Numerical analysis of ground-shotcrete interaction in tunnelling

The increasingly frequent use of shotcrete as primary support in tunnelling is not always accompanied, at the design stage, by analyses capable of describing its mechanical behaviour. In the first part of this paper the experimentally observed behaviour of shotcrete and its main properties as a supporting material are outlined, together with a critical overview of the general approaches adopted in modelling ground-support interaction in tunnelling. The results of three-dimensional parametric analyses of tunnel excavation and support are then presented. The analyses have been performed in axi-symmetric conditions, assuming an elastic constitutive law characterised by a time-dependent stiffness for the shotcrete. The numerical results are then compared in order to determine the influence of the following factors: constitutive law, strength parameters, tunnel advancement rate, relative ground-support stiffness. In the final part of the paper, simplified techniques are proposed to introduce the main features that emerged from the axi-symmetrical approach in the standard two-dimensional analysis.