13. Design of shotcrete shells within the framework of the New Austrian Tunneling Method (NATM)
In the NATM, a shotcrete shell is used as primary support for the freshly excavated stretch of a tunnel. Elaborate monitoring devices allow for the estimation of the internal forces in the tunnel shell, e.g. through a hybrid method by which displacement vector fields are approximated from measured displacement vectors at discrete points of the tunnel shell and used as boundary values in a three-dimensional finite element model of the tunnel shell. Structural computations require an elaborate material model for shotcrete, accounting for mechanical properties that change because of hydration. Recent advances in micromechanics and nanotechnology have opened a new gateway to relations between the degree of hydration and mechanical properties: Based on mathematical descriptions of the material morphology within representative volume elements, shotcrete-independent constituents (cement, hydrates, water, air) and their hydration degree-dependent dosages define the overall material behavior.
Good agreement between model-predicted evolutions of elasticity and strength and corresponding experiments across water-cement ratios ranging from 0.35 to 0.60 shows that (i) elastic properties of mature cement paste can be estimated reliably on the basis of both spherical or acicular shaped hydrates, but that (ii) the development of a reliable strength model does require consideration of non-spherical hydrates. At the structural level, the micromechanics model, when combined with 3D displacement measurements, shows that a decrease of the water-cement ratio results in an increase of the safety of the shotcrete tunnel shell.