Abstract
Computer modeling can be used to explore and gain new insights into the impacts of rock bolt intersecting joints in rock masses, and to estimate the effectiveness of the rock reinforcement system. In order to achieve this goal, numerical simulation was performed using FLAC3D to investigate the stress evolution in the rock bolts. The coupling algorithm is based on the analytically-derived interface behavior between a rock bolt and the rock material for grouted rock bolts. The shear force generated by slippage along the interface is assumed to have a linear relationship with respect to the relative slipping distance between the rock bolt and the rock. The linear elastic criterion is applied to determine the material behavior of rock bolts before the axial stress reaches the yield value. The pullout tests are simulated to verify the coupling algorithm and the effects of the proposed rock bolt elements. The simulation results show that the proposed rock bolt models can predict the shear forces and axial loading along the rock bolts. For the rock bolts observed in this study, the max axial force was within the design limit of the bolts, thus the support design was shown to be acceptable.
1. INTRODUCTION
Rock bolts are used as primary reinforcement to strengthen the jointed/fractured rock mass in civil and mining engineering excavations. Windsor (1997) [1] categorized the current reinforcement devices into three fundamental classes: Continuous Mechanically Coupled (CMC), Continuous Frictionally Coupled (CFC) and Discretely Mechanically or Frictionally Coupled (DMFC). The fully encapsulated rock bolt system belongs to CMC, while expansion shell bolts belong to the DMFC system and the Swellex and Split set rock bolts to the CFC system. Because of the simple installation procedure, effective rock reinforcement and economic factors, cement or resin grouted rock bolts are the most commonly used ground reinforcement method. A series of analytical solutions [2, 3, 4] have been made to study the load transfer mechanism in the rock bolting system. Farmer (1975) [5] presented an analytical model to predict the shear stress distribution of fully grouted rock bolts before decoupling occurs in the bolt grout interface. Li and Stillborg (1999) [6] proposed a model for describing the behavior of rock bolts by dividing the rock bolt into elastic, softening and de-bonding zones. Ren et al. (2010) and Blanco MartÍn et al. (2011) [2, 3] presented analytical models for fully grouted rock bolts based on a tri-linear bond-slip model. Ma et al. (2013) [4] introduced a non-linear bond-slip model to analytically simulate the behavior of fully grouted rock bolts. However, these analytical models have difficulty in representing the true interaction between rock bolts and the in situ rock mass as the solutions are based on simplified bolt-collar loading experiments. In this paper, a rock bolt element with both tension and shear restrictions is developed and implemented into the three dimensional FDM program. A series of pullout tests considering the end effect and bond-stiffness effect of the rock bolt are simulated by using the proposed rock bolt element. Thereafter, the effects of shear debonding are simulated by assuming two kinds of constitutive laws for the interface behavior. The debonding process at the nodes and the deformability of a rock bolt are also discussed in this paper.
