Effect of epoxy grouting-induced interfacial bond strength on the critical loading angle and mixed-mode I-II fracture behavior of fractured sandstone

Abstract

Understanding the fracture behavior of grouted fractured rock masses is critical for enhancing structural stability and fracture control. This study investigates the mixed-mode I–II fracture behavior of fractured sandstone grouted with epoxy resin, focusing on the effects of grout strength and loading angle. A theoretical model—termed the Critical Loading Angle Model (C-LAM)—was developed to predict the critical loading angle (β_c) for pure Mode II fracture, incorporating grout–rock interfacial cohesion. Brazilian splitting tests were conducted on center-cracked Brazilian disc (CCBD) specimens grouted with epoxy grout at varying mix proportions. Fracture processes were monitored using acoustic emission and digital image correlation techniques. The results show that both interfacial bond strength and crack inclination modulate the internal stress field and jointly influence the fracture behavior. High-strength grout enhanced interfacial cohesion, forming a reinforced zone that redirected the main crack to bypass the grouted region. In contrast, low-strength grout led to plastic-dominated failure, with crack morphology resembling that of ungrouted specimens. Crack inclination further affected fracture response, with the main crack either bypassing or penetrating the grout depending on the loading angle. The C-LAM model predicts a nonlinear decrease in β_c with increasing interfacial cohesion, indicating an inverse correlation. Crack-type classification based on internal and external monitoring data validated the model’s predictive capability. These findings elucidate the mechanism by which grouting modifies fracture mode transitions and provide a predictive framework for Mode II failure in reinforced rock systems.