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The Tunnel Masters: Digging the impossible

Creating tunnel developments is an extremely complex industry, but one that is continuing to grow. As cities run out of space above ground, the only place to build new infrastructure is underground.

Creating a tunnel might seem a basic process to many, but at Anderselite, we understand the huge engineering challenge it creates - often new solutions have to be developed for specific issues on sites. Developments over the past few years have enabled more and more difficult projects to take place and that’s why we wanted to sit back, take stock and look at two of the most intricate tunnelling projects of recent years: the Big Dig in Boston and London's Crossrail scheme.

The Big Dig

If you’re unfamiliar with the Big Dig, it consisted of a series of road tunnels which now connect South Boston and Logan Airport. The aim of the project was to relieve congestion above ground and to make it easier to get around the city.

The Ted Williams Tunnel was completed in 1995 and was one of the most complex within the project. The tunnel needed to be created through an extremely deep harbour, so they used the cut-and-cover method of construction. This involved constructing 40ft-diameter steel tubs off-site. They were then floated to the site, filled with water and lowered into the trench. Once they were securely anchored in place, the water was pumped away and the pieces of the tunnel were connected together.

Further along the road, there was another difficult tunnel to construct. This section needed to go through the Fort Point Channel, which is 400ft wide, creating an enormous challenge for the engineers involved. They were unable to use the same steel tubes as they did for the Ted Williams Tunnel as they couldn't float them under some of the bridges along the river.

This meant that they had to build a dry dock by the side of the site in order to manufacture concrete tunnel sections. It was the first time this particular method of tunnel construction had been used in the US! The tunnels were sealed to ensure they were watertight and the dock was flooded to float them out. When it finally came to lowering the sections into place, engineers faced another problem. Just below where the tunnel would run was a subway line, which needed to be protected. As a result, the tunnel sections were supported on columns to keep them from damaging the subway.

London's Crossrail Project

This huge compound of 26 miles (42km) of tunnels through central London is Europe's largest civil-engineering project. Not only are there a large number of tunnels to construct, but engineers also have to deal with London's underground network. This includes Tube lines, the oldest of which is 149 years old, as well as sewers, utilities and underground rivers. The closest that the new tunnels will get to any existing infrastructure is less than one metre, which is a huge feat when you consider the size of the tunnel boring machines.

In total, there will be eight tunnel-boring machines in operation and 40 construction sites are needed to run the project. The machines have been custom-made to create exactly the right size tunnels and work within specific conditions. They each weigh 1,000 tonnes and are 150 metres long. The machines cut into various materials, remove the debris and create a sealed concrete tunnel as they move along the network.

They have to cut through different ground environments, so two types of machines are used. Earth Pressure Balance Machines will be used for clay, sand and gravels and a Slurry Machine in clay surroundings. These machines are running almost continuously, 24 hours a day, 7 days a week, 365 days a year, in order to reduce the risk of settlement.

The project has been designed to be as sustainable as possible and one of the aims was to use almost 100% of the debris. Once the machine has collected the material, it's transported to the site of a new nature reserve.

The Crossrail scheme has also had to deal with archaeological issues, including the discovery of the Bedlam Burial Ground. Here experts are working to excavate 4,000 skeletons which date back to between the 1600s and 1800s. The project will also include the refurbishment of the Connaught Tunnel, which is 135 years old.

Whether the tunnel runs under buildings, water or other natural structures, it requires complex planning and engineering methods. Without the technical knowledge of these experts, many of today's most famous tunnels wouldn't have reached the construction phase. That’s what we love about construction here at Anderselite - not only is it inventive, innovative and forever-evolving - it’s also tenacious and never gives in to a challenge!

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