Investigating Load Arches and the Uplift Capacity of Rock Anchors: A Numerical Approach

Rock anchors, which can withstand substantial forces and are essential for securing the foundations of the built environment, can fail due to steel tensile failure, bond detachment, or rock mass fracturing through uplift. In this study, uplift failure of the rock mass surrounding rock anchors was simulated using both physical and numerical models. The physical models used a specially designed testing rig with predefined sizes and different joint layouts to simulate rock mass uplift failure. The numerical models were calibrated using the physical models and used to investigate scenarios involving increased complexity and greater depths. This paper presents the load distribution within rock masses with orthogonal joints when exposed to an anchor load. Extensive analysis of the numerical models revealed that symmetrical load arches were induced in rock masses that contained a joint set oriented parallel to the loading direction. However, joints that were obliquely oriented relative to the anchor axis induced asymmetrical load arching. The greatest displacement occurred in the direction parallel to the joint set with the smallest angle to the anchor loading axis. Large-scale models with zero or near-zero in situ horizontal stress revealed that the load capacity in a rock mass with continuous vertical joints was relatively low, especially when compared to rock masses with discontinuous vertical joints. It was observed that the uplift capacity of the rock mass around an anchor decreased if the load arches of adjacent anchors overlapped; consequently, a decrease in anchor spacing was found to correspond to a decrease in uplift capacity. Symmetrical load arches are induced within a rock mass if the joint sets are oriented parallel to or close to parallel to the anchor axis. An oblique orientation of the joints relative to the anchor axis results in asymmetrical load arching in the rock mass when the anchor is loaded. Rock masses with continuous vertical joints have a low load capacity, especially compared to rock masses with discontinuous vertical joints. In a row of rock anchors, the rock mass uplift capacity of individual anchors is decreased if the anchor spacing is too low.