The UK’s flagship railway project has completed the 3 year, 26 mile tunnel excavation beneath the streets of London on schedule.
The last of the 18 tunnels at Farringdon Station was completed on the 4th June 2015 by one of Crossrail’s immense 980 tonne cutting machines, named Victoria. The fleet of eight £10 million machines carved 6.2 metre wide tunnels under historical landmarks and passed within mere centimetres of existing tunnels in a truly remarkable feat of planning and engineering.
The tunnelling machine Victoria - Image courtesy of Crossrail
Farringdon will become the hub that links the London Underground and Thamselink making it, when it comes online in 2018, one of the busiest stations in the country.
In addition to the 26 miles of tunnels, an additional 8.4 miles of passenger, platform and services tunnels have been constructed using a technique called sprayed concrete lining. Essentially a special mix of concrete is applied to the substrate using compressed air. The process is very quick and ideal of high strength, high durability structures.
The 148 metre long TBMs not only excavate but extract the rock and soil in order to maintain a consistent pressure at the cutting face. Were that not impressive enough, these sophisticated machines uses powerful hydraulic arms to position precast concrete wall sections into position to line the tunnel. It then uses that ring as purchase to drive itself forward.
A completed tunnel - Image courtesy of Crossrail
Two-hundred and fifty-thousand concrete sections were used to line the 26 miles of tunnels, the human crew adding bolts and bleed valves as the behemoth machines lumbered forward, covering approximately 100 metres a week.
The scale of the project goes far beyond the excavators. Crossrail employed a sophisticated laser-based monitoring system that spread over the city like a net to watch for settlement. Settlement is essentially a shifting of the ground in response to the excavation beneath which can cause subsidence in buildings. The response is compensation grouting. Lines spread across the city from central grout shafts points like spider webs. Using the detection network, Crossrail could respond to the slightest shift directing teams pump compensation grouting along the required line to firm up the ground.
The £14.8 billion railway project is a staggering example of project planning, team work, logistics and technology all working in perfect harmony which will add 10% to London’s rail capacity. This may not seem like much but that 10% represents 200 million passengers a year. That’s the entire population of the United Kingdom travelling through the city three times each.
With the tunnelling complete the construction firm Bechtel will begin fitting out the stations and tunnels with everything required for Crossrail to operate in 2018.
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