Effects of fissure inclination and spacing on crack evolution and mechanical behavior of 3D-printed horseshoe tunnel models: Experiments and SPH simulations

To elucidate the regulatory mechanisms of prefabricated fissures on the mechanical behavior and crack evolution of horseshoe-shaped tunnels, this study employs sand-based 3D printing technology to fabricate tunnel models containing prefabricated fissures with varying inclinations (θ = 0°–90°) and spacings (L = 30–65 mm). Uniaxial compression tests, coupled with digital image correlation (DIC) and improved smoothed particle hydrodynamics (SPH) simulations, are conducted to systematically investigate the influence of fissure parameters on crack initiation and propagation patterns, peak stress–strain responses, and failure mechanisms. The results demonstrate that fissure inclination governs crack types by modifying the ratio of normal to shear stresses along the fissure plane. At θ = 15°, shear stress predominates, resulting in the lowest peak strength (∼3.0 MPa), whereas at θ = 75°, tensile stress dominates, yielding the maximum peak strength (∼4.8 MPa). Fissure spacing influences failure modes via stress field interaction: significant stress superposition at L = 35 mm leads to pronounced strength degradation (peak stress ∼ 2.5 MPa), while minimal stress interference at L = 50 mm corresponds to optimal mechanical performance (∼4.5 MPa). Observations from DIC and numerical results from SPH simulations collectively validate a chain mechanism linking fissure geometry, stress concentration, and crack propagation. Specifically, small inclinations (0°–30°) and narrow spacings (30–40 mm) facilitate multi-crack coalescence and extensive damage zones, whereas large inclinations (75°–90°) and wider spacings (50–65 mm) favor localized brittle failure. These findings offer quantitative guidance for tunnel stability design under complex geological conditions.