Punching fracture mechanism and strength formula of early-age shotcrete

Abstract In tunnel construction, shotcrete is sprayed on excavated surfaces to strengthen the walls and prevent rocks from falling off the tunnel face. Soon after, workers enter the face to erect the steel arch support. However, before entering, they must know the maximum rock size supported by the early-age shotcrete. This study aims to reveal the resistance and fracture mechanism of early-age shotcrete when a rock exfoliated from the tunnel face punches through shotcrete. To this end, we developed a novel experimental apparatus in which the strength deformation characteristics of shotcrete were evaluated through a laser scanner by punching a disc rock against the shotcrete on the substrate rock. The punching experiment was simulated, and the fracture mechanism was revealed using the discrete element method. The stress state of shotcrete sequentially reached the tensile failure criterion at the elements adjacent to the edge of the loaded zone. The elements of shotcrete progressively failed at the fracture process zone with punching. Finally, the punching strength of early-age shotcrete was formulated based on its fracture mechanism.