Experimental insights into anchorage performance of en-echelon joints under cyclic shear loading

Abstract

Understanding the anchorage performance of en-echelon joints under cyclic shear loading is crucial for optimizing support strategies in jointed rock masses. This study examines the anchorage effects on en-echelon joints with various orientations using laboratory cyclic shear tests. By comparing unbolted and bolted en-echelon joints, we analyze shear zone damage, shear properties, dilatancy, energy absorption, and acoustic emission characteristics to evaluate anchoring effects across shear cycles and joint orientations. Results reveal that bolted en-echelon joints experience more severe shear zone damage after cycles, with bolt deformation correlating to shear zone width. Bolted en-echelon joints exhibit faster shear strength deterioration and higher cumulative strength loss compared to unbolted ones, with losses ranging from 20.04% to 72.76%. The compressibility of en-echelon joints reduces the anchoring effect during shear cycles, leading to lower shear strength of bolted en-echelon joints in later stages of shear cycles compared to unbolted ones. Bolts reinforce en-echelon joints more effectively at non-positive angles, with the best performance observed at 0° and –60°. Anchorage accelerates the transition from rolling friction to sliding friction in the shear zone, enhancing energy absorption, which is crucial for rock projects under dynamic shear loading. Additionally, rock bolts expedite the transition of the cumulative AE hits and cumulative AE energy curves from rapid to steady growth, indicating that strong bolt-rock interactions accelerate crack initiation, propagation, and energy release.

1. Introduction

En-echelon joints are a common fracture system in rock masses [1], characterized by overlapping or stepped, parallel or subparallel fractures within a narrow, elongated zone. As weak structural surfaces, they play a critical role in controlling fault slip and rock mass destabilization [2]. Periodic dynamic forces from earthquakes or excavation interacting with these fractures further reduce their shear resistance, posing a serious threat to rock engineering projects [3–5]. Rock bolts, a mostly used support method, can effectively reinforce jointed rock masses by limiting block slip along shear surfaces through pin effect and additional normal forces due to bolt deformation [6]. An in-depth study of the anchorage performance of en-echelon joints and their anchoring mechanisms can offer valuable insights for optimizing support designs in engineering rock masses, with significant research and practical implications.

Extensive research has focused on the shear strength deterioration and asperity degradation of rock joints through laboratory cyclic shear tests. Lee et al. [7] explored the degradation behavior of asperities on smooth and rough rock joints during shear cycles, confirming the exponential relationship between sliding friction work and asperity angle. Based on experimental results, they proposed the concept of an equivalent asperity angle to refine the Plesha degradation model [8] and develop an incremental constitutive equation for shear cycles. Jafari et al. [9] examined the evolution of shear strength of triangular and natural joints subjected to shear cycling, revealing a negative correlation between shear strength and both cycle number and stress amplitude. Using a 3D laser profilometer, Belem et al. [10] measured the evolution of joint surface topography under periodic shear loading and found that high normal stress levels accelerate asperity degradation. Mirzaghorbanali et al. [11] investigated the effects of shear rate and initial normal stress on the shear strength evolution of triangular joints under constant normal stiffness conditions. They observed that higher shear rates and initial normal stresses significantly influence the degradation of shear strength, with implications for understanding joint behavior under dynamic loading.