Gas Occurrence and Emission Characteristics in Highway Tunnels Through Non-Coal-Bearing Strata: A Case Study of the Linchang Tunnel

The Western Sichuan Plateau is characterized by pronounced topographic relief, intense tectonic activity, and widespread metamorphic sandstone, slate, and locally carbonaceous strata, which create a potential for abnormal gas emission during highway-tunnel construction. Gas in non-coal-bearing formations can occur in concealed, spatially discrete sources and localized release zones whose structural connections are difficult to characterize fully by conventional investigation. The objective of this study is to identify the geological settings and inspection locations associated with localized methane enrichment in a low-gas highway tunnel through non-coal-bearing metamorphic strata, and to translate these findings into construction-stage control priorities. The Linchang Tunnel of the Jiuzhi–Maerkang Expressway is examined using geological investigation, gas-composition testing and periodic concentration records. The Linchang Tunnel mainly traverses metamorphic sandstone and slate of the Triassic Zhuwo and Zagunao formations. Locally developed carbonaceous slate provides a potential hydrocarbon source, whereas complex folding and relatively confined surrounding-rock conditions favor local gas preservation. The classification-stage absolute methane emission value was 1.185 m3/min, placing the tunnel in the low-gas category. On the basis of representative tunnel projects in Western Sichuan, China, gas emission from non-coal-bearing strata is generalized into three basic modes: direct exposure of hydrocarbon-generating strata, connection of deep gas reservoirs by gas-conducting structures, and indirect exposure of structurally controlled gas accumulations. Periodic field inspections show a cross-sectional concentration ordering at the tunnel face: CH4 concentrations are typically 0.04–0.05% at the crown, approximately 0.03% at the haunches, and approximately 0.02% at the arch feet; relatively high concentrations also occur locally in fractures and collapse cavities. Gas-composition tests confirm methane in the sampled gas–air mixtures. An explicitly idealized transport calculation and a three-grid comparison examine how source intensity, ventilation and effective mixing influence local enrichment; these calculations are distinguished from field observations of crown concentrations. Accordingly, a process-wide framework integrating advanced identification, dynamic monitoring, enhanced ventilation, borehole drainage, grouting sealing, and tiered management is proposed. The findings provide a basis for gas-hazard identification and construction-safety control in highway tunnels through non-coal-bearing metamorphic strata on the Western Sichuan Plateau, China, and in comparable geological settings.