Geotechnical investigations and remediation design for failure of tunnel portal section: a case study in northern Turkey Geotechnical investigations and remediation design for failure of tunnel portal section: a case study in northern Turkey

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Vol14 No.6: 1140-1160

Title】Geotechnical investigations and remediation design for failure of tunnel portal section: a case study in northern Turkey

Author】Ayberk KAYA1*; Kadir KARAMAN2; Fikri BULUT3

Addresses】1 Recep Tayyip Erdogan University, Faculty of Engineering, Department of Geological Engineering, 53100, Rize, Turkey; 2 Karadeniz Technical University, Faculty of Engineering, Department of Mining Engineering, 61080, Trabzon, Turkey; 3 Karadeniz Technical University, Faculty of Engineering, Department of Geological Engineering, 61080, Trabzon, Turkey

Corresponding author】ayberk.kaya@erdogan.edu.tr

Citation】Kaya A, Karaman K, Bulut F (2017) Geotechnical investigations and remediation design for failure of tunnel portal section: a case study in northern Turkey. Journal of Mountain Science 14(6). DOI: 10.1007/s11629-016-4267-x

DOI】10.1007/s11629-016-4267-x

Abstract】Mass movements are very common problems in the eastern Black Sea region of Turkey due to its climate conditions, geological, and geomorphological characteristics. High slope angle, weathering, dense rainfalls, and anthropogenic impacts are generally reported as the most important triggering factors in the region. Following the portal slope excavations in the entrance section of Cankurtaran tunnel, located in the region, where the highly weathered andesitic tuff crops out, a circular toe failure occurred. The main target of the present study is to investigate the causes and occurrence mechanism of this failure and to determine the feasible remedial measures against it using finite element method (FEM) in four stages. These stages are slope stability analyses for pre- and post-excavation cases, and remediation design assessments for slope and tunnel. The results of the FEM-SSR analyses indicated that the insufficient initial support design and weathering of the andesitic tuffs are the main factors that caused the portal failure. After installing a rock retaining wall with jet grout columns and reinforced slope benching applications, the factor of safety increased from 0.83 to 2.80. In addition to slope stability evaluation, the Rock Mass Rating (RMR), Rock Mass Quality (Q) and New Austrian Tunneling Method (NATM) systems were also utilized as empirical methods to characterize the tunnel ground and to determine the tunnel support design. The performance of the suggested empirical support design, induced stress distributions and deformations were analyzed by means of numerical modelling.Finally, it was concluded that the recommended stabilization technique was essential for the dynamic long-term stability and prevents the effects of failure.Additionally, the FEM method gives useful and reasonably reliable results in evaluating the stability of cut slopes and tunnels excavated both in continuous and discontinuous rock masses.

Keywords】Portal failure; Stability analysis; Finite element method; Tunnel support design; Remedial measures Rock Mass Rating (RMR);Rock Mass Quality (Q); New Austrian Tunneling Method (NATM)