ABSTRACT:

To investigate tri-axial unloading mechanical properties of granites under different stress paths, the tri-axial unloading tests are conducted on specimens with initial confining pressures of 10, 20 and 30 MPa and unloading rates of 0.05 MPa/s in three stress paths. The stress-strain curves, the deformation characteristic and the strength criteria were studied. The results show that the rate of lateral strain is about 3–5 times as much that of the axial strain, the strong dilatation leads to the rock failure along unloading paths. Deformation modulus increases slowly along with the decline of confining pressure, but decreases rapidly when unloaded to yielding strength. In the same unloading stress path, the reduction of deformation modulus increases with increasing initial confining pressure. Poisson ratio increases with the decrease of confining pressure during the process of unloading. Rock failure primarily features as shear failure, the features of the rock strength is well reflected by using power function type of Mohr strength criterion.

1 INTRODUCTION

Underground stope excavation results in unbalancing of rock stress. With continuous propulsion of mining face, the exposure area of surrounding rock is increasing constantly. The effects of unloading by excavation make continually deterioration of rock mechanics properties (Yoshinaka et al. 1997). Currently, it uses static and dynamic loading methods to widely studies deformation characteristics of rocks during underground geotechnical projects (Huang 2014).

However, unloading and loading correspond to two different stress paths; subsequently the mechanical properties exhibited are substantially different (Marco 2008, Yin et al. 2012). To better understand the unloading mechanical properties during underground rock excavation, numerous previous scholars have been studied the unloading rock mass. These authors formed excellent contributions in their results, e.g., the concept to the time-dependent expansion of rock damage and earthquakes (Sun 2001), tri-axial unloading confining pressure (You et al. 1998); marble unloading confining pressure strength characteristics (Chen et al. 2008); a real simulation of underground excavation tri-axial experimental study on the rheological properties of shale unloading (Guo 2012); the high confining pressure unloading expansion properties of rock (The Manh et al. 2013); an experimental study on shale unloading rheological properties (Hu et al. 2011) and salt rock unloading confining pressure tests (Reza et al. 2010, Wang et al. 2012, Ha 1998, Liu et al. 2012, Zhou et al. 2009). The unloading confining pressure stress-strain relation process, deformation characteristics and laws have been conducted and studied. However, neither an in-depth study of the deformation characteristics nor the laws of different stress unloading paths have been previously researched. And there is no comparative analysis of deformation parameters or fracture characteristics in different stress paths.

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