How does shotcrete application contribute to immediate ground support after TBM excavation?

Shotcrete application provides immediate ground support after TBM excavation by creating a thin, hardened layer on the exposed rock or soil surface. This layer acts as a temporary skin, preventing or minimizing the immediate deterioration and loosening of the ground. It achieves this through several mechanisms. First, it provides confinement to the exposed surface, preventing small pieces of rock or soil from falling out. This is especially important in fractured or weak rock masses. Second, it seals the surface, reducing the ingress of air and water, which can cause weathering and erosion. This helps to preserve the rock's or soil's strength and stability. Third, it bonds to the surface, creating a composite structure that is stronger than the individual components. This bond helps to distribute loads and prevent localized failures. The rapid setting time of shotcrete is crucial for providing immediate support. Accelerators are often added to the shotcrete mix to speed up the setting process. Fiber reinforcement, such as steel or synthetic fibers, is often added to the shotcrete to increase its tensile strength and ductility. This helps to prevent cracking and improve its ability to withstand deformations. Shotcrete application is typically performed immediately after excavation, using specialized equipment that sprays the shotcrete onto the surface. The thickness of the shotcrete layer depends on the ground conditions and the expected loads. The term 'shotcrete' refers to concrete that is pneumatically projected onto a surface. 'Ground support' refers to measures taken to stabilize the ground around an excavation. 'Confinement' refers to the act of restraining movement or deformation.