The Dangerous Condition of Ground during High Overburden Tunneling (A Case Study in Iran)

Knowledge of the ground condition and its hazards can play an important role in the selection of support and suitable excavation method in underground structures. Water transport tunnel is one of the most important structures with regard to the goal of excavation, special conditions and limitations considered in the design and execution of them. Beheshtabad Water Conveyance Tunnel with 64930 meters length, 6 meters final diameter is the largest water Conveyance tunnel in Iran. Because of high over burden and weak rock in the most of tunnel path, the probable hazardous of the ground condition such as squeezing and rock burst must be studied. Squeezing stands for large timedependent convergence during tunnel excavation. This phenomenon occurs in weak rocks and deep conditions. Besides, the height of overburden in some of the zone tunnel is about 1200 meters. The occurrence of this phenomenon is always together with the instantaneous release of strain energy stored in the rock materials, causing the harm to the personal equipment and the collapse of underground structures. The existence of high thickness overburden in some the zones of this project indicates the high potential of rock burst hazard. In this research, the length of the tunnel has been partitioned into sections using the interpreted geological, geophysical studies and borehole data. After evaluating rock burst and squeezing potential with alternative analytical and experimental methods for each section, the results of different methods were compared with each other. Results predict low to moderate squeezing potential and moderate to high rock burst potential for some panels of the tunnel.