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

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 challenges solving are very difficult and costly. During investigation of the situation, one of the main tasks is prediction or evaluation of the risk of H2S gas and selection of the best methods to tackle its engineering and environmental problems. 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 the tunnel are presented in brief.

In order to determine the source of gas and to select the best methods to mitigate its hazards and problems, in addition to investigation of the same experiences, concentration of various gases was recorded by fixed 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 specific laboratory for chemical analysis. At the same time, concentration of the gas in the air and water of the tunnel was measured.

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 sulfur springs, organic traces, organic shales, exposure of H2S odor from fresh surface of rock, and smelling of H2S during 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 the investigations, the source of the gas is relevant to hydrocarbon formations inthe area.

Geological formations related to hydrocarbon resources are very important in the formation and reservation of H2S gas. Since the gas is in solution form and is emitted promptly, controlling the inflow of groundwater into the excavation, diluting the concentration of H2S and training the workers are a series of methods used to decrease the risks and problems associated with tunnel excavation in an H2S-bearing environment.