Numerical study of the role of fibres in the preliminary lining of tunnels

The design of preliminary lining has a fundamental role in the stabilization of tunnel working, before the completion with the final concrete lining. Among the several types of lining, shotcrete is one of the most widespread, but since its limited tensile strength it is usually reinforced using welded steel fabric or coupled with steel ribs and anchorage systems in the soil. Nowadays, the designers, instead of using welded steel fabric, prefer more and more often the use of standard concrete reinforced with steel and/or synthetic fibres, which are usually added during material mixing. Recent studies [1] have shown that the use of fibres, considering proper volumetric percentages, fibre lengths, aspect ratios and bond properties, also mixing, if necessary, different types of fibres (hybrid fibres), has a positive effect on all FRC mechanical properties, and particularly on its tensile strength, ductility, workability and shrinkage. As the material shows a larger toughness, the presence of fibres especially improves the properties which are typically related to the post-cracking behaviour; so fibres are often used instead of traditional reinforcement in several applications. In this work the attention will be focused on preliminary lining realised using shotcrete. A series of numerical analyses aimed to determine the role of fibres with respect to traditional welded steel mesh on the structural behaviour of preliminary lining will be carried out by performing transversal and longitudinal bidimensional tunnel finite element models having different kind of lining (concrete, traditionally and fibre reinforced concrete. In order to perform realistic simulations, the local behaviour of concrete reinforced by fibres and ordinary bars must be properly modelled to realistically describe the soil-structure interaction forces, stress and strain fields of the single materials as well as crack pattern and failure modes of structural elements. To this scope the PARC [2, 3] constitutive model, which is able to describe the behaviour up to failure both of ordinary and fibre reinforced concrete structures, will be adopted to perform non linear finite element analyses