Seismic isolation of tunnel lining - a case study of the Gavoshan tunnel in the Morvarid Fault”

Abstract

Strike slip faults are wide spread around the world more than the other kinds of faults. Therefore, huge structures such as dams, tunnels, power plants, etc. are more likely to meet these faults. When the fault is active, special treatments should be considered. Most of the design code suggest avoidance of active fault crossing. Nevertheless, ASCE in 1998, by taking into consideration that crossing these kinds of faults is inevitable, has suggested a useful flowchart about design process of under ground structure in front of fault rupture hazard and seismic events. In order to analyze the shape and amount of fault rupture effect on tunnel lining, it was necessary to build an accurate model of the tunnel. Therefore, Gavoshan tunnel in west of Iran, which is in construction status now, was chosen. The mentioned structure is a 20 km, water supply tunnel 4.2 m in diameter, horseshoe shaped, and lined with reinforced concrete. This tunnel crosses the Morvarid fault, which is one of the young Zagros fault's branches. The fault is active and strike slip. For the modeling of the tunnel, finite elements method is one of the accurate possible solutions. The ANSYS software was selected to model and run the problem. For the first step, according to seismology studies the fault movement during earthquake was estimated then the concrete lining and the surrounding broken rock mass, was modeled. Intact rock was chosen as the boundaries of the model. After applying the fault rupture load to the model, together with in situ and induced stress of excavation, distribution of stresses and failure mode in rock and lining was obtained. In this study, two solutions for the lining protection were selected. In the first solution, an adequate gap between lining and rock was considered, and in the second one a soft coating (rubber) injected into the gap between the lining and the rock. Both of solutions were modeled and studied. In order to reach to the most effective stable state of lining and study the behaviour of lining during fault rupture, several analyses with different conditions and thicknesses of soft coating were performed. Finally, a suitable solution and some comments for the protection of lining in seismic zone are suggested. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.