The latest in Fircroft’s ‘engineering feat of the month’ series takes a look at Crossrail- an epic civil engineering project that is tunnelling deep beneath the streets of the UK’s capital city.
Construction on the Crossrail project began in 2009 at Canary Wharf with a plan to create 10 new stations and upgrade 30 more by integrating new and existing infrastructure.
At this point the project is 75% complete, being delivered on time and within the funding. With a £14.8 billion investment budget, 118 km of railway lines and 42 km of new tunnels the project is due to start running services towards the end of 2018 and be fully open and functioning in late 2019.
The beginning of the project started with the naming of eight Tunnel Boring Machines (TBM). As part of tunnelling tradition, a TBM must have a name before starting work and all the names were submitted by members of the public. Once Ada, Phyllis, Victoria, Elizabeth, Mary, Sophia, Jessica and Ellie were officially named, they could begin tunnelling in their pairs for 24 hours a day, 7 days a week with a team of 20 people operating in 12 hour shifts.
The tunnelling phase of the project had to take into account the geology of the material underground – particularly waterlogged ground underneath the Thames – as well as the buildings above ground. This meant the opening phase was hazardous and open to possible errors. Using hydraulic cylinders that were pressed against the tunnel face to cut and scrape away and form the tunnel, the TBM’s moved between 90 – 150 metres per week with an accuracy of within the nearest millimetre. Engineers on board the TBM’s had to continuously monitor the pressure sensors that check the turning power of the cutting wheel and screw conveyor as well as the excavated material which is taken away on conveyor belts – the use of which we’ll come onto later.
Once the tunnelling phase is complete, the TBM’s take on their next significant role in the Crossrail project – the ring building phase. The pre-built reinforced concrete segments that make up the ring are built above ground and transported into the tunnel on rail carts. A vacuum is used to lift the concrete segments into place whilst the hydraulic cylinders are momentarily retracted to allow the segments to be fitted.
The importance of the accuracy of the ring segments comes into realisation here, as the vacuum places the concrete parts with millimetre accuracy.
As with any large scale projects such as Crossrail, challenges were to be expected. And when tunnelling as deep as 42 metres beneath a capital steeped in history – there was sure to be a surprise or two! Or even 10,000 – that’s how many archaeological objects were unearthed during the excavation process, ranging from medieval floor tiles to Roman period horse skulls and shipyard chains.
The most substantial excavation took place at Liverpool Street, where over 3,000 skeletons were uncovered including a mass burial site for victims of The Great Plague of 1665. The majority of the human remains were excavated from the ‘Bedlam’ psychiatric hospital where a team of 60 archaeologists worked on uncovering the remains from the infamous burial ground. This phase of the excavation included working with local volunteers who used parish records to assist in creating a database of names which contributed to the first comprehensive (but not exhaustive) list of people buried at Bedlam.
From the 7 million tonnes of material excavated as part of the tunnelling process, 98% of which will be re-used. Notably, 3 million tonnes of material was transported by road and river to Wallasea Island, Essex. Crossrail worked in partnership with the Royal Society for the Protection of Birds (RSPB) to restore the land back into its original habitat of mud flats, lagoons and salt pastures to home tens of thousands of birds. Using materials that lay undisturbed for 55 million years.
The size of the Wallasea Island site is 2.5 times of the size of City of London, with 1,500 ships transporting 2,000 tonnes of material as part of this unique partnership with the RSPB and the construction industry.
The Crossrail project is expected to increase central London rail capacity by 10%. Connecting London and the South East with an estimated 200 million passengers per year, bringing an estimated £42 billion to the UK economy.
The new 200-metre-long trains, will allow space for 1,500 passengers and reduced journey times. The journey from London Heathrow to the City of London will fall from 55 to 34 minutes, making the Crossrail project a revolutionary addition to London and South East transport links.
Crossrail is just the beginning of the rail development overhaul, with potential plans gaining Government support for ‘Crossrail 2’. The second phase aims to reach further to connect London and the South West and alleviate growing concerns about the increasing London population.
Image source: http://www.crossrail.co.uk