A combined experimental and DEM study on the mechanical and failure characteristics of fissured sandstone under complex stress conditions.

In geotechnical engineering, fissured sandstone is frequently subjected to cyclic loading-unloading under complex construction conditions. Its mechanical properties and catastrophic failure characteristics directly influence the stability and safety of engineering structures. The mechanical behavior and failure mechanism of fissured sandstone under uniaxial and biaxial cyclic loading-unloading were studied using laboratory experiments, CT scanning, and numerical simulations. First, uniaxial compression experiments on intact and fissured sandstone specimens were conducted to obtain the stress-strain responses and peak strengths, providing the basis for subsequent numerical model calibration. This was followed by uniaxial cyclic loading-unloading experiments on the fissured specimens. By combining CT scanning and numerical simulation, the fissure evolution process was revealed from a macro-meso perspective. The reliability of the numerical model was verified. Based on the validated model, biaxial cyclic loading-unloading simulations were further carried out to investigate the mechanical responses and failure characteristics of fissured sandstone under complex stress conditions. Under uniaxial cyclic loading-unloading, the peak strength and elastic modulus ascended as the fissure angle grew, while crack initiation stress a trend of first decreasing and then increasing. Moreover, the extent of rock damage increased with the fissure angle. Under biaxial cyclic loading-unloading, an increase in fissure angle does not significantly change the peak strength and elastic modulus of the specimens but aggravates the failure dominated by shear. Compared with uniaxial cyclic loading-unloading, specimens subjected to biaxial cyclic loading-unloading exhibited higher crack initiation stresses, more extensive crack propagation, and more severe damage at the same fissure angle. The findings of this study provide theoretical support for the stability assessment and failure analysis of fissured rock masses under cyclic loading-unloading conditions.