Formation processes, accumulation patterns and risk mitigation strategies for harmful gases in tunnels within the complex geological structure area of Southwest China

Abstract

The mountainous landscapes of southwest China are renowned for their complex geological settings, a region where an extensive network of tunnels, including railway to highway and hydraulic engineering tunnels, is planned for construction. The area has witnessed several severe incidents resulting in casualties caused by underground harmful gases during tunnelling activities. With ongoing development, it is critical to investigate the risks associated with harmful gases in tunnelling projects within the region. Current research on the formation and enrichment mechanisms of harmful gases remains insufficient. This study investigates the relationship between harmful gases and the geological structures and strata by conducting through geological surveys, sampling tests, field observations, and employing advanced geological forecasting in both surveyed areas and existing tunnels. The findings identified three main mechanisms for the generation of harmful gases: hydrocarbon generation, thermochemical reactions, and hydrothermal migration. Methane, carbon dioxide, carbon monoxide, and hydrogen sulfide are the predominant gases encountered during tunnel construction. These harmful gases tend to migrate upwards, with their migration distance and rate of diffusion were controlled by faults and/or fractures. This migration can lead to localized accumulations of harmful gases, which we categorized as fault-controlled and joint-controlled accumulation types. This study interprets the disaster-causing mechanisms of harmful gases, suggesting that risk identification and advanced forecasting methods of tunnel harmful gases establish classification criteria for assessing gas risks during construction, and forms a comprehensive risk management system spanning the entire process from investigation, design, to construction. Future research directions should focus on the application of artificial intelligence for predicting harmful gas risks, the management of high-pressure gases in tunnels exceeding 1000 m in depth, and the development of strategies to mitigate the toxicity of harmful gases. These findings offer a theoretical basis and technical guidance for the safe construction of tunnels.