Analysis of methane emissions in a tunnel excavated through Carboniferous strata based on underground coal mining experience

During the excavation of different segments of the Variante de Pajares tunnels by means of single shield TBMs (tunnel boring machines), methane has been found. The analysis of the recorded data has allowed to prove that the natural gas emission in a tunnel excavating through Carboniferous strata is similar to that which occurs in an underground coal mine. In this paper, two mining parameters for the characterisation of methane emission, the methane emission rate (volume of methane emitted per ton of excavated rock) and average methane flow rate (volume of methane emitted per day), are introduced. Following, using the measurements of TBM advancing rate, quantity air flow and methane concentration carried out during the tunnel excavation through San Emiliano geological formation these two parameters have been determined for the tunnel. The results were very similar to those observed in coal mines, which means that mining experience can be used in order to predict the methane inflow into a tunnel excavated through a Carboniferous rockmass which is very useful for the design of the ventilation system, something very significant for safety.