The Role of Geological Conditions and Petrology of Formations on Excavation Hazards (Case Study of Ghomroud Tunnel)

In the present case study، a thick overburden above the Ghomroud tunnel locally reaching about 600 meters plus the existence of many fault/crushed zones along the tunnel led the full-face Tunnel Boring Machine (TBM) to encounter squeezing and collapsing of materials. In parts 3 and 4 of the tunnel، a combination of these problems caused long-term and frequent pausing in digging processes (about 600 days). The plan is situated in the Sanandaj-Sirjan geological zone. Because of intense tectonic deformations through time، this zone consists of series of rock units، which have experienced moderate to high metamorphisms that caused schistosity and recrystallization of minerals. In the present paper، in addition to preparing thin sections of rocks for microscopic studies، the role of geological conditions with an emphasis on formation petrology is investigated to explore excavation hazards along the tunnel. Hence، by traversing along the axis of the tunnel، several samples were taken from the rocks exposed on surface and from those in the tunnel. Most problems occur in the foliated Jurassic rock units and in sections where faults and relatively thick tunnel overburden (high in-situ stress) have made the geological condition unfavorable. Results show that the petrological properties (texture، mineralogy، etc.) of the extensively foliated rock masses (schist، slate، graphite schist) and their interbedding with strong units (Quartz and quartzite veins) along with other geological factors have been effective on the occurrence of hazards and TBM pausing. Results of Cerchar abrasiveness index test also indicate high hardness and strength of quartzite veins، which played an important role in creating mixed face conditions and amplifying the hazards in easily squeezing and collapsing materials.