Sources of high-temperature water and gas inrush during tunnel excavation: A case of Bangfu tunnel in Southwest China

Abstract

Cases of simultaneous inrush of high-temperature water and harmful gases are infrequently reported in areas without geothermal anomalies, hydrocarbon source rock, or coal measures. For this, we investigated the origin, development, and formation of the high-temperature water and harmful gases that rushed into Bangfu tunnel, Southwest China. During excavation of the Bangfu tunnel through the F1-2 fault in sandstone, a significant incident occurred involving a sudden influx of high-temperature water (45.4 °C) of Na–HCO3 type and harmful gases (CO2, H2S). An extensive geological examination uncovered a fault network extending from the crust to the mantle in the tunnel site area. The site features a substantial presence of both surface water and groundwater. Furthermore, within the middle crust at depths ranging from 19 km to 23 km, there are high-temperature ductile melts enriched with fluids and gases. Monitoring and experiments conducted on the harmful gases reveal that the primary source is identified in the crust, with the mantle source being secondary, followed by the atmospheric source being a minimal contribution. The hydrochemical and isotopic composition characteristics of the high-temperature rushed water indicate its evolution was formed through the infiltration of atmospheric precipitation from cold groundwater of the Ca–HCO3 type. The mechanism underlying the formation of the inrush high-temperature water and harmful gases can be outlined as follows. The fault network, spanning from the crust to the mantle, serves as a migration pathway for the inflow substances. Mantle-derived volatiles and high-temperature melts make heat energy facilitate the inrush activity, while groundwater contributes to heat transfer and acts as a medium for gas transport. As mantle-derived volatiles migrate towards the surface through the fault network, they mix with high-temperature melts and crust-derived gases, forming a crust-mantle mixed gas. Through processes such as deep hydrothermal circulation, shallow hydrothermal circulation, water/rock reaction, near-surface mixing, and dilution, Ca–HCO3 type cold groundwater transforms into high-temperature water of Na–HCO3 type. The methodologies and findings of our research offer insights into the route selection, investigation, and construction of mountain tunneling projects under similar geological conditions.