Gas Detection in Construction of Crossrail Tunnels

Mar 21, 2018

Gas Detection in Construction of Crossrail Tunnels

by | Mar 21, 2018


A network of rail tunnels was built to carry Crossrail’s trains in North London. Each tunnel is 21 kilometres/13 miles long, and up to 40 metres below ground.

Crossrail’s tunnelling construction took over 3 years to complete. Eight 1,000-tonne tunnel boring machines (TBMs) bored 42km of new rail tunnels. The machines were operated by “tunnel gangs” of 20 people, working in shifts around the clock to construct the tunnels.


During the construction of the 6.2m diameter tunnels, a number of vehicles were used to assist with moving materials into place. The vehicles’ exhaust fumes posed a threat to the safety of workers as the application is classed as a confined space.

As stipulated by the Health & Safety Executive (HSE) and the BTS (British Tunnelling Society) continuous gas detection is required to ensure workers’ safety at all times.

They have specified the requirement to monitor Oxygen (O2), Nitrogen Dioxide (NO2), Carbon Monoxide (CO), and Nitrogen Oxide (NO), and to include data logging & reporting software.


a1-cbiss known for their complete project management of gas detection systems in construction projects were commissioned for the project.  to work alongside a leading integrated support services company to design, install, commission, service and maintain a new fixed gas detection system.

The gas detection system was installed by a1-cbiss inside two rail tunnels during the construction phase.

The gas detection panels are set to alarm at 19.5%, 25ppm, 30ppm, and 5ppm respectively. Diesel engines used by MEWPs (mobile elevating work platforms) are to be used within the tunnels which will emit CO, NO, and NO2.

To help flush out contaminants, the two tunnels are constantly fed with fresh air by four 1600mm, 60kW axial flow fans. It was necessary to install six detection points, three in each tunnel, one in the middle of each tunnel and two at either end.


a1-cbiss supplied a gas detection system to continuously monitor for all four gases and log their readings every 60 seconds. The PLC controller and data logger operator are able to easily download the data on a weekly basis via a USB key and download the data to their laptop.

The GRP enclosure used to house the gas detector sensors was designed to withstand the harsh environment. The enclosure was also strong enough to take any impact from heavy moving machinery. The gas detection system was powered by a diesel-generated 110v power supply. For circumstances when power was temporarily down, a1-cbiss installed a backup battery.

As soon as the alarm levels were breached during operation, the gas detection panel activated tunnel-grade alarms. Due to the conditions (loud and fairly dark), 114dB multi-tone sounders with integrated LED beacon were installed.

During the 3 year project, a1-cbiss provided full service and support to ensure the continuous operation of the gas detection system.

There were no reported incidents involving exhaust fumes from the MEWPs.