Experimental and Numerical Investigations of Mechanical Properties and Fracture Behaviors in Fluvial‐Lacustrine Sedimentary Rocks Under Uniaxial Compression

ABSTRACT Fluvial‐lacustrine sedimentary rocks with complex depositional origins and microstructures are widespread in underground engineering strata in western China, yet the mechanisms underlying differences in their mechanical properties and fracture behaviors remain unclear. Four representative rocks from the Huanglong Coalfield in the Ordos Basin were investigated using polarizing microscopy (PM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), uniaxial compression with acoustic emission (AE) monitoring, and two‐dimensional Particle Flow Code (PFC2D) simulation. Key findings include: (a) The littoral‐shallow lacustrine mudstone deposit (LSLMD) specimen contains 42.2% clay minerals and is characterized by argillaceous cementation and locally developed submicron intercrystalline micropores in the selected SEM field; the channel deposit (CD) specimen contains 56.3% quartz and feldspar and comparatively open intergranular pores with apparent dimensions of 5–25 µm in the selected SEM field. The natural levee (NL) and CD specimens show higher strength and brittle shear failure; the channel bar (CB) and LSLMD specimens show lower strength and tensile‐shear composite or tensile failure. (b) Gaussian mixture model (GMM) classification yields cumulative tensile‐microcracking proportions of 84.5%, 63.7%, 70.6%, and 69.4% for NL, CB, LSLMD, and CD, respectively. Shear microcracking increases during late loading in NL and CD. (c) The PFC2D models reproduce peak strength and corresponding axial strain with absolute relative errors of 0%–7.95% and capture the observed macroscopic failure modes. Shear microcracks localize and coalesce along inclined directions in NL and CD, whereas tensile microcracks interconnect approximately vertically in LSLMD. These results establish multiscale links among depositional microstructure, microcrack evolution, and macroscopic failure.