Experiences of TBM operation in gas bearing water condition – A case study in Iran

• The Nowsud tunnel, in west of Iran, is studied.
• The CH4 and H2S bearing water put a adverse effect on tunnel.
• The remedial works performed to isolate water and sufficient ventilation system.

The Nowsud tunnel (Lot 2B) project is a 25.7 km length water conveyance tunnel which was bored with a 6.73 m diameter double shield TBM. The tunnel consisting of 6.0 m inside diameter which lined with 25 cm thick, 4 pieces honeycomb precast segments. The geology is consisted of limestone, limy shale, black shale and Shally limestone of Cretaceous Garu and Pliocene Gurpi formations. During heading of tunnel, it was encountered with CH4 gas emission and H2S bearing water inrushes, up to 890 l/s. A source which could not be plugged with grouting and the heading had to be stopped for several days and months. As far as the author is aware, this condition has not been seen in double shield TBM tunneling.This case study attempts to discuss firstly the quality and origin of poison gases and water ingress into the excavations. Then, the destructive effects of the poison gases and water ingress on different parts of TBM, concrete corrosion, considerable delay in tunnel progresses and negative impact on tunnel personals productivity have been evaluated.Finally, with respect to restricted space in TBM and back up, a executable solution methods have been pursued to abatement and prevention of the poison gases and water ingress into the excavations, while the construction is in progress by double shield TBM.