A historic tunnel in central London has been equipped with an array of the latest sensing technology to assess the impact of newer construction work on pre-existing infrastructure.

Engineers from the University of Cambridge are using a historic tunnel dating back to the early 20th century to monitor the construction of a new Crossrail tunnel running directly beneath it, in the hopes of better preserving historic infrastructure.

The Royal Mail tunnel, situated 25 metres below the streets of central London and measuring 2.5 metres in diameter, was originally used to convey mail across the UK capital during the period from 1927 to 2003.

The location of a huge new Crossrail tunnel measuring 11 metres in diameter happens to pass very near to the Royal Mail Tunnel around the vicinity of Liverpool Street Station, running a few metres beneath and parallel to it for a distance of over 100 metres.

This marks the first time in London’s history that two tunnels have been built so near to each other, making the obsolete structure an ideal site for monitoring and assessing the construction and excavation of the new structure.

Engineers from the Centre for Smart Infrastructure and Construction (CSIC) at the University of Cambridge have installed hundreds of sophisticated, low-cost sensors along some 30 metres of the Royal Mail Tunnel where it runs parallel to the Crossrail site. This will allow them to measure and assess the impact of construction on the older structure.

Four different low-cost sensors are being used in tandem, enabling the researchers to detect movements as small as a hundredth of a millimetre. The data collected will provide a comprehensive account of the state of the whole tunnel as opposed to just an isolated site.

The technologies include optical fibre running along the length of the tunnel to measure deformation and bending wireless displacement transducers which measure the displacement of parts of the tunnel relative to others; photogrammetry techniques which permit the digital visualisation of changes to the tunnel as a whole; and sensors which measure temperature, humidity, acceleration and tilt, which consume minimal amounts of power and can be left operating independently for years at a time.

The sensing technology being used marks a big advance upon current monitoring techniques, particularly with respect to their low cost, ease of installation and power consumption.

“It is the first time so many of our revolutionary devices have been used to monitor the movement of an existing tunnel,” said CSIC director, Dr. Jennifer Schooling. “It will also mean that we will see what effect such a large-scale excavation will have on a cast iron tunnel for the first time, almost in real time.”

The researchers hope the highly sensitive monitoring will enable them to answer a number of critical questions in relation to tunnel engineering, including how much movement is occurring as a result of the excavation and construction, the form that this movement is taking and whether it can be kept within acceptable limits in order to avoid compromising pre-existing infrastructure.

“Together, the sensors paint an incredibly accurate and detailed picture of how the older tunnel is behaving, which will inform the best way to protect and maintain it,” said Cambridge PhD student Mehdi Alhaddad.

Alhaddad points out that the technology has tremendous potential for monitoring the impact of construction work on older infrastructure dating from the 20th century and earlier, such as the many miles of cast iron tunnelling which comprise the London Underground.