This study developed a wet-mix shotcrete mixture incorporating a superabsorbent polymer (SAP) to enhance water-leakage resistance and evaluate its field applicability. Accordingly, the type and dosage of SAPs were selected to maximize self-sealing while minimizing compressive strength loss. The developed SAP shotcrete mixture was evaluated through laboratory tests of its rheological properties, mechanical performance, and shrinkage behavior. Subsequently, large-scale shotcrete placement was performed on a tunnel mock-up structure (3.0 m in width, 3.2 m in height, and 2.5 m in length) to assess its field applicability in terms of mechanical properties and water-leakage resistance. The test results showed that the incorporation of 1 % acrylamide–acrylic acid copolymer SAP by cement weight led to a reduction in mechanical strengths; however, it markedly decreased water leakage by 68.3 %. In addition, the shrinkage was reduced, thereby improving the crack resistance. However, during the water absorption stage of the SAP, the static yield stress, dynamic yield stress, and plastic viscosity increased, resulting in decreased constructability. Conversely, when absorbed water was released from the SAP, it delayed the time-dependent increase in the dynamic yield stress and induced shear-thinning behavior, which partially improved the shotcrete placement performance. Therefore, in the application of SAP-incorporated shotcrete, time-dependent rheological changes should be considered when selecting the SAP type and dosage, as well as in determining the appropriate construction window.

Tunnels have been extensively constructed to improve the efficiency of human mobility and material transport by penetrating natural obstacles, such as mountains, seabeds, and rock masses . They are typically excavated using methods such as blasting, followed by the installation of linings to ensure structural resistance against external forces during both construction and operation. Among the various techniques, the shotcrete method is widely employed because of its ability to provide immediate support and high constructability. Shotcrete refers to a technique in which flowable concrete is pneumatically projected at high velocity through a nozzle and adheres immediately to the excavation surface with the aid of a set accelerator, enabling rapid strength development . In shotcrete lining systems, groundwater leakage remains a critical issue that can significantly undermine tunnel stability and durability. Prolonged leakage can lead not only to the functional degradation of the structure but also to catastrophic failure. Accordingly, a range of strategies—such as the application of waterproof membranes and impermeability enhancement through crack control—have been investigated and implemented to address leakage issues. However, the demand for more effective and construction-friendly technologies continues to grow.