Analysis of Tunnel Liner Response in Squeezing Heterogeneous Rockmasses
Branscombe, Laura C.
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Squeezing ground in tunnelling is a unique method of failure, typically occurring in ductile rock at depth. To accommodate this mechanism of failure, liners have been designed such that they reduce in circumference as the support takes on initial load. Technological development in this area is all the more important for tunnels mined in heterogeneous rockmasses. Currently, the state of practice is to design support based on a normalised average of the strength parameters of the rock units present in the tunnel face. Under many configurations of heterogeneity, this approach is perfectly valid, however there are styles and magnitudes of heterogeneity that require deeper analysis. The purpose of this study is to introduce an empirical relationship between liner response in heterogeneous rockmasses and liner response in the homogenised equivalent. A suite of parameters that define heterogeneity were numerically modelled, using finite element analysis, such that a comprehensive set of the various permutations was evaluated. The three styles of heterogeneity that were modelled were chaotic structure, folded structure and laminar structure, these represent common and discrete structures found in alpine regions. Laminar structure was considered at three orientations, and all structures were considered under hydrostatic and differential stress regimes. Granularity, or scale, and contrast ratio of units were also varied. These data were analysed with respect to mean axial thrust, range of axial thrust and range of moment on the liner. Liner response analysis was also conducted on the case study of St. Martin-la-Porte, an adit to the Lyon-Turin Ferroviaire base tunnel in southern France. This adit has been excavated in a highly variable, heterogeneous rockmass and provides realistic examples of the nature of heterogeneity present in current tunnelling projects. Ultimately, three equations were developed to provide preliminary predictions of heterogeneous axial thrust and moment variability from the analysis of homogeneous equivalents.