Study on the failure mechanism of surrounding rock in upper and lower parallel tunnels regulated by horizontal spacing and joint dip angle

Horizontal spacing between twin parallel tunnels and surrounding rock joint dip angle are critical to tunnel stability. To clarify the underlying mechanisms, uniaxial compression tests were conducted on rock-like specimens with parallel holes (spacing: 0, 20, 40, and 60 mm) and prefabricated joints (dip angle: 0°, 30°, 45°, 60°, and 90°), with acoustic emission (AE) and digital image correlation (DIC) employed to monitor crack propagation and full-field strain evolution. Results show horizontal spacing affects specimen strength by regulating inter-hole stress superposition, with peak stress showing a nonmonotonic trend of first rising then falling and 20 to 40 mm spacing optimizing stress distribution to enhance load-bearing stability; joint dip angle dominates failure modes via crack propagation paths, as a 45° dip induces the strongest compression-shear coupling to maximize bearing capacity and increase mixed-mode cracks while shear cracks prevail at other angles. A boundary line with a slope of 100 enables quantitative distinction between different crack types, thus confirming the dynamic influence of these two parameters (horizontal spacing and joint dip angle) on crack modes. Additionally, numerical simulations conducted via COMSOL Multiphysics confirmed the existence of a “low-stress bridge” effect between the two parallel holes and elucidated the compression-shear coupling mechanism along the joint surface.