Experimental study on grouting-lining synergistic support mechanism of horseshoe-shaped tunnel specimens with prefabricated cracks

Underground construction is advancing into greater depths and more complex geology, and tunnels in fractured rock frequently exhibit insufficient bearing capacity and uncontrolled deformation. Grouting reinforcement and lining support are widely adopted to improve the surrounding rock quality and provide passive support. To explore the synergistic mechanism between grouting and lining, this study conducted uniaxial compression tests on rock specimens containing a horseshoe-shaped opening with prefabricated cracks. The tests evaluated various combinations of grouting materials, namely polyurethane/water glass (PU/WG), ordinary Portland cement (OPC), and calcium sulfoaluminate (CSA), with lining materials including PU/WG, gypsum (GY), and CSA. The results show that grouting materials with high elastic modulus significantly improve the overall stiffness of the support system, increasing the bearing capacity by up to 66.8%. The contribution of grouting to strength enhancement substantially exceeds that of the lining alone. Ductile linings such as PU/WG effectively restrain the relative horizontal widening Δ W / W 0 , reducing the Δ W / W 0 to as low as 0.49%. The rigid lining, such as CSA, is more conducive to controlling the relative vertical convergence Δ H / H 0 , lowering Δ H / H 0 to 0.86%. GY lining, due to its low tensile strength, tends to develop tensile cracks at the crown, resulting in significantly larger Δ H / H 0 . The optimal performance was achieved with a rigid-flexible combination of CSA grouting and PU/WG lining, which was limited to Δ H / H 0 =1.1% and Δ W / W 0 =0.18%. The failure pattern is transformed from the penetrating failure along the prefabricated crack to the crack initiation at the haunch and springing. These results highlight the importance of grouting materials with high elastic modulus for reinforcing defect zones, providing an experimental basis for synergistic grouting-lining design in fractured rock tunnels.