Post Office rail tunnel turned into high-tech sensor lab

Installing underground infrastructure in cities could become cheaper and faster thanks to technology being tested in part of the old London Post Office railway.


Cambridge University engineers have turned a section of the capital’s former underground delivery line into a “smart tunnel” laboratory for trialling a range of new sensors that measure disturbances coming from nearby construction works.

The tunnel runs very close to developments for the new Crossrail underground railway and the new technology will enable the researchers to study how existing infrastructure can move and come under strain from other construction.

Being able to better predict what impact a new tunnel would have could make the construction process much more efficient, said PhD student Mehdi Alhaddad, a researcher at the Cambridge Centre for Smart Infrastructure and Construction (CISC).

An array of sensors is being used to monitor movement, acceleration, tilt, temperature and humidity in the tunnel.

‘When you design a tunnel, you’re conservative because of all the unpredictable things you can’t measure when you excavate,’ he told The Engineer.

‘But if you could measure how much the existing tunnels will move, how much the buildings on top will move with high level of confidence, then you could be more efficient in construction, you could be quicker, use less material, loosen the safety procedures and that would save huge amounts of money.’

The Post Office tunnel, which was used for 75 years to transport letters across London and a part of which is now due to be opened to the public, is just over 2.5m in diameter, while the Crossrail tunnel is 11m wide and the two are just 20cm apart in certain places.

The sensors have been designed to use minimal power so can be left in place for potentially years before the batteries run out.

Like most existing London Underground tunnels, the Post Office subway is made of cast iron, which can become brittle over time, and movement of just 1cm could cause it to crack, said Alhaddad.

The researchers say the four new sensors being trialled represent a lower cost monitoring system than existing technology.

Optical fibre installed along the length of the tunnel can show if it deforms or bends, while wireless displacement transducers measure displacement of one part of the tunnel relative to the next and transmit the data to a receiving station.

The Post Office railway tunnel is in places just 20cm from the new Crossrail tunnel works.

PhD student Heba Bevan used her experience working for microchip designers ARM to develop an electronics architecture and manufacturing techniques for an ultra low-power sensor that measure temperature, humidity, acceleration and tilt.

The 20g sensor’s long lifetime makes it ideal for underground monitoring, said Bevan. ‘It uses a 220 miliamp battery and has so far lasted 14 months. The current sensor is the size of a brick and uses a 19 amp battery that needs changing every two months.’

Part of its low-power operation is down to software that allows it to shut down much of its functionality until it detects movement, which it then records and transmits wirelessly – rather than constantly making and sending data.

Better monitoring of existing tunnels could help improve the design of new ones.

The final sensor uses photogrammetry, or computer vision, to visually detect movements as small as 0.1mm in the tunnel, using off-the-shelf digital camera equipment much cheaper than conventional technology, which can cost tens of thousands of pounds.

Instead of firing a laser between a number of prisms set along the tunnel to detect movement, the new sensor uses algorithms to calculate movement as captured in camera images. Its low cost also means it can be deployed in large numbers and collect data about more of the tunnel.

The trial has been underway for over a year and is due to carry on for at least six more months. Alhaddad said he couldn’t yet reveal what the findings were but said the results were meaningful.

The sensors’ low-cost means they can be deployed more widely than existing technology.

‘This will change the way we look at tunnels when they are going to be affected by construction,’ he said. ‘I think it will change not only the design but also the monitoring.’