Mechanical properties and failure mechanisms of rock analogs under unilaterally confined conditions: Decoding borehole layout effects via AE and DIC monitoring

The hole layout critically governs the borehole-induced stress redistribution and the consequent mechanical behavior of the surrounding rock. Rock analog samples with various borehole configurations were prepared and tested under unilaterally confined compression using a self-developed L-shaped steel mold to quantify the effects of the hole diameter, depth, and spacing. The mechanical responses, acoustic emission (AE) signatures, and fracture evolution of the samples were captured using AE monitoring and digital image correlation (DIC). The key findings revealed that increasing the hole diameter or depth, or reducing the spacing led to decreases in both the peak strength and elastic modulus. When the hole diameter increased from 6 mm to 10 mm, the peak stress decreased by 18.5–24.7 %, and the elastic modulus decreased by 9–12 %. Large diameters and small spacing promoted a 20–35 % increase in pre-peak AE event rates. Deep holes exhibited tensile-dominated, localized energy release. Moreover, deep holes exhibited tension-controlled localized energy release. Small-diameter or shallow holes induced tensile failure at hole mouths, whereas deep holes or large-diameter holes caused 40–60 % more network cracks. Small spacing accelerated crack bridging and penetration, which was delayed by medium depths. Overall, the failure scale was governed mainly by the hole diameter, structural complexity by depth, and synergistic effect of spacing. A rational combination of a shallow-to-medium hole depth and moderate spacing reduced the peak stress by 10–15 % and improved structural stability. These findings provide a theoretical basis for optimizing borehole decompression under asymmetric boundary conditions in deep, high-stress rock masses.