London, UK, has embarked on what is described as a "one in one-hundred-and-fifty-year project." The maneuvering of a crane into place at Blackfriars on the River Thames marks the start of major work on the Thames Tideway Tunnel, a "super sewer" that will drastically reduce sewage overflow into the river.

Large parts of London's sewer system were constructed over 150 years ago by Sir Joseph Bazalgette. Although the brick-constructed tunnels are said to still be structurally sound, they were not built to manage the capacity that is required today.

In the 1860s, during the construction of Bazalgette's sewer system, London had a population of around two million. Today, the population is over eight million. Not only is more sewage produced and more water used, but there is also less green space to soak up rainwater.

Bazalgette's combined sewer system was designed to carry both foul-water and rainwater away to be treated at sewage works around London. As part of of the system, he built in Combined Sewage Overflow (CSO) points on the banks of the river into the River Thames through which overflow runoff could be discharged in the event of the system becoming over capacity.

While they were designed to stop streets and homes flooding with sewage on the rare occasions it was necessary, the CSOs are now opened on average once a week and over 39,500,000 cu m (1,400,000,000 cu ft) of sewage is emptied into the River Thames as a result each year. The Thames Tideway Tunnel is aimed at reducing this output to around 2,400,000 cu m (84,800,000 cu ft).

The £4.2 billion (US$6.3 bn), seven-year project project will see a 25-km (16-mi) tunnel constructed that will run west to east from Acton to Abbey Mills, generally following the route of the Thames. From there, it will be connected to the Lee Tunnel, which is already under construction and will transfer the sewage to Beckton Sewage Treatment Works.

This scale of the required construction and tunneling is comparable to that of the recently completed Crossrail tunnels, although those are longer still. The Thames Tideway Tunnel will be 7 m (23 ft) in diameter and will be built between 25 m (82 ft) and 65 m (213 ft) below ground.

There are 50 CSOs along the Thames in total and the new tunnel will link the 34 most polluting outlets, siphoning away sewage that would otherwise have been directed into the river. The system will also act as a huge storage tank, capable of containing up to 1,600,000 cu m (56,500,000 cu ft) of sewage until it can be treated.

There will be 24 sites across London during the construction of the tunnel, with three main "drive sites" where tunnel boring machines (TBMs) are lowered into the ground. Preparation of the sites will include setting up offices and barge facilities, after which a vertical shaft will be dug from which the tunneling will start.

Tunneling will be undertaken 24 hours a day and will be taking place at several different sites at any one time. Much like those used for the construction of the Crossrail tunnels, the TBMs will both excavate and lay concrete segments around the freshly dug tunnel as they go. A concrete coating will be added to complete the process.

The tunnel is designed to get a meter (3.3 ft) deeper for every 790 m (2,592 ft) that it travels, so as to allow the sewage to flow naturally from one end to the other. In addition, it must dodge other existing tunnels underneath London, such as those for the Underground train lines. Excavated material will be transported out of the tunnels, processed and loaded onto barges for removal.

The crane at Blackfriars is being used to construct a new pier for Thames Clipper users, which is being moved to accommodate the tunnel construction. Despite only being of significance because it signals that start of major work, getting the crane into position was described as "no mean feat."

"The 20-m (66-ft) wide barge had to be carefully threaded under seven bridges and through the traffic on one of the busiest working rivers in the world," explains central project delivery manager for Tideway Andy Alder. "Alongside the man-made challenges, the crane also needed perfect tidal water levels and good weather before the movement could go ahead."

Tunneling activity for the Thames Tideway Tunnel is expected to begin in 2017 and to be completed by 2021. The tunnel is scheduled to be fully complete by 2023.

The video below provides an overview of the Thames Tideway Tunnel project.

View gallery - 8 images