Back Analysis of a Tunnelling Case Study in Weak Rock of the Alpine System in Northern Greece: Validation and Optimization of Design Analysis Based on Ground Characterization and Numerical Simulation
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The backdrop for this research paper is the tunnelling that is currently nearing completion in the Epirus region of Northern Greece, as part of the Egnatia Odos Highway construction. Highly deformed and altered sediments and low grade rock masses dominate the near surface environment creating a variety of technical challenges for tunnelling. Accurate equivalent rock mass performance reductions for tunnels in these materials is complicated by geomorphologic peculiarities such as those found in Flysch materials. The mechanisms of rock-support interaction related to face or near-face reinforcement systems are poorly understood at this time. As well, the mechanics of weak rock materials in the complex deformation regime in advance of a tunnel face are not robustly integrated into current 2D design models. Design decisions are currently possible using empirical techniques and simplified models, but a true optimized and mechanicsbased design process for the various support technologies are not fully developed. This research addresses elements of such issues, such as: use of the Longitudinal Displacement Profile (LDP) of the Convergence-Confinement method of tunnel design, relating 2D numerical models to their distance from the face using the size of the plastic zone as an indicator, near face tunnel support analysis in weak rock masses, boundary condition assessment for numerical modelling of such weak rock masses, the influence of plasticity zones surrounding tunnel excavations, and modelling optimization techniques for weak rock tunnelling in order to optimize the design of such underground structures and better predict near-face deformation and yield development. This work involved the use of 2D and 3D numerical models of tunnel sequencing for numerical simulation of composite material behaviour and sequential tunnel deformation response.