Gas Disaster Mechanisms in Bitumen‐Bearing Sandstone Tunnels: Structural–Lithological Coupling Controls and Excavation‐Triggered Chain Evolution

ABSTRACT Gas disasters in tunnels crossing bitumen‑bearing sandstones are a distinct geohazard, fundamentally different from coalbed methane risks. The Huangjialiang Tunnel on the Xi'an–Chengdu high‑speed railway (Longmenshan foreland and Sichuan Basin) provides an exceptional case study of combined gas–bitumen outbursts. Through an integrated approach combining geological characterisation, real‐time drilling monitoring, high‐precision geochemical fingerprinting ( δ 13 C and biomarkers) and process‐based logical deduction, this study establishes the geological genesis and engineering disaster mechanisms of such events. Carbon isotope data show that methane ( δ 13 C 1 = −33.8‰) and bitumen extracts ( δ 13 C 1 = −34.2‰) share a common origin from Lower Cambrian source rocks. Gas occurrence is controlled by structural–lithological coupling: The Longmenshan fault system provided migration pathways; dual‑porosity sandstones act as reservoirs; mudstone interbeds serve as local seals; and bitumen‑plugged fracture networks create high‑pressure ‘gas pockets’ via Jamin effects, supported by numerical simulations showing a 40% permeability reduction. Excavation triggers a four‑stage chain: (1) stress unloading generates tensile microcracks, (2) multiphase fluids ingress, (3) H 2 S accumulates and corrodes concrete, and (4) safety thresholds are exceeded. The mean H 2 S concentration of 5.92 vol.% highlights acute toxicity and long‑term structural corrosion risks. This study synthesises existing knowledge into an integrated conceptual framework for this under‑explored hazard type, offering practical guidance for risk mitigation in similar petrogenic formations worldwide.