The Investigation of Twin Tunnel Stability: Effect of Spacing and Diameter

The underground excavation became the backbone of developing as well as developed country due to unavailability of surface area and increase demand of space for transportation, water channeling, waste disposal and storage. Except for transportation, the crystalline rocks (high strength, low porosity and less permeability) are considered most suitable and safe space for the storage of radioactive waste as well as petroleum. The role of spacing between twin tunnel and tunnel's diameter in its stability was carried out using finite element method. Not much research has been done on twin tunnel spacing in hard rock. In the present study, a total of ninety simulations were performed to investigate critical spacing of the twin tunnel in granitic rock mass. Twin tunnel diameter is varied from the 2 m to 10 m and their spacing varied from 0.2 to 2 times of diameter. It was concluded that minimum spacing should be 0.8 times of the tunnel diameter. It was also found that small diameter tunnel effects only near field but large diameter tunnel effect near field as well as far field. The vertical hole, approximately 3m diameter, containing radioactive waste canister should be drilled at the interval of more than 2.4m.