Fracture behavior of compacted clay under mixed-mode I/II loading: Size effect analysis

Compacted clay serves as a critical material for the core wall of earth-rockfill dams. The investigation into its fracture behavior is paramount for ensuring the operational safety and long-term stability of these structures. Pertaining to the size effect on soil fracture characteristics, prior investigations have predominantly focused on Mode I fracture. Conversely, research addressing the mixed-mode I/II fracture behavior, particularly concerning scale dependency, remains sparse. This study employs the cracked single-edge notched deep beam (CNDB) specimen configuration to execute mixed-mode I/II fracture tests on compacted clay. Macroscopic fracture characteristics, including the load-displacement curve, crack initiation angle, and critical stress intensity factors (K_If and K_IIf), were systemically characterized under the influence of the size effect. Furthermore, the Digital Image Correlation (DIC) technique was utilized to analyze the full-field displacement and strain fields, thereby quantifying the deformation state in the vicinity of the crack tip. The results demonstrated that the bearing capacity of the specimens exhibited an increasing trend with the enlargement of the specimen size. The size effect showed no statistically significant influence on the crack initiation angle. With the increase in the pre-fabricated crack inclination angle, K_If exhibited a decreasing trend, while K_IIf first increased and then decreased; both critical factors clearly manifested size dependency. Under pure Mode II loading, the failure mechanism of CNDB specimens was characterized as a tensile-shear fracture, primarily governed by the tensile stress component. The length of the fracture process zone (FPZ) was observed to increase with the pre-fabricated crack inclination angle, a trend further exacerbated by the enlargement of the specimen size. Finally, the Tangential Stress Contour (TSC) method was successfully validated for its accuracy in predicting the fracture load across different specimen dimensions.