Understanding the compression failure mechanism of rock-shotcrete composites using X-CT and DIC technologies

In recent decades, the permanent single-layer tunnel lining has been developed to meet the requirements of the deep-buried and over-long tunnel projects. In this study, macro-synthetic polypropylene (PP) fiber-reinforced shotcrete was designed for the permanent single-layer tunnel lining. Uniaxial compressive tests were carried out on the three rock-shotcrete composites (G-NSC, S-NSC and S-FSC) with two types of rocks [granite (G) and sandstone (S)] and two types of shotcretes [normal shotcrete (NSC) and fiber-reinforced shotcrete (FSC)] to investigate the failure mechanism of the composites. Combined with X-ray computed tomography (X-CT) and digital image correlation (DIC) technologies, the meso-structures of shotcrete, the strain development, crack propagation and damage evolution laws of different composites were analyzed. The results showed that macro-synthetic PP fiber increased the porosity and the volume of large-diameter pores of the shotcrete. The microcracks mainly emerged and propagated in the shotcrete for G-NSC, but occurred and gathered near the rock-shotcrete interface of sandstone-based composites. Besides, macro-synthetic PP fibers can greatly increase the ductility of the composite, and cracks tended to propagate along with the interface between aggregates and mortar in FSC. Finally, damage calculation model based on the dissipated energy was proposed for the rock-shotcrete composites.