TBM Tunnelling in Hydrogen Sulfide Gas Bearing Ground and its Solutions

In the late December of 2006 while Zagros tunnel project in western Iran was advancing according to the schedule, a sudden rush of groundwater accompanied by a nauseating odor similar to that of rotten egg intruded the tunnel. Some workers complained from eye and respiratory tract irritation. The presence of hydrogen sulfide (H₂S) gas as high as 200 ppm was soon tested positive by gas detectors and subsequently the ventilation fans (2*110 kW) speed were boosted to 1,450 rpm in order to dilute the gas concentration to safe levels (10 ppm). Nonetheless, the work continued at a rather moderate pace for another 11 days and 134 more meters excavated in the gas infested grounds before 4 men died during a damage assessment survey of the TBM after a power failure that had forced the tunnel ventilation system to temporarily shut down. This paper is to discuss hazards and geological sources of H₂S gas in of Zagros Water Conveyance Tunnel and to recommend practical solutions to prevent or mitigate the gas destructive effects on human and machinery, as well.