The Silvertown Tunnel, which crosses the Thames to link east London with the Greenwich Peninsula, opened to traffic on 7 April, on schedule and within budget.
The Silvertown Tunnel, which crosses the Thames to link east London with the Greenwich Peninsula, opened to traffic on 7 April. The Times reports that the successful construction process of the first Thames tunnel in 50 years was delivered on schedule and within budget.
The Silvertown Tunnel took just under five years to complete, and it has been constructed to ease congestion on the Victorian-era Blackwall Tunnel, which is frequently closed and a source of bottlenecks several miles long. This is a source of air pollution, and also increases journey times and congestion in other areas as drivers seek new routes.
The new tunnel is 1.4km long and has two lanes in each direction, with dedicated zero-emissions bus lanes. This will open up more opportunities for crossing the Thames on public transport, which will give the population wider access to jobs and services. The faster journey times will also benefit businesses and the economy.
How the construction took place
The project was managed by the Riverlinx consortium, and the majority of the funding was raised by private finance. Transport for London will manage the tunnel, and the crossing charges will be used to repay costs and to finance the maintenance and operation of the tunnel.
The tunnel boring process made use of one boring machine (named Jill, after Jill Viner, London’s first female bus driver). Over 1,860,000 tonnes of earth were excavated from the site, and transported by river rather than road to reduce road congestion in the area. The first tunnel took about four months to complete, progressing at about 22 metres per day.
The concrete segments used to create the tunnel lining were put in place by the boring machine directly after the earth had been excavated, which means that the tunnel was immediately stabilised. This was particularly beneficial as the ground under the Thames is formed of an unstable mixture of rock and soil, making it challenging to work with.
Furthermore, the standard approach to create the second tunnel would have been to dismantle the existing boring machine or assemble a new one, and both of these processes would have been very costly and time consuming.
The engineers working on the Silvertown Tunnel instead used a rotation chamber at Greenwich to turn the boring machine 180 degrees, so it was facing the right way to complete the second tunnel. This was the first time this innovative technique has been used in the UK.
The passages between the tunnels (necessary for safety and maintenance purposes) were created with another innovative technology known as ground freezing. The ground was too unstable to support a regular tunneling process, so pipes were inserted and a coolant applied around them to freeze the soil solid, so it was able to withstand tunnelling.