Geoenvironmental effects on distribution characteristics of bed-perpendicular joints in sandstone-mudstone interbedded strata: a case study from the Ordos basin

Abstract Background Bed-perpendicular joints (BPJs) are widely developed in soft-hard interbedded strata and reduce the integrity and mechanical properties of rock masses, which may further lead to increased permeability, insufficient bearing capacity and loss of local stability. Methods In the sandstone-mudstone interbedded strata of the Ordos Basin, more than 3500 BPJs were surveyed from 30 exposures. The effects of geoenvironmental factors, including stress environment, sandstone layer thickness ( T ), upper and lower mudstone layer thicknesses ( d 1 and d 2 ), and equivalent overburden sandstone thickness ( h ), on the distribution characteristics of BPJs were analyzed. Results Results showed that the strike of the BPJs is regionally controlled by tectonic stress and locally affected by slope strike. The joint spacing ( S ) is linearly related to the sandstone layer thickness ( T ) and the square root of the upper and lower mudstone layer thicknesses ( d 1 and d 1 ); however, it has a negative exponential relationship with the equivalent overlying sandstone depth ( h ). According to the statistical relationships between S and geoenvironmental factors, a comprehensive geoenvironmental factor $$\frac{{T\sqrt {{d_1}{d_2}} }}{h}$$ is proposed, which is used to establish an empirical nonlinear model for evaluating the joint development characteristics. The model contains two segment lines, indicating the nonlinear effects of geoenvironmental factors on BPJ development. Conclusion By theoretical analysis, it can be found that the model’s nonlinear form is caused by the nonlinear variation of maximum tensile stress in the sandstone layer, changing with the increasing thickness of the sandstone and mudstone layers. The new model indicates that the empirical linear relationships obtained from a small thickness range of the sandstone layer may have restrictions and limitations. A comprehensive investigation of geoenvironmental factors influencing BPJ distribution provides critical insights into their formation mechanisms, aiding in the prediction of joint development patterns in soft-hard interbedded strata.