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Using RFPA software, the hydraulic pressure resistance and damage process of the grouted stone body of a tunnel in the high-pressure karst zone were analyzed, and the hydraulic pressure resistance of the composite surrounding rock and composite structure when the tunnel was expanded through the cavern under the effect of hydraulic coupling was investigated. Then, a new parallel heading expansion scheme for the New Yuanliang Mountain Tunnel was innovatively discussed, and the reliability of the results of the calculation model was verified by a field test. Lastly, the model calculation results guided the project to adopt a 5 m grouting range and advanced secondary lining to ensure that the composite structure’s water pressure resistance could reach 3 MPa; by adopting the improved technology of parallel heading expansion based on the CRD method, the safety of tunnel expansion through the cavity was effectively ensured, which can provide a reference for similar projects in the future.