The impact of rock block size on cable bolt performance has been assessed using the Universal Distinct Element Code (UDEC). The results indicate that the interaction between rock mass and reinforcement changes with variable block volumes and highlights that block size plays a key role when modelling and designing support systems. A simple statistical approach for calculating the forces acting along the length of the bar demonstrated that bolt design should not solely based on the maximum predicted loads that commonly concentrate along sliding discontinuities, because this reaction may not represent the overall bolt behaviour and may have been triggered by unrepresentative fracture patterns. Because fracture frequency is difficult to be controlled in UDEC due to interacting joint sets, a simple method for controlling the input block size is suggested to transform borehole or scanline survey data into more realistic block volumes.

1 Introduction

The choice of appropriate techniques to evaluate the response of the structural elements used as rock reinforcement in mining and civil engineering projects is both a critical decision and a very subjective matter. In many cases, a simple empirical relationship, a theoretical expression or even the designer's practical experience may be adequate while, in other cases, sophisticated numerical modelling, in-situ and laboratory testing, validation and redesign may be required to arrive at an effective and economical support solution.

The most common types of reinforcement used to restrict deformation and improve the self-supporting capacity of blocky rock masses are rock bolts, cables and ground anchor bars. Their behaviour is typically assessed based on the maximum predicted loads and a decision is taken by considering the maximum resistance the structural element can display in rock mass deformation. When rock joints are considered explicitly in numerical models, the maximum load-displacement concentrations commonly occur along that portion of the reinforcement where shearing and/or opening or closing discontinuities interact with the support system (Itasca 2011). Although, this observation may result in representative rock-support responses, it may lead to misleading evaluations and the utilisation of conservative support solutions. This is because high loads developed along relatively short lengths of the bar due to localised individual joint movements may not represent the general response of the rock-support system. Additionally, unrealistic high loads may have been triggered due to unrepresentative joint geometrical parameters (e.g. spacing, persistence, number of joint sets, etc.). Therefore, the accurate representation of the structural geology is extremely important when modelling rock masses because the size and shape of rock blocks influence the deformation of the disturbed zone around the excavation (Shen & Barton 1997) which in turn controls the rock-support response and design.

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