Alborz Service Tunnel in Iran: TBM Tunnelling in Difficult Ground Conditions and its Solutions

Alborz Service Tunnel is the longest tunnel (6.4 km) along Tehran Shomal Freeway. A service tunnel with diameter 5.20 m is excavated in advance of two main tunnel tubes to be excavated subsequently. The purpose of the service tunnel is for site investigation, drainage of the rock mass, providing access for main tunnel excavations and for service, ventilation and drainage during operation of the complete tunnel system. Site investigation for the service tunnel was done by surface mapping and surface geoelectrical survey, but without direct investigations like core drilling, since the overburden is up to 850 m. During excavation by open gripper TBM, the following major extraordinary difficulties have been encountered: methane gas, squeezing ground and blocked cutter head and TBM shield, multiple fault zones with large crown instabilities, karst with water in gypsum / anhydritic rock, water ingress of more than 550 l/s over months and H2S gas of >100 ppm, released from water and from dry rock. The paper describes the measures taken to negotiate these difficulties. They include modifications on the ventilation system, adapted working procedures at CH4 gas, manual excavation of bypasses around the shield and the cutter head, foam and cement injections for rock stabilization, extensive drainage measures to handle the water quantities and various protective equipment against cold water and H2S gas.

Keywords:

TBM , water ingress , H2S , CH4 , Alborz , karst