Scientific Research and Essays Vol. 6(31), pp. 6566-6583, 16 December, 2011 Available online at http://www.academicjournals.org/SRE DOI: 10.5897/SRE11.1691 ISSN 1992-2248 ©2011 Academic Journals Full Length Research Paper Analysis of support requirements for a tunnel portal in weak rock: A case study from Turkey Ayberk Kaya 1 *, Fikri Bulut 2 , Selçuk Alemdağ 1 and Aytuna Sayin 3 1 Department of Geological Engineering, Faculty of Engineering, Gümüşhane University, 29100, Gümüşhane, Turkey. 2 Department of Geological Engineering, Faculty of Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey. 3 10th Division of Turkish General Directorate of Highways, 61310, Trabzon, Turkey. Accepted 18 October, 2011 In this paper, engineering geological properties of the entrance portal section of Konakönü Tunnel, located on Black Sea Coastal Highway, Turkey, is studied. Moderately weathered basaltic and andesitic tuffs orderly to be driven by the entrance portal section. Tunnel portal design in weak rock presents some special challenges to the designers, since misjudgements in the design of tunnel can lead to very costly failures. The entrance portal was analyzed by empirical and numerical methods. Both field and laboratory studies were carried out during this study. Field studies involved geological mapping, detailed discontinuity surveying and sampling. Laboratory tests were carried out to determine the physico-mechanical and elastical properties of rock units. For this purpose, tunnel stability analyses were performed at the entrance portal section. The RMR (rock mass rating), Q (rock mass quality), RMi (rock mass index), NATM (new Austrian tunnelling method) and GSI (geological strength index) systems were employed for empirical rock mass quality determination and preliminary tunnel portal support design. The parameters calculated by the empirical methods were used as input parameters for the FEM (finite element method) analysis. The results from the two methods were compared and preliminary support designs were carried out. This comparison suggests that for more reliable support design, empirical and finite element methods should be combined. Key words: Finite element method, rock mass classification systems, tunnel portal, weak rock. INTRODUCTION Determination of the most suitable and economical support system contribute to applicability of engineering projects for both design and construction stages. Sometimes, even a small misinterpretation in the design stages can lead to costly and time-consuming failures at the construction phases (Sari and Pasamehmetoglu, 2004). Empirical and numerical methods are commonly used methods when underground engineering structures are designed. Rock mass classification systems are very useful tools for the preliminary design stage of a project, when very little detailed information on rock mass is available. *Corresponding author. E-mail: ayberkkaya@hotmail.com, ayberkkaya@gumushane.edu.tr. Tel: +90-456-2337425/1227. Fax: +90-456-2337427. The RMR, Q, RMi, NATM and GSI rock mass classification systems have been used by many researchers and have gained a universal acceptance (Barton, 2002; Ramamurthy, 2004; Basarir et al., 2005; Hoek and Diederichs, 2006; Gurocak et al., 2007; Gurocak, 2011). The rock mass classification systems have been originally obtained from many tunnelling case studies and they have been successfully applied to many tunnel construction designs in the past decades (Sari and Pasamehmetoglu, 2004). Although rock mass classification systems are very useful during the preliminary design stage, they do not adequately calculate stress distributions, support performance and deformations around the tunnel (Genis et al., 2007). Underground projects will be safer and more economical, if empirical approaches supported by numerical approaches.