A note on the design of fibre-reinforced shotcrete linings for underground support

The ability of sprayed concrete to hold the rock mass around an opening, avoiding progressive loosening and destabilization, is widely recognized. On the other hand, design of shotcrete supports is still more of an art than a science, with the majority of designs based on experience, on empirical recommendations or on monitoring and adapting during construction. In order to improve both the mechanical and the durability characteristics of sprayed linings, a recent trend is to replace the ordinary mesh reinforcement with steel fibres. It is well-known, in fact, that the usage of high-strength fibres (e.g. steel fibres) in the concrete mixture permits to maintain a high compressive strength while considerably improving the toughness and thus the structural ductility. In this paper, the evaluation of the strength and ductility characteristics of fibre-reinforced shotcrete linings is performed by adopting suitable theoretical and numerical models. Analyses show that failure of a shotcrete support is generally a two-step process, initially generated by loss of adhesion to the rock, followed by flexural failure. Flexural strength and toughness play a critical role in the post-peak region once adhesion is lost. After analysing the stress state in traditional underground configurations, the bridged crack model is used in order to define the correct softening curve of the material, based on the fibre content. By means of a numerical code based on the cohesive crack model, the post-peak behavior of the structure is investigated. It is shown that the fibre reinforcement greatly enhances the flexural and punching shear strength, as well as the energy absorption capacity.