コアディスキング現象に関する研究

Abstract

Discing of rock cores refers to the formation of discs of relatively uniform thickness which fracture on surfaces approximately normal to the axis of the core. The purpose of the present paper is to study the mechanism of this phenomenon which is best observed when drilling into highly stressed hard rock.For the purpose of the theoretical study, the state of stress in the region where core discing initiates has been analyzed through the use of Finite Element Method and the results of this analysis have been compared with the stress conditions, as determined experimentally, required to produce core discing in specimens subjected to the axial stress Pz and the radial biaxial stresses Px and Py. This comparison show the discing initiates not at the bottom of kerf area but at the root of the core where subjected to the maximum tensile stress σz, across a plane perpendicular to the axis of the core, which increases as radial stress component 1/2 (Px+Py) increases and diminishes with increasing axial stress Pz. Moreover, the magnitude of this tensile stress, as determined analytically, is fairly equal to the Brazilian tensile strength St. As the result of above investigations, the relationship among the stresses Px, Py and Pz required to occur discing, is determined, that is1/2 (Px+Py)≥-4St+Pz, 2.5≥Px/Py≥0.4Moreover, the authors have studied on the practical implications of core discing and on the similar fracture of the hollow core which could be obtaind by the over drilling, with the same axis to the previous drilling. It is experimentally comfirmed the lower stress conditions of hollow core discing than the usual and its conditions are mainly concerned with the ratio of diameters. Finally, it is pointed out that the application of these phenomena to determine the rock stress is quite possible.