Assessment of the dynamic stability of the portals of the Dorukhan tunnel using numerical analysis

Instability problems may arise during the construction and operation of tunnels depending on the quality of the rock mass. In particular, determining instability problems at the portals of a tunnel is of utmost importance during excavation and operation of the tunnel. Slope instability and rockfalls are the most frequent instability problems that may be encountered at tunnel portals. Such instability problems can be triggered by dynamic effects such as earthquakes, blasting, etc. This study investigates the stability of the portals of the Dorukhan Tunnel connecting the provinces of Zonguldak and Bolu in the close vicinity of the North Anatolian Fault Zone (NAFZ), which is a well-known strike-slip 1500 km long fault. The effect of an earthquake that may occur in the NAFZ on the extent of failure has also been studied by numerical analyses. In the analyses, a three-dimensional dynamic analysis computer code, based on the finite difference method, has been used. According to results of the dynamic numerical analyses, the Devrek portal was found to be more stable than the Mengen portal. Moreover, it was found that, for acceleration values of dynamic wave higher than 0.5g, tunnel portal slopes and the areas as far as 20–50 m from the tunnel entrance may suffer serious damage. Wave propagation perpendicular to the tunnel axis was established to cause more damage around the openings and in the portal slopes as wave compared to propagation parallel to the tunnel axis.