ABSTRACT:
Rock around an underground excavation is enabled to reach its full stability by controlling the loosening zone. Thus in practice, it is necessary to estimate the extent of the inelastic zone(s) and to determine the allowable limit of convergence through convergence measurements. The authors analyzed in detail approximately 500 convergence curves observed in six motorway tunnel projects, which were driven through shale and sandstone, granitic rock and slate formations. Also, they investigated the relationship between the width of loosening zone, the allowable limit of convergence and Tanimoto’s rock classification, which indicated relationships between support load, initial deformation rate and final deformation. Consequently, the proposed relationship is useful to predict the final deformation, the extent of loosening zone and the targeted value of convergence based on the result of convergence measurements observed at earlier stages of excavation.
1 INTRODUCTION Since Terzaghi (1946) proposed his relationships between rock load and tunnel support, several rock mass classification systems have been developed. The RMR system by Bieniawski (1974) and the Qsystem by Barton et al. (1974) have been used for rating rock mass characteristics and to provide empirical support guidelines for on-site engineers. Hoek et al. (1995) introduced the Geological Strength Index (GSI) to estimate the reduction in rock mass strength for different geological conditions, which in turn can be used to calculate rock mass strength/deformation behaviour. In general, these systems are not focused specifically on rock mass behavior in tunneling but rock mass properties, such as lithological characteristics, joint roughness and frequency, strength of intact rock and so on. Rock behavior around an opening reaches full stability by means of mobilizing the ground arch. This can be achieved by controlling a loosening zone, namely by providing a resultant inner pressure within a certain limit of deformation.
