Groundwater and gas inflow toward the tunnel being excavated in Aspar Anticline (at the west of Kermanshah, IRAN) cause some problems during construction. Aspar Anticline consists of limestone units from Illam Formation which in Iran is known as a resource formation for Petroleum and natural gas. Detailed investigations (based on result of physical and chemical tests performed on different samples taken from the water and air of the Tunnel) show that about 1 lit H2S gas can be solved in 100 lit water and enter to the Tunnel. It is obvious that solved gas can released in Tunnel by decreasing water pressure and disturbance of water. This study shows that there is a strong relationship between geological and hydrogeological parameters and concentration of gas in the tunnel. These parameters can be considered as indices for identifying potential of gas and water inflow toward the tunnel before excavation. Also, in this paper, the problems related to gassy and water inflow in the tunnel, being constructed with double shield TBM machine are discussed.
This tunnel is designed to transfer 70 m3/s and is located at western of Iran and in Zagros mountains. The 6.73 m diameter tunnel is 26 km long and had breakthrough in February 2006 and is still under construction. Groundwater inflows into tunnels constructed in fractured bedrocks not only constitute an important factor controlling the rate of advancement in driving the tunnel but may induce environmental impacts. Drawdown in groundwater level caused by tunnel construction may also induce geotechnical and environmental impacts. The changes in the hydrologeology condition of the area caused by tunnel construction were also studied, using measurements of the changes in groundwater levels of the borehole and well and discharge of the springs. Due to special properties of clay minerals; clay bearing rocks beside water have more problems and their engineering properties, depending on their clay type and content. Gassy ground condition can be another important problem in tunneling, especially in cases with dangerous and corrosive gases. Natural gases are encountered rarely in tunneling. However when natural gases enter tunnels and other underground openings, they pose a particularly severe hazard that can, and often has, resulted in death to workers. In this study impacts of ground water condition and gassy ground (H2S and methane) on the tunnel were investigated, and characteristics, sources and geological and hydrogelogical indicators of H2S were presented.
The geology of studied area consists of sedimentary rocks that are exposed in form of folds, faults, fractured zones and joints. The Aspar anticline is one of the main structures at the project area. The axis of the anticline extended western-north to eastern-south. The area known as folded Zagros province. The oldest geologic unit along the tunnel alignment is brownish gray limestone of Illam formation (ch: 03 + 710 to ch: 04 + 927) that is located in the core of Aspar anticline (Figure 1).