The Stress Distribution around Fully Grouted Rockbolts in a Fractured Rock Mass

Abstract

A fully grouted rockbolt is the most common method for rock-mass reinforcement, of which stress distribution is an important basis for the design of supporting works. To investigate the stress distribution around a rockbolt in a fractured rock mass, in this study, laboratory-similar material tests and numerical simulation [discrete-element software PFC2D (two-dimensional Particle Flow Code, version 3.0)] were conducted on rockbolt-supported intact and fractured rock masses, taking the Hongtoushan Copper Mine as the background. By comparing the stress distribution of rockbolts in intact and fractured rock masses, fractures were found to affect the stress distribution around a fully grouted rockbolt. When a certain rock deformation is reached, a tensile stress relief zone is formed in the rockbolt near the fracture, leading to lower stress at this location compared with the rockbolt on the two sides of the fracture. Meanwhile, the stress distribution curve exhibits double peaks instead of a single peak, and the peak stress in the deep part gradually exceeds that in the shallow part. When the surrounding rock mass is damaged and becomes unstable, the stress in the rockbolt near the fracture suddenly increases, becoming the only peak in the stress distribution curve. Therefore, field monitoring of the stress in the rockbolt shall be conducted. If the stress in the rockbolt near a fracture gradually changes from the local minimum to the local maximum, instability or failure of surrounding rock mass is imminent, and safety measures should be implemented. When tunnels, subways, mine slopes, and other projects are excavated, the rock will deform and be prone to danger. In order to improve the stability of rock mass, rockbolt support is usually carried out on rock mass, and rockbolts of different types, different diameters, and different lengths are used according to site engineering conditions. Knowing the stress distribution around a fully grouted rockbolt is helpful to improve the support design and the support effect. The research of this paper is carried out in view of the characteristics of joints and fractures in on-site rock mass. It is found through laboratory-similar material tests and numerical calculations that, compared with the stress distribution around the rockbolt in a homogeneous rock mass (it increases first and then decreases), the existence of fracture has a greater influence on the stress distribution around the rockbolt, which makes the stress distribution curve wavy with double peaks. At the same time, the stress value of the rockbolt decreases in the fracture area. In addition, fractures can reduce the anchorage force of a rockbolt. The research results of this paper can provide guidance for engineering support design and the stability monitoring of anchored rock mass in the later period.