Hazards of Tunnel Excavation in H2S-bearing Grounds: (Case study of Aspar Tunnel)

Subject Areas : environmental management

Hossein Mirmehrabi ¹ , Mohammad Ghafoori ² , Gholamreza Lashkaripour ³ , Jafar Hassanpour ⁴

1 - Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
2 - Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
3 - Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
4 - SAHEL Consulting Engineers, Tehran, Iran

Received: 2010-06-29 Accepted : 2010-10-20 Published : 2014-12-22

Keywords: Gassy grounds, Geological hazards, Mechanized excavation, Hydrogen sulfide,

Abstract :

AbstractIntroduction: Encountering hydrogen sulfide gas (H2S) during excavation is one of the important engineering,geological and environmental hazards during the tunnelling. Tackling this hazard and solving its challengessolving are very difficult and costly. During investigation of the situation, one of the main tasks is prediction orevaluation of the risk of H2S gas and selection of the best methods to tackle its engineering and environmentalproblems. In this study, water conveyance tunnel of Aspar excavated in H2S bearing environment is discussed.This tunnel is excavated in the hydrocarbon formations. In this paper, hazards, characteristics, safety regulations,and geological sources of H2S as well as the methods to decrease the risks and problems in excavation of thetunnel are presented in brief.Methods: In order to determine the source of gas and to select the best methods to mitigate its hazards andproblems, in addition to investigation of the same experiences, concentration of various gases was recorded byfixed stations on the machine and by mobile sensors at the beginning, midpoint and end of each working shift.Moreover, sampling of the polluted air and water was implemented. The samples were sent to a specificlaboratory for chemical analysis. At the same time, concentration of the gas in the air and water of the tunnel wasmeasured.Results: Experiments raleted to the tunnel showed that the gas caused an unacceptable condition for workers.For predicting the risk of H2S gas in underground spaces, it is possible to use some evidences such as sulfursprings, organic traces, organic shales, exposure of H2S odor from fresh surface of rock, and smelling of H2Sduring boreholes drilling. Results of the analysis show that the gas enters the tunnel along with water,dominantly. Also considerable amount of the gas is released to the air at the beginning. According to theinvestigations, the source of the gas is relevant to hydrocarbon formations inthe area.Conclusion: Geological formations related to hydrocarbon resources are very important in the formation andreservation of H2S gas. Since the gas is in solution form and is emitted promptly, controlling the inflow ofgroundwater into the excavation, diluting the concentration of H2S and training the workers are a series ofmethods used to decrease the risks and problems associated with tunnel excavation in an H2S-bearingenvironment.

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