Margate, Dover, Folkestone, Rochester, Gillingham, Sheerness, Faversham and other destinations in Kent via 3rd Rail as the Class 345's Aventras are dual voltage EMU's that could operate on DC 750v 3rd rail as well on AC 25kv OHL.

James Palma.   27/03/2017 at 20:19

Good god Andrew!! Dont sat that. I will be getting on it at the second station in and dont want people getting a seat before me!!! Its bad enough having people getting on the train at all. Pmsl

Acton   28/03/2017 at 11:50

The class 345 EMUs are not dual voltage so therefore cannot run beyond the wires at Abbey Wood.....

Andrew Gwilt   28/03/2017 at 23:34

So the Elizabeth Line Class 345's will operate on 25kv AC Overhead only with some of the London Overground Class 710's also to operate on AC 25kv Overhead on the Gospel Oak-Barking, Romford-Upminster and Lea Valley metro services between Liverpool Street-Cheshunt, Enfield Town and Chingford as the rest of the 710's will be dual voltage just like the Class 378's on the Watford-Euston Line and North London Line except the 378/2's are DC 750v 3rd Rail only used on the East London Line and South London Line.

Chris M   29/03/2017 at 03:24

So..... Andrew wants to send a London metro train with rock hard seats all the way from Abbey Wood to Folkstone without a power source. Imagine a three-hour journey stopping at all stations! Perhaps a combination of sails and solar panels could provide forward momentum? Lol! Returning quickly to the world of sanity, there are several videos on youtube showing in detail the work that Linda and her team carried out on the structure of the Connaught tunnel in a very short timescale. They can be proud of what they achieved under pressure, especially as the dense brickwork was very sturdy and it took some time to chip away..

Andrew Gwilt   02/04/2017 at 01:08

Ok. The Elizabeth Line (Southeast section) between Whitechapel and Abbey Wood will only go as far as Abbey Wood. As the Class 345's are to operate on 25kv Overhead only as Bombardier are currently building the Class 345's with some already been delivered to Ilford Depot for test runs between Ilford Depot and Shenfield and between Liverpool Street and Shenfield and Ilford Depot before the Class 345's are to operate on the Liverpool Street-Shenfield route to replace the Class 315's from May. And the Class 345's are formed as 7-cars before being extended to 9-cars on the other routes before the Central section between Liverpool Street and Paddington is opened and the Class 345's could extend to 10-car trains once the central section is fully operational.

Noblelox   29/04/2017 at 07:56

It's London, let throw lots of money at it. Sickening.

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