Monitoring of Gavoshan Tunnel Using Instrumentation Data

In this paper, using the data obtained from the measurement of convergence of Gavoshan Tunnel and its access galleries, the ratio of horizontal to vertical in-situ stresses have been estimated from back calculations. This tunnel with a length of about 20.2km is located in the west of Iran, and in the vicinity of the main road of Sanandaj to Kermanshah. This tunnel has been excavated from inlet and outlet and from eight faces created by four access tunnels including Haltooshan, Tavankesh, Sabenav and Kachaleh galleries. Several instruments have been used for monitoring the behavior of the tunnel including tape extensometers, rod extensometers, total pressure cells, piezometers and load cells. The convergence of the tunnel has been measured using tape extensometers in several stations in the main tunnel and the access galleries. For the numerical modeling, the computer software Plaxis with computational modeling of jointed rock (JR) and triangular elements of 6 nodes in plane strain has been used. Considering that Gavoshan Tunnel is located in the zone of Sannadaj-Sirjan fault, assessment of the ratio of horizontal to vertical stress is important. Therefore a back analysis to calculate K value has been carried out. The results obtained from convergence measurements are analyzed for the different stations and are combined with the numerical analyses to evaluate the coefficient of lateral pressures. It is concluded that in the outlet region of the tunnel, Sarbenav, Haltooshan and Tavankesh access galleries, the ratio of horizontal to vertical in-situ stress exceeds one and in the inlet region of the tunnel, this ratio is less than one.