Crossrail – Creating a Lasting Legacy: Part 2.
Images coutesy of Crossrail. © Crossrail.
Work on the central tunnels began in May 2012. Eight tunnel boring machines (TBMs), each 100 metres long and managed by experienced teams, worked around the clock over a three-year period. Skilled engineers had to weave these machines through London’s subterranean labyrinth of existing train tunnels, sewage systems, utilities and other infrastructure – constantly monitoring London’s historic buildings above for signs of impact as a result of settlement.
Bechtel Project Manager Nisrine Chartouny was involved with one of the three teams responsible for all the tunnels, portals and shafts on the central section. “When we started the project, we saw tunnelling as being the riskiest activity,” she explains. “But we managed to build 42km of tunnels under some of the most expensive real estate in the world, without any real significant damage resulting from it.”
Where tunnels were too small, large or irregular for TBMs to create, Crossrail applied a mining technique known as sprayed concrete lining. This process involves spraying layer after layer of quick-dry concrete onto freshly excavated ground, making the tunnels watertight and highly durable. The method was used to create everything from platform tunnels to ventilation shafts throughout the central section of the route.
An astonishing 7 million tons of earth were excavated. And, in line with Crossrail’s emphasis on environmental sustainability, 99% of that went to beneficial re-use. For example, some 3 million tons of excavated material were transported by ship and used to create a new bird sanctuary at Wallasea Island in Essex.
Aerial view of Wallasea Island in Essex.
A journey through history
Besides negotiating tight spots through London’s dense underground infrastructure, Crossrail also had to carefully excavate and manage archaeological finds. Some of these were anticipated, such as 3,000 skeletons uncovered at what were the original burial grounds of Bethlehem Hospital. Others came as a surprise: a Tudor bowling ball, several prehistoric animal remains – even what was part of the original West Ham football complex.
From tunnels to train stations
Of course, creating the tunnels was only the start. The next step involved laying down the concrete on which to lay track, and installing electrical and mechanical systems, lighting and cabling. Because the tunnels themselves are unique, Crossrail commissioned a fleet of custom machines with which to fit them out. These included a giant, 465-metre long concreting train, four gantries for laying down track, a drilling rig that placed over 250,000 holes in the tunnel lining, and a shuttle for transporting concrete along the tunnels.
Even as the rail tunnels were being constructed, huge excavated caverns were being transformed into light, modern stations – each one sharing the same design features while also reflecting the character of its local neighbourhood. The dark cinematic design of Tottenham Court Road’s western ticket hall, meanwhile, reflects the contrasting Soho neighbourhood directly above it.
Breathing new life into old structures
Of course not all of Crossrail’s tunnels are new. A major feature of the project involved re-commissioning existing infrastructure, such as the disused Connaught Tunnel in the city’s East. Built in 1878, this beautiful piece of Victorian engineering was robust and relatively intact – but too small to fit the new, larger Crossrail trains. Enlarging it was no small feat. Above ground, a cofferdam was installed and 13 million litres of water then drained from the South Docks so that engineers could enter from above and widen the tunnel crown. The entire structure was deepened, widened and reinforced to ensure it lasts for the entirety of the route’s 120-year design life.
Linda Miller was Bechtel’s Project Manager for the Connaught Tunnel upgrade. She described this three-year endeavour as “a job I will never forget’’, citing the quality of the people involved and the collaborative team spirit as stand-out aspects. This collaboration enabled more effective working – and a safe environment. “Safety doesn't just happen on complex construction sites” she explains, “it needs vigilant work each day. Having a well-bonded team is one of the strongest starting points for ensuring that safety culture.”