Rock Uniaxial Compression Test and Axial Splitting

Fakhimi, Ali (New Mexico Institute of Mining and Technology) | Hemami, Behzad (New Mexico Institute of Mining and Technology)



Axial splitting is an important failure mechanism in rock engineering. This failure mechanism is particularly observed in the vicinity of free surfaces of rock structures. Different possibilities such as existence of defects, pores and micro-cracks in the rock specimen have been attributed as the cause of axial splitting in the literature. Based on some physical uniaxial compression tests on Pennsylvania blue sandstone, an alternative theory for axial splitting is proposed. In this theory, axial shear cracks are considered that are induced because of non-uniform deformation of the specimen ends. Dislocation along these axial shear cracks is the cause of rock lateral dilation which results in radial stresses. The radial stresses push out the cylindrical thin shells of the specimen. Consequently, radial cracks are developed which subsequently cause rock spalling in mode-I fracture. The proposed failure mechanism is discussed within the framework of a simplified theoretical model. The prediction of the model is then compared with the physical observations. In addition, a discrete element bonded particle system is utilized for rock simulation in uniaxial compression testing. The prediction of the numerical model is consistent with the physical observations.

1. Introduction

Axial splitting is a complicated failure mechanism that is normally observed in uniaxial compressive testing of rock. Wawersik and Fairhurst [1] conducted some uniaxial compressive tests and defined local fracturing that is mostly parallel to the specimen axis and both local and macroscopic shear faulting as two possible failure modes. In the work of Horii and Nemat-Nasser [2], a pre-existing inclined crack in the rock specimen was considered. As a result of axial loading, wing cracks can extend from the tips of the preexisting crack that eventually become parallel to the specimen free surface. Holzhausen and Johnson [3] examined several possibilities in uniaxial compressive testing of rock which result in induced lateral tensile stresses and axial splitting. The envisioned mechanisms included buckling of vertical rock slices due to presence of internal flaws, induced axial cracks due to imperfection in the specimen geometry, sliding on an inclined internal crack which can result in tensile cracking, and development of axial cracks due to presence of elliptical holes parallel to a free surface. Dey and Wang [4] considered the stress inhomogeneity and cracks interactions as the cause of rock facture under compressive loading. Bazant and Xiang [5] introduced a fracture mechanics based model for the compression failure of quasi-brittle materials. In their model, a pre-existing crack was considered to be parallel or inclined with respect to the specimen axis.