The performance of a TBM in a squeezing ground at Uluabat, Turkey Nuh Bilgin a,⇑ , Melih Algan b a Mining Engineering Department, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey b Zemar, Consulting, Geological and Geotechnical Studies, Istanbul, Turkey article info Article history: Received 25 April 2011 Received in revised form 29 December 2011 Accepted 1 May 2012 Available online 12 June 2012 Keywords: Squeezing ground Jamming of the TBM Rescue galleries abstract Squeezing characteristics of the ground in the Uluabat Tunnel, Turkey, are discussed and the analysis of TBM performance data around zones where the TBM jamming problems occurred is presented. Detailed analysis shows that a squeezing index of the ground can be defined which is related to the change in thrust force, time and length of tunnel where the increase of thrust starts and terminates with squeezing of TBM. This index is found to increase linearly with overburden and although the relationship is not strong, particularly with decreasing RMR values. It is also shown that the delays in tunnel excavation or TBM stoppages also play an important role in the squeezing of TBM shield. It was possible to mitigate the squeezing problem by means of bentonite injection through the shield. As a result, a methodology of predicting critical zones for shield blockage and problems caused by squeezing characteristics of difficult ground is discussed in detail. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Squeezing behaviour of the ground may adversely affect the TBM performance even questioning the feasibility of a TBM exca- vation since even small convergences may cause jamming of the shield (Ramoni and Anagnostou, 2010a). Therefore the prediction of the squeezing characteristics of the geologic formations is cru- cial in order to take mitigating measures such as injecting benton- ite through the shield or overboring. Literature survey on the subject with detailed theoretical and numerical investigations, as well as reference to practical experi- ence, were carried out by Ramoni and Anagnostou (2010a,b, 2011a,b). However, published case studies always help academi- cians to verify their theoretical work and numerical analysis. The main objective of this paper is to contribute to the area by analyz- ing the causes and the impact of squeezing ground on the opera- tional parameters of TBM for the Uluabat Tunnel and to evaluate the effects of bentonite injection and interruptions in the excava- tion on the squeezing of TBM. The following basic works are of general character dealing with squeezing ground. According to the International Society for Rock Mechanics (ISRM), squeezing is time dependent large deformation which oc- curs around the tunnel associated with creep caused by exceeding a limiting shear stress (Barla, 2001). Aydan et al. (1993), took into account the experience gained in Japan, suggested that squeezing occurs if the ratio of uniaxial com- pressive strength of intact rock to overburden pressure is less than 2. Hoek and Marinos (2000a,b) proposed that the ratio of rock mass uniaxial compressive strength to the in situ stress may be used as an indicator of tunnel squeezing characteristic. Kovari and Staus (1996) found that the low strength and the high deformability of the rock as well as the presence of pore water pressure facilitate squeezing and the following rock types are spe- cially prone to developing large pressure and large deformations; altered gneiss, schist, phyllite, clay, shale, mudstone and tuff, spe- cially with high content of such minerals as micas, chlorite, serpen- tine and clay. Panthi and Nilsen (2007) combined the semi-analytical method suggested by Hoek and Marinos (2000a,b) and the empirical formula proposed by Panthi (2006) for Himalayan rock mass conditions. They concluded that squeezing characteristics of the ground may be predicted using some statistical analysis concerning engineering geological properties of rock formations along tunnel alignment. The study of the behaviour of TBM in squeezing ground is the main objective of this paper, and this topic is investigated in details in the following works. Farrokh et al. (2006), using convergence-confinement method, introduced a new approach to calculate ground pressure on TBM shield in the vicinity of the tunnel face in the Ghomroud tunnel in Iran. Based on the results of the analysis, the required TBM thrust was calculated along various geological units. The study concluded that the adopted approach for predicting squeezing characteristic produced realistic results provided that reliable data on rock mass property and ground stress conditions were available. Farrokh and Rostami (2009) emphasized that the existence of weak structures in rock mass resulted in high rate of convergence 0886-7798/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2012.05.004 ⇑ Corresponding author. Tel.: +90 212 2856159; fax: +90 212 2856131. E-mail address: bilgin@itu.edu.tr (N. Bilgin). Tunnelling and Underground Space Technology 32 (2012) 58–65 Contents lists available at SciVerse ScienceDirect Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust