Loose circle and rock mass strength tests of extremely thin layered surrounding rock in large deformation tunnels

The scope of the surrounding rock loosening and its mechanical properties can significantly affect the analysis of the excavation loosening range, stress-strain state partitioning, and surrounding rock mass stability. Investigating the mechanical properties and failure modes of the surrounding rock mass is of significant engineering value for optimizing support design and ensuring the safety of tunnel construction and. Therefore, the tests on loose circle and mechanical properties of the rock mass were conducted, yielding the following results. (1) The sonic wave testing method and ground penetrating radar (GPR) detection were more suitable for loose circle testing of extremely thin-layered phyllite than the True Reflection Tomography (TRT) method and borehole camera method. (2) The loose circle ranges for the test sections were 4.9 m and 6.8 m, respectively. (3) The cohesion and internal friction angle of the test sections were less than 0.3276 MPa and 19.0°, respectively. The test results were consistent with the predominant phyllite composition of the surrounding rock. This study combined in situ tests and site conditions to analyze the causes of tunnel deformation and instability, offering recommendations for tunnel support. These findings served as valuable guidance for similar tunnel support design and construction, and play a crucial role in ensuring constructor safety.