Instability Mechanism of Expansive Soil Slopes Based on the BExM Model: A Study on the Control Effects of Macro and Micro Parameters

The presence of expansive soil interlayers poses a severe threat to the long‐term stability of engineered slopes due to their complex hydro‐mechanical behavior. To investigate the intrinsic instability mechanism, this study develops a two‐dimensional finite element model of a three‐layer slope with an expansive soil interlayer. The slope's response under coupled excavation and impoundment is simulated using the Barcelona Expansive Model (BExM), which effectively captures the double‐structure characteristics of expansive soils. A systematic sensitivity analysis is conducted to examine the controlling effects of macroscopic strength and microstructural swelling parameters. The results identify the interlayer as the core weak zone governing slope deformation. The numerical simulations reveal a progressive failure mechanism where plastic strain preferentially initiates at the slope toe and propagates deep along the interlayer, forming a continuous shear band. This internal damage evolution manifests macroscopically as a distinct differential deformation pattern characterized by heave at the toe and settlement at the crest. Furthermore, the study reveals a dual‐control mechanical mechanism where slope stability is determined by the interplay between the intrinsic macroscopic shear resistance and the internal wetting‐induced swelling drive. In terms of practical engineering, these findings imply that traditional strength‐based assessments are insufficient. Effective slope design must explicitly quantify microstructural swelling parameters to accurately predict long‐term deformation and prevent progressive failure triggered by hydrological changes.