Multidisciplinary methodology used to detect and evaluate the occurrence of methane during tunnel design and excavation. An example from Calabria (southern Italy)

The occurrence of high volumes of methane during tunnelling operations is a critical safety factor that can influence the choice of different technical approaches for tunnel design and construction. Moreover, gas accumulation can be influenced by fluid migration along spatially focused preferential pathways (i.e. points along faults and fracture zones) that can result in highly variable gas concentrations along the tunnel trace. This paper proposes a methodological approach to minimise the risks, and costs, related to tunnel construction in rocks with potentially high methane concentrations. This approach combines soil gas geochemistry and structural geology surveys along and across the main faults and fracture systems that occur in the study area. The procedure is based on near-surface sampling and consists of a two-pronged approach: the measurement of fault zone gas emissions and their classification as barrier or conduit zones. Moreover, it is illustrated the importance of measuring a wide spectrum of different gas species, not just methane, for a more accurate interpretation of the geological, geochemical, and structural system. This is due to the potential for multiple gas origins, different gas associations, and various alteration and oxidation processes (e.g., CH4 oxidation into CO2) that can modify the geochemical signal along the flow path as gas migrates towards the surface.