The possibility of encountering dangerous gases during the construction of a tunnel, or in the completed works, needs to be addressed during the investigation and construction stages of a project. The presence of explosive or toxic gases is significant in relation to the health and safety of the workforce and the selection of appropriate plant and equipment to be employed. In recognition of this and the fact that the Channel Tunnel is underlain by coal bearing strata, a detailed desk study was commissioned to review the geological conditions, structure and history of the formations at and below the tunnel horizon. This included information from recent site investigations in the Channel and was supplemented by gas detection surveys conducted within the tunnel workings.

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