The tunnel group of the “818 Project” spans approximately 30,000 m², located within the park area of Tapo Mountain. The project comprises seven horizontal tunnels, connecting tunnels, and nine ventilation shafts, forming a complex cavern system characterized by shallow burial, large cross sections, and complex construction conditions. This study aims to ensure construction safety and surrounding rock stability in this super-large cross-section and closely spaced complex tunnel group situated in karst areas.

The acoustic wave, Barton rock mass quality (Q) classification, and high-density resistivity methods were adopted to investigate the surrounding rock classification and karst development in the tunnel group at planar and spatial intersections. The study area’s surrounding rock grade and karst characteristics were assessed to provide technical support for tunnel construction and support design.

The acoustic wave method was used to measure the compressional wave velocities of the intact rock and rock mass. The crack coefficient of the surrounding rock was 0.266–0.301, indicating a fractured rock mass. The Q classification method yielded key parameters, including rock quality designation, number of joint sets, joint roughness coefficient, joint alteration coefficient, joint water reduction coefficient, and stress reduction coefficient. The classification results showed that the surrounding rock of Tunnels A10–A17 falls within Grades Ⅳ and Ⅴ, the shallow-buried portal sections are Grade Ⅴ, and the deep-buried tunnel sections are Grade Ⅳ, presenting extremely poor surrounding rock quality. A total of 14 longitudinal and transverse survey lines were measured using the high-density resistivity method. Apparent resistivity (ρ_s) contour maps and high-density resistivity profiles were plotted using Surfer software and facilitated geological structure identification through data inversion. The results showed that the highly weathered bedrock is well developed, with dense joints and fractures and a severely broken rock mass. Bedrock is exposed at the surface with well-developed karren and grikes. The thickness of the overburden varies from 0 to 5 m, whereas the thickness of the highly weathered layer ranges from 2 to 14 m, with four karst caves detected onsite.

The comprehensive geophysical prospecting combining the abovementioned three methods achieves excellent detection, thereby accurately interpreting the geological conditions of the surrounding rock in the complex tunnel group. This approach provides a scientific basis for designing, constructing, operating, and maintaining complex tunnel groups in karst areas.