Study on the discrete element mesoscopic mechanisms of anchorage composite layered rock masses under cyclic loading

As the scale of rock engineering projects continues to expand, the stability of composite layered rock masses and anchors breakage caused by deformation damage in weak layers have become increasingly prominent issues. Meanwhile, water level rising and falling or rainfall is the main force source that causes the rock masses anchorage system to be subjected to cyclic loading and unloading conditions. It is crucial to recognize the anchoring and micro-deformation mechanisms of composite layered rock masses with weak layers for designing slope engineering stability control. Therefore, combining two-dimensional(2D) discrete element method and indoor pullout test, the resistance of anchor, the displacement field and micro-fractures expansion characteristics within the rock masses under different loading methods are investigated at the mesoscopic level. The results reveal that cyclic loading effect is equivalent to a kind of "time acceleration", significantly shortening the time required for the rock mass anchoring structure to reach its preset service life. The displacement field of the rock mass is symmetrically distributed in a “U” shape, and there is shear contraction of the anchor. Convergent micro-fractures are distributed at the end of the anchor, and the extension range of the micro-fractures increases with the increase of the number of cycles. When the rock masses contains mudstone layers, the mudstone rock particles shows significant shear dilatation under cyclic loading. With increasing thickness of the mudstone layer, the peak pullout resistance of the rock-anchorage system decreases to 60.93%, and the distribution characteristics of micro-fractures change from uniform distribution to an "inverted V" shape distribution. Under the same mudstone layer thickness, the number of micro-fractures is more in high-frequency cyclic loading compared to low-frequency cyclic loading, and its distribution is mainly in the mudstone rock interior.