Executive Summary : | Tunnel infrastructure in India is growing rapidly with a robust pipeline of projects in the urban mass transit, railways, metros, roads, and highway sectors. Currently, precast segmental tunnel lining construction with a tunnel boring machine (TBM) is the most prevalent tunnel construction technology in India. In this technology, precast concrete segments are consecutively installed in the form of rings within the TBM skin. Further, the installed tunnel ring serves as the reaction frame against which TBM exerts force for tunnel advance. In this process, segments subjected to TBM jack thrust forces may crack due to bursting and spalling stresses which, can affect the long-term performance, durability, and service life of the tunnel. To prevent cracking in tunnel segments, steel fibre reinforced concrete (SFRC) is used which has higher fracture toughness and better crack width control compared to plain concrete. While SFRC is advantageous in controlling crack width, it does not have the same load-bearing capacity as that of conventional steel-reinforced concrete against the TBM jack thrust forces. Further, the design of SFRC segments is based on the quasi-brittle fracture mechanics concepts where the fracture toughness of SFRC is computed based on the standard bending tests. However, the segments under the TBM jack thrust forces are under biaxial stress state comprising of traverse tensile stresses and compressive stresses parallel to the crack. Recent results of novel gap test revealed that the presence of crack parallel stresses in a beam under bending leads to a significant reduction in the fracture toughness of the material. This result has direct relevance in the tunnel segments as the material in the tunnel segments is in a similar state of biaxial stress. Therefore, while employing SFRC in tunnel segments, it is imperative to understand its fracture behaviour under bursting and spalling stresses. The proposed research aims to investigate the mechanics of bursting and spalling fractures in SFRC through a rigorous experimental program followed by a numerical study. |