Numerical Study on the Scale Effect in the Creep Mechanical Behavior of Intact Sedimentary Soft Rocks in Ordos Basin

ABSTRACT Although scale effects in jointed rock masses are well studied, the rheological behavior of intact sedimentary soft rocks (ISSRs) at different scales remains unclear due to experimental challenges and complex microstructures. This study explores the scale‐dependent creep behavior of ISSR, controlled by mineral heterogeneity and anisotropy. Using numerical modeling techniques, including the grain‐based model in particle flow code (GBM‐PFC) and ball assembling method (BAM), the creep characteristics of coarse sandstone and mudstone are captured. A theoretical model considering “anisotropy effect” and “volume effect” is proposed to predict long‐term strength. Simulation results indicate that larger samples exhibit increased isotropy, delaying accelerated creep onset, facilitating uniform stress distribution, and reducing deformation under equivalent stress. Consequently, long‐term strength and elastic moduli increase with sample size, stabilizing at an REV of 10–12 m. A “soft rock hardening” phenomenon is identified, linking enhanced isotropy and uniform stress distribution to improved strength. Furthermore, the study provides insights for assessing roof water inrush mechanisms. Comparative analysis of mining cases shows that the thickness of effective aquiclude critically influences the types of mine water inrush in Ordos Basin: Thinner effective aquiclude experience accelerated creep and leads to the gradual water inrush with no preceding water level drop, intermediate thicknesses lead to rapid inrush following water level drop with quick flow attenuation, while thicker effective aquiclude either result in sustained water level drops with possible delayed inrush or stable water levels with no inrush.