ORIGINAL PAPER Understanding the mechanism of slope failure on a nearby highway tunnel route by different slope stability analysis methods: a case from NE Turkey Ayberk Kaya 1 • Aykut Akgu ¨n 2 • Kadir Karaman 3 • Fikri Bulut 2 Received: 13 March 2015 / Accepted: 4 July 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract The Arakli tunnel is located in the eastern Black Sea region where the most mass movement is observed in Turkey. Following the tunnel entrance portal excavations in basaltic tuffs on nearby the Konakonu res- idential area, an impending failure occurred. Because of the developed tension cracks and deformations on the ground, five houses and their gardens were damaged completely. The present study aims to investigate the mechanism of the failure. In order to do this, kinematic, limit equilibrium, and numerical stability analyses were carried out. Firstly, the kinematic analyses were performed taking into account the main joint sets for the slopes. The results of the kine- matic analyses showed that planar and wedge failures were possible on the portal slope and no failure occurred on the cut slope. However, the limit equilibrium analysis showed that neither the planar nor wedge failures were expected to occur on the portal slope. The numerical stability analyses were performed to determine if circular failure is to occur in the slopes. The Phase 2 programme was used in the numerical analyses, and the Strength Reduction Factors (SRF) of the slopes were determined. According to the numerical stability analyses, the failure mode for the portal slope is composite starting with a circular surface and following a linear surface and circular for the cut slope. The stability analyses indicated that the failure mechanism was not directly controlled by the joints and might be related to the low strength parameters of the rock mass and joints. Finally, precautions were determined to make the region stable using the Phase 2 programme. After support installation, the SRF values for the portal and cut slopes increased from 1.21 to 1.63 and from 1.32 to 1.71, respectively. These results showed that the proposed sup- port units prevent the effects of failure and were essential for the long-term stability. Keywords Slope stability  Mass movement  Numerical analysis  Support design Introduction Stability analysis of rock slopes is crucial for safety design in geotechnical engineering and open-pit mining. Various methods are performed for evaluation of the slope stability. Kinematic, limit equilibrium, and numerical analyses are widely used by engineers for the designing of safe slopes. The kinematic analysis deals with the motion of the bodies without consideration of the forces that give rise to motion (Kliche 1999). The analysis is generally used in the initial evaluation of the slopes. If the slopes were determined to be unstable based on the kinematic analysis, additional analyses, such as the limit equilibrium method, would be implemented for the evaluation of rock slopes (Karaman 2013). Limit equilibrium analysis takes into account the shear strength along a failure surface, the influences of water pressure, and the effect of external activities, such as seismic accelerations or reinforcing units (Kentli and Topal & Aykut Akgu ¨n aykut.akgun@ktu.edu.tr 1 Department of Geological Engineering, Faculty of Engineering, Recep Tayyip Erdogan University, 53100 Rize, Turkey 2 Department of Geological Engineering, Faculty of Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey 3 Department of Mining Engineering, Faculty of Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey 123 Bull Eng Geol Environ DOI 10.1007/s10064-015-0770-5