Investigation of a methane flare during the excavation of the Silvan irrigation tunnel, Turkey

摘要

Mechanized methods are being increasingly used in tunnel excavations to such an extent that to ensure safe construction and the economic viability of projects the need for design-stage geotechnical studies has considerably increased. However, unexpected problems, such as gas inflow, can be experienced during excavation when the geological conditions are insufficiently investigated. In fact, even if construction is taking place in a known oil-natural gas basin or coal-bearing strata, the possibility of methane flare/explosion events can be minimized if a suitable excavation method and equipment are used. However, little published literature is available on this topic, resulting in the potential problem of encounters with sources of methane during construction that have not been considered in the planning of the tunneling operations. We have studied a methane (natural gas) flare incident that occurred in 2015 during the excavation by full-face hard rock tunnel boring machine of the Silvan irrigation tunnel, south-eastern Turkey. During the planning and pre-construction stages no consideration was given to the fact that the tunnel passes through a natural gas basin and, consequently, the selection of excavation method and machine equipment was made without taking into account the high possibility of natural gas being present. During excavation a significant methane gas flare occurred, resulting in 13 workers being injured and abandonment of the project. Subsequent investigations revealed that the proposed route of the tunnel passed through a natural gas basin and that Turkish Petroleum Corporation (TPAO) had carried out natural gas exploration in the area. Here we provide details on the geological background and the flare incident itself and come to the conclusion that the pre-construction ground investigations for this project were grossly inadequate. We also suggest that in order to facilitate economical and safe tunnel construction, consideration should always be given to the possible presence of methane and other gases at the ground investigation stages of tunneling projects and that all previous geological and technical studies related to the study area should be taken into consideration during the pre-construction stage.