On the basis of ascertaining the classification bases of tunnel collapses, i.e. type of tunnel, cause of collapse, speed of collapse, location of collapse in the tunnel, scale of collapse (including length and volume), form of destruction, economic loss and casualty, time-related characteristics, various tunnel collapses were systematically classified in this paper through the collection and analysis of the tunnel collapses. They can be classified as mountain tunnel collapses, urban tunnel collapses or submerged tunnel collapses; collapses caused by stability of surrounding rock or caused by insufficient supporting intensity; sudden collapses, intermittent collapses or slow deformation collapses; tunnel portal or tunnel truck collapses; minor collapses, medium-size collapses or large-scale collapses; local collapses, arch collapses, irregular collapses, penetration collapses; general collapses, large collapses, very large collapses or extremely large collapses; collapses before preliminary bracing, collapses before secondary lining and collapses after the completion of support. Finally, the formation mechanism and typical characteristics of main tunnel collapses were also analyzed.
During the large-scale construction of tunnels in China, accidents occur frequently, which causing serious economic losses and casualties, and among that, collapse is the most common accident in tunnel construction. According to the statistics from 2001 to 2016, collapses account for 56 percent of all the accidents during the tunnel construction, some of which in China are listed in Table 1.
In spite of numerous researches on prevention and control of tunnel collapses (ZHANG Dingli et al., 2014, 2017a, 2017b; SUN Jun, 2006; WANG Yingchao, 2010; QIAN Qihu, 2012), the situation remains severe as a whole. It is generally accepted that collapses of different triggers require dissimilar control measures, as a result of which, classifying those accidents is vital in accordance with respective characteristics (WU Qiang et al., 2013). According to the form and scale of collapse, it is divided into four categories: penetration collapse of large scale, collapse of more than 3 meters, local collapse of minor scale and block falling from roof (FU Siming and LI Yuan, 1998). According to the cause of collapse, it is divided into four categories, namely collapse caused by unfavorable geological condition, groundwater, ground stress and improper design and construction (ZHENG Yuxin, 1999). According to the shape of collapse, it is divided into local collapse, arched collapse, irregular collapse, large deformation collapse and rock burst (ZHENG Yuxin, 1999; GUAN Baoshu, 2007); it can also be divided into local collapse, arched collapse and penetrated collapse (WANG Chengbing, 2007). According to the mechanical behavior of collapse, it is divided into five categories, namely collapse subjected to gravity, fracture loosening, tension, bending and shear (XU Linsheng et al., 2002). Classifications of tunnel collapse have generated considerable recent research interest, but most of the researches concentrate merely on one or two aspects in their study, making the process oversimplified and unsystematic.