ABSTRACT:

The classic rock mechanics is limited to evenly fractured rock mass at shallow depths wherein works of Mohr, Coulomb, Hoek and Brown, Drucker-Prager, Wiebols and Cook and more consider isotropic structural effects of joints seen at shallow depths. However, hardly any failure criterion is applicable to massive rocks available at deep depths. At deep tunnels, as one moves from the excavation surface into the rock, the fracture mechanism changes from extensional to shear cracking. The representative Kannur limestone is observed to exhibit exquisite extensile fractures as columnar shards are blown away in a manner much similar to strainburst in deep tunnels. The aim of the paper is to discuss the confinement dependency of limestone when subjected to triaxial stresses. Also, an improvement is suggested to Mogi criterion. This will further help to decide upon the multistep unloading of confining pressure observed during an excavation process in tunnels.

INTRODUCTION

High geo-stress and complex geological conditions such as varying rockmass properties, geological structures, discontinuities, and rock types, as well as some induced seismic and blasting activities may trigger rockburst, spalling, caving, or buckling on the side walls and excavation faces of the tunnels or vertical boreholes. This paper discusses a special case of polyaxial stress state i.e., triaxial stress state in deep tunnels, wherein massive limestone (say Kannur limestone) could be present. The rock type under consideration is ideal to strainburst as it exhibits exquisite extensile fractures at the exposed surface of the specimens, independent of applied confinement. The term ‘extensile’ is used as it produces splendid columnar fragments under the compressive stresses acting on all sides of the specimens. These ‘shards’ are aligned parallel to the major principal stress. The shards are of the length of the 54 mm diameter specimen, tested for uniaxial compressive strength, which produces columnar structure.

The initiation of tensile cracks was first studied by Griffith (1924) and then modified by McClintock & Walsh (1963) to account for shear mechanism by considering the friction acting at the surface of the closed cracks. However, since Griffith criterion does not consider the extension of the tensile crack, it needed to be modified as explained in Hoek & Martin (2015). Herein the dimensions of the columnar fractures were found to be related to modulus of elasticity and peak stress, applicable to brittle rocks.

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