The Effects of Unstable Fault Zones on Design Parameters of Tunnels (The Case of Erzurum Kırık Tunnel)

The Kırık Tunnel in Erzurum is an important route in the highway network designed to connect the northern and southern regions of Turkey. The tunnel, planned to be 7 km long, 10 m high and 7 m wide, passes through a geologically complex region. Preliminary investigations have revealed that the tunnel route is predominantly composed of clayey limestone and marl succession with variable slopes and orientations. During the tunnel construction works, a cave-in occurred during excavation after the water flow and a fault zone containing sandy-gravelly and organic matter was encountered at the 1+825+70 km section of the tunnel. The discovery of the crushed zone due to faulting necessitated a change in the tunnel design and it was decided to apply the back analysis method to understand the collapse mechanism. The deformation data affecting the tunnel were obtained with the help of load cells and back analysis was performed to determine the parameters of the collapse material from this deformation value at the time of collapse. Rock mass classifications were made using the parameters obtained from field and laboratory observations. As a result, with the help of Rocscience RS2, the collapse in the Kırık Tunnel was analyzed by finite element method and back analysis was performed. During the back analysis, the lateral vertical stress pressure acting on the faulted weak zone had to be re-evaluated. The k value changes in the numerical calculation model were examined from the existing approach methods and the comparison of these methods depending on the deformations in tunnel construction was given within the scope of the study. As a result of these data, new design parameters were found and grouting and support recommendations were given to overcome the stability problems in the tunnel. Accordingly, the lateral vertical stress ratio was taken as 1.6, which is normally 0.3, and the lateral pressure in the tunnel was determined as higher than the vertical pressure. With the new parameters, the unstable zone in the tunnel was theoretically crossed without any problems.