Geological Control Factors and Accumulation Patterns of Harmful Gas in Tunnels in Northwest Hunan, China, and the Sustainable Development of Tunnel Engineering

This research addresses the critical safety issue of harmful gas influx during tunnel excavation through unconventional gas-bearing structures. It focuses on six tunnels affected by shale gas influx along the Zhangnan Expressway of northwestern Hunan, China. The research reveals the geochemical characteristics, distribution patterns, and accumulation mechanisms of harmful gas in the shale formations of northwestern Hunan. The research adopts an integrated approach of “geological background analysis—multi-parameter testing—comprehensive evaluation”. It is based on geological and borehole data, field geological surveys, as well as laboratory and field tests. The research systematically analyzes the gas-bearing structural characteristics, geochemical parameters, and reservoir physical properties of the shale gas area in the Zhangnan Expressway. The geological regularities are summarized. The results show that the Longmaxi Formation of the Silurian system and the Qixia Formation of the Permian system serve as source rocks in the tunnel sites. The reservoirs are characterized by ultra-low porosity and permeability, with limited late-stage hydrocarbon generation potential. The gas-related hazard during tunnel construction and operation is primarily associated with the release of existing free and adsorbed gas. According to the calculation standards for absolute gas emission rates during construction, three tunnels are classified as micro-gas tunnels and three as non-gas tunnels. Two accumulation patterns are proposed: the self-sourcing composite accumulation pattern with micro-scale migration, and the accumulation pattern of self-generated and self-storage type of water pressure confinement. Enhanced monitoring, ventilation, and grouting sealing are recommended. This study develops an integrated “geology—testing—evaluation” assessment method for shale-gas-bearing tunnels. It provides important guidance for controlling harmful gas hazards in such tunnels and guaranteeing the sustainable development of tunnel construction.