Joint Induced Anisotropic Deformation and Supporting Effects of Rockbolts for Tunnels Excavated in Regularly Jointed Rock Masses

Liu, Hsiao-Hua (National Taipei University of Technology) | Wang, Tai-Tien (National Taipei University of Technology) | Huang, Tsan-Hwei (National Taiwan University)

OnePetro 

Abstract

Based on a three-dimensional non-linear constitutive model, this study investigates the anisotropic deformation of a tunnel excavated in jointed rock masses and associate supporting effects of rockbolts. Using a numerical model capable of describing the influence of mechanical behavior of intact rock, the spatial configuration of joint sets, and mechanical behavior of the joint plane on the non-linear behavior of a rock mass, this manuscript reveals the joint-induced anisotropy in failure zone and deformation surrounding a tunnel. The support effects of a rockbolt are generally limited. However, when the rockbolt is pre-stressed and acts perpendicular to a joint plane, the shear displacement and associated dilation of the plane is constrained. And the joint sliding failure caused by tunnel excavation can be reduced. The rockbolt provides a beaming effect. Such an effect is significant in locations where the joint planes parallel to tunnel wall and influenced by the numbers of joint sets and joint strength.

Introduction

A rockbolt is an innovative support element in modern tunneling, providing active pre-stress support and associated constraining effects to sustain a potential falling rock wedge and to minimize shear dilation on fractural planes, which benefits the limitation on the development of excavation disturbance zone (Spang& Egger, 1990; Huang et al., 2002; Bobeta & Einsteinb, 2011). However, conventional mechanical models and numerical modeling generally simplify rock masses as homogeneous isotropic medias and stipulate the behavior after excavating a tunnel for support design, which disregard the discontinuity induced anisotropic behavior in rock strength and deformation, neither insufficiently access the effect of tunnel supports. The support effects of modern tunneling have not beenwell evaluated yet.

Focusing on a rockbolt, this study aims to elucidate its diverse mechanism for supporting a rock tunnel using numerical approach. The two-dimensional numerical implementation, which embedding the three-dimensional non-linear constitutive model for a rock mass containing ubiquitous joint sets, is utilized. The constitutive model (Wang & Huang, 2009) takes the mechanical behavior of intact rock, the orientations of joint sets, and the mechanical behavior of joint planes into account, and therefore the adopted numerical code is capable of describing complete stress-strain relationship and strength and deformability anisotropies of rock masses induced by joint sets. Thus, the rock support effects can be quantitatively analyzed and well elucidated.