ABSTRACT

Rock bolts have been widely used as a primary support system to stabilize rock masses around tunnels, underground mine galleries, slopes and others structures. To model the interaction between fully grouted bolts and the rock mass a numerical procedure is developed called „enriched finite element method (EFEM)?. Conceptually if a solid finite element is intersected by a grouted rock bolt, it becomes an „enriched? element. Nodes of enriched elements have additional degrees of freedom which are used to determine displacements and stresses in the bolt. Stiffness of enriched elements is formulated based on properties of the rock mass, bolt rod and grout, orientation of the bolt and borehole diameter. This paper quantitatively evaluates bolt performance in different shapes of underground openings viz circular, rectangular and D-shaped, using the proposed enriched finite element method (EFEM) combined with elasto-plastic behaviour of rock mass and grout material. In addition, a comparative study of bolt performance is also presented considering both coupled and decoupled behaviour of rock bolts.

INTRODUCTION

Rock bolts have been widely used as a primary support system to stabilize slopes, hydro dams and underground structures such as tunnels and mine workings and others structure made in rock masses. The term “rock bolt” is defined in geomechanics as a form of mechanical support that is inserted into the rock mass with the primary objective of increasing its stiffness and/or strength with respect to tensile or shear loads. In general, rock bolts reinforce rock masses through restraining the deformation within rock masses and reduces the yield region around the excavation boundary. During the last four decades, different types of rock bolts have been practiced, out of which fully grouted active/passive bolts were the most common types. For a fully grouted passive rock bolt installed in deformable rock masses, a neutral point exists on the bolt, where shear stress at the interface between the bolt and grout material vanishes. Based on neutral point concepts, shear stresses and axial loads developed along a bolt rod are analytically formulated by many researchers. Bolt grout interactions around a circular tunnel in Hoek-Brown medium have been formulated analytically considering a bolt density factor. Considering different approaches to bolt performance Stille presented a closed form elasto-plastic analytical solutions of grouted bolts. Based on the shear lag model (SLM), Cai et al. derived an analytical solution of rock bolts for describing the interaction behaviours of rock bolt, grout material and rock mass. Brady and Lorig. numerically analyzed the interactions of bolt grout in Mohr Coulomb media using the finite difference method (FDM) technique. In addition, numerous studies have been published on the analytical solution of stresses and displacements around a circular tunnel considering elasto-plastic rock mass with Mohr-Coulomb yield criterion. Elwi and Hrudey and d?Avila et al., proposed embedded finite element method for reinforcing curved layers and concrete structures respectively. Finite Element Method (FEM) and/or FDM based procedures are also developed for the analysis of the said problem and have been presented in many references for solving geotechnical problems.

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