CASE REPORT Engineering Geological Study of NWCT Tunnel in Iran with Emphasis on Squeezing Problems Masoud Shamsoddin Saeed • Parviz Maarefvand Received: 23 May 2013 / Accepted: 26 August 2013 Ó Indian Geotechnical Society 2013 Abstract This research is concerned with the assessment of squeezing potential along of North Water Convey Tunnel in Iran (NWCT) and its stability analysis has been carried out. In this study, the most suitable methods are utilized for the stability analysis and design of support of the tunnel. For the empirical investigation, the rock mass were classified based on RMR, RMi, GSI and Q systems. The geomechanical properties of the rock mass were determined from the laboratory and field investigations. The results obtained from the analysis show that the tunnel is highly unstable due to the presence of a fault and hence strong supports are need in these regions. Because of high overburden (up to 600 m) and the presence of faults and crushed zones, it was necessary to evaluate squeezing potential along the critical section of the tunnel (Lot2). Therefore, empirical and semi empirical approaches and a new method as critical strain have been used for evaluation of squeezing potential. Some experimental equations were used for estimation of critical strain and also Flac2D pro- gram has been used to calculate tunnel convergence. Consequently, squeezing index was estimated. This index is used for determination of squeezing degree. As a result, fair squeezing potential was found in crushed zones along the tunnel route. Keywords Engineering geology  Rock mass classification  Squeezing  Critical strain  Flac2D Introduction Squeezing is a large time dependent deformation which occurs around the tunnels. It is essentially associated with creep caused by exceeding a limitation of shear stress. Deformation may terminate during construction or continue over a long time period [7]. The effects of squeezing of rock, as the redistribution of stress and formation of a plastic zone around a deep tunnel were first identified by Wiesmann [45] during construction of the Simplon tunnel in Switzerland. Recently, the squeezing phenomenon was addressed and extensively investigated by Singh et al. [42], Aydan et al. [1], Dube [16], Barton and Grimstad [8] and Goel et al. [17, 20, 21]. Prediction of squeezing conditions is of great importance to a designer for designing a stable support system of the tunnel. A number of relatively extensive studies have been carried out on the prediction of squeezing problem [1, 2]. In this study, in addition to numerical modeling and finite ele- ment methods, prediction of squeezing has been done using experimental and critical stress methods [9, 15, 38]. A threshold value of tangential strain at the tunnel periphery above which instability and support problems are likely to occur existed. The threshold value of strain is termed as the critical strain [1]. Using Flac2D program, deformation of tunnel has been calculated. This value is employed to predict squeezing index (SI). In this paper, in addition to critical strain, empirical and semi empirical equations have been used for endorsement squeezing ana- lysis in the critical section of the NWCT. Study Area Characteristic The NWCT tunnel, located in north of Iran, provides a part of water requirement to the north Iran tropical plains M. Shamsoddin Saeed (&)  P. Maarefvand Department of Mining and Metallurgical, AmirKabir University of Technology, Tehran, Iran e-mail: Masoud_shams90@yahoo.com P. Maarefvand e-mail: parvizz@aut.ac.ir 123 Indian Geotech J DOI 10.1007/s40098-013-0078-y