The Performance of Fully Grouted Rock Bolts Subjected to Combined Pull and Shear Loads Under Constant Normal Stiffness Condition

Abstract

The natural discontinuities in rock masses, which form unstable rock blocks, have a profound impact on the stability and safety of mining structures. The most commonly used means of rock block reinforcement in the field is fully grouted rock bolt because of its high tension capacity and its efficient anchoring. Rock bolting system forms a self-supporting structure in rock mass through reinforcing loosened rock blocks, improving shear strength of rock joints. According to field observations the failure of rock bolts occurs due to a combination of both pull-out and shear forces. Thus, understanding the failure mechanism of bolted rock joint under such a mixing loading condition is essential for rock support system design. The surface roughness characteristics, Constant Normal Load (CNL) and Constant Normal Stiffness (CNS) conditions, and the presence of infill material within rock joint can significantly influence its shear strength. Moreover, the mechanical and failure behaviour of rock as a heterogeneous material is controlled by various microstructural parameters, such as grain shape and size, type of minerals, and the existence of pre-existing flaws. Any damage due to the mine roof fall (e.g. rock block collapse in roadways and tunnels) or the failure of rock in open pit slopes can hinder mining activities, and results in penalties being imposed on mining companies. Therefore, an appropriate evaluation of rock block instability and response of rock bolting system is critical when designing both surface and underground mining structures. Recent developments in computational mechanics and distinct element numerical method (DEM) enable more efficient and faster design of mining structures. However, a promising DEM framework requires a robust and rigorous contact constitutive model, which is capable of mimicking the failure and mechanical response of material at microscopic scale. The key aspect of DEM contact model is its contact force-displacement law, which is responsible for capturing the essential macroscopic features of material failure and deformation. For rock joints reinforced with fully grouted rock bolts, these macroscopic features include brittle or softening behaviour of rock and grout, the cohesive or non-cohesive behaviour of infill material during shearing, and the failure of bolt-grout interface due to tension load. In the case of polycrystalline rock (e.g. granite), the inter- and intra-granular micro-cracking behaviour should also be taken into considerations. The focus of this study is on development of a DEM-based cohesive contact model for simulating the failure behaviour of rock, cohesive infill material (e.g. clay), grout, and bolt-grout interface. The proposed DEM-based cohesive model couples damage mechanics and plasticity theory in both modes I and II, and features an exponential decay damage function that considers the influence of both normal and shear stresses in reproducing a gradual, post-peak softening response in DEM contacts. Unlike conventional contact models such as Parallel Bond Model (PBM), flat-joint model (FJM), and smooth joint model (SJM), which feature no gradual degradation of contact strength after yield point, the cohesive softening behaviour incorporated in the new contact model inhibits the abrupt contact failure that enhances the macroscopic softening response of the DEM model. The proposed contact model is implemented in DEM code (PFC2D) to develop a cohesive DEM framework. A Stepwise Pull-Shear Test (SPST) scheme is developed to investigate the influence of pretension load, rib angle, and CNS boundary condition on the ultimate shear resistance of rock joints. The SPST approach allows simulation of bolted rock joints subjected to a combined pull-shear load, which is more realistic compared to previous shear testings that neglect the impact of simultaneous pull-out and shear loads. The proposed cohesive contact model is also incorporated into a Grain Based Model (GBM) to develop a cohesive GBM framework for simulating the micro-cracking behaviour of polycrystalline rocks. The numerical validations against a range of laboratory tests demonstrate that the proposed cohesive DEM and GBM frameworks are effective in reproducing the mechanical and failure behaviour of rock and grout materials as well as bolt-grout interface, the cohesive macroscopic response of clay-infilled rock joints, and micro-cracking behaviour of granitic rocks. The proposed modelling method, in conjunction with the SPST scheme, provided an efficient and inexpensive numerical framework that can be used by designers and geotechnical engineers for carrying out realistic experiments (i.e., combined pull–shear loads). Doing so will give them new insights into the mechanical performance of fully grouted rock bolts and failure behaviour of rock mass.