Methane ingress during Yangtse River outfall construction

A US$345 million project to upgrade and extend Shanghai’s sewerage system was completed in 2007. The tunnelled component of this scheme includes 30 km of sewer tunnels with diameters varying from 2·4 to 3·5 m i.d.—a major tunnelling project in its own right. When methane gas was encountered during construction of the twin-bore 2·4 m i.d. outfall tunnel, the client recognised the serious safety implications and, being keen to complete the works without incident, requested expert advice from both national and international experts. This paper demonstrates knowledge sharing and the application of best practice in China through a case study of how the client, in partnership with national and international experts, dealt with the methane problem, enabling the outfall tunnels to be constructed without incident despite the onerous ground conditions. This case study must be viewed in the context of the absolute need for the development of Shanghai wastewater facilities with knowledge of the difficulties experienced in earlier similar projects.