Computational Multiscale Model for NATM Tunnels: Micromechanics-Supported Hybrid Analyses

Knowledge of the stress state in shotcrete tunnel shells is a necessary requirement to assess the safety of these structures. Estimation of these stresses from measured 3D tunnel shell displacement requires material models for shotcrete. Preferably such models should be able to deal with on-site adaptations of the water-cement and aggregate-cement ratios. Therefore, we recall the fundamentals of continuum micromechanics and its application to shotcrete in order to upscale elastic and strength properties from the micron scale (where unhydrated cement can be discerned from its reaction products with water, called hydrates), via the intermediate scale of cement paste, to the shotcrete scale. Comparison of model predictions with experimental data shows that the elasticity and strength evolutions of hydrating shotcrete can be predicted reasonably well from mixture- and hydration-independent elastic properties of aggregates, clinker, hydrates, water, and air, and from strength properties of hydrates. At the structural level, the micromechanics models are combined with 3D displacement measurements of a shotcrete tunnel shell. This hybrid approach provides insight into the load-carrying behavior of the tunnel, and it allows for a safety assessment of the shell's structural integrity.