Acoustic emission analysis of shear processes in red sandstone with soft red bed interlayers

Landslides in the red beds are associated with potential slipping along structural planes, which is further aggravated by the presence of weak interlayers within these planes. These weak interlayers, characterized by soft red bed materials with loose structures, play a critical role in the slope slide. However, the precursors to sliding caused by these weak interlayers remain unclear. To address this, the study employed acoustic emission (AE) monitoring—an approach widely recognized for its effectiveness in detecting precursory signals—to investigate potential indicators of slip. AE monitoring was conducted during direct shear tests on red bed interlayers containing soft red rock layers to analyze their behavior prior to slip. The findings reveal that during shearing, the soft red bed in the interlayer first consolidates and then transitions into frictional sliding, resulting in complex and diverse stick-slip events. AE signals show a marked increase in AE activity preceding stick-slip events. Additionally, the friction coefficient increase during shearing is correlated with a decrease in the AE b -value. The AE analysis divides the shearing process into three distinct stages: the initial stage is marked by low AE rates and the highest b -value, the second stage shows increasing AE rates with the lowest b -value, while the last stage, characterized by the highest AE rates, corresponds to a relatively low b -value. Moreover, the RA-AF value distribution of AE events indicates that the shear process in the interlayer is predominantly governed by tensile fractures, along with contributions from shear fractures and tensile-shear composite fractures. Furthermore, direct shear tests were conducted on red bed interlayers with varying water contents (10%, 20%, and 30%) to examine the influence of moisture on shear behavior. Results show that higher water content also weakens AE activity, reduces the number of AE events and energy release, and shifts the failure mode from tension-dominated to shear-dominated. These results suggest that the evolution of AE rate, b -value, and event distribution provides effective precursory indicators for instability in red bed slopes containing soft interlayers.