Rehabilitation of Portal Subsidence of Heybat Sultan Twin Tunnels: Selection of Shotcrete or Geogrid Alternatives

Abstract

The occurrence of karst phenomenon is one of the common problems in carbonate rocks in the presence of water. The rock masses constituting the ground surface are mostly of sedimentary types among which carbonate rocks are widely observed. It is therefore necessary to accept such a geological hazard in many projects. Some of the rehabilitation methods for karstic subsidence include grout injection, filling with concrete or shotcrete and making use of geosynthetic products. Few studies have been carried out on the application of the geosynthetic products. Limy rocks form the main lithology of a large part of the 72 km long access road tunnel designed for the Iraqi-Kurdistan. The occurrence of a karstic subsidence with a volume of about 2250 m3 in the portal of Heybat Sultan tunnels revealed the necessity of examining and selecting one of these rehabilitation methods. Concerns about the repetition of such collapses in other tunnels, especially in the 6740 m long Korek twin tunnels that is the longest tunnel of the Middle East located in the vicinity of the project, has doubled the importance of the issue. This paper aims to render an account of the use of shotcrete and geogrid to rehabilitate the subsidence of the portal of Heybat Sultan tunnels. The modeling results with the FLAC numerical finite difference code showed that the displacement amount of the host rock mass after implementing the second method would be 2.9 times less than the first method. Geogrid also reduces the axial forces exerted to the tunnel supporting system up to 43 tons.