Abstract

The behaviour of jointed rock mass is highly dependent on the properties of rock joints. One of the most important property used in identifying the behaviour and performance of rockmass is the joint stiffness and the deformation behaviour of tunnels in jointed rocks is dependent on this joint stiffness. As the stiffness parameter rises to a very high value, the joint attains the strength comparable to an intact rock and behaves as a welded joint. Many studies under static as well as in dynamic conditions have been conducted to understand the effect of joint stiffness on rock mass behaviour. In this study, the wave propagation characteristics and transmission coefficient have been studied on single jointed rock mass under the influence of varying joint stiffness using Universal Distinct Element codes (UDEC). The analysis has been further extended to understand the effect of joint stiffness on the tunnel in jointed rocks with shake table model testing and subsequent comparison with dynamic loading with a sinusoidal input. A parametric study is performed to understand the effect of joint normal stiffness and joint shear stiffness on the performance of jointed rock mass is investigated. It is found that the tunnel deformation is linearly varies with the joint normal stiffness under static conditions but it varies exponentially under dynamic loads.

1.
Introduction

Rock mass encountered in the field is usually found with joints. These joints being unavoidable, needs special attention in the execution of big rock engineering projects. The presence of joints reduces the strength of rocks and the joint strength becomes the factor determining the performance of the rock mass. The main factor affecting the strength of the joints is the joint stiffness parameter. Unlike many other rock parameters, the estimation of joint stiffness is difficult mainly due to the absence of any proper specifications or guidelines (Kutalilake et al., 2016). A lot of research has been done on laboratory experiments to identify the joint displacement behaviour (Goodman, 1974, 1976; Bandis, 1983; Barton and Bandis, 1980, Jing et al., 1994).

Joint normal stiffness (Goodman, 1968) can be defined as the ratio joint normal stress to the normal displacement. Finding the joint stiffness in a rock mass medium is considerably tough and studies established the importance of insitu stresses on the joint normal stiffness. Many linear and nonlinear models were developed to define the relationship between joint stress and joint displacements. Study by Hsuing et. al. (1993) highlighted the importance of normal stress in joint normal stiffness and joint shear stiffness and emphasized on the importance of normal stiffness on shear displacement. Most of the studies being done under laboratory conditions to understand the joint behaviour, Barton (1981) gave an equation correlating joint stiffness to the rock mass modulus and intact rock modulus.

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