Exploration on Formation Mechanism of Core Discing Based on Energy Analysis

Abstract

The basic premise of deep in situ fluidized mining is to study the in situ stress state of deep rocks, and coring is one of the basic physical means. However, core discing often occurs in the process of coring. And core discing is one of the symbols of high in situ stress in deep engineering area, and revealing the mechanical response of core stub under different conditions is an important premise to explore the formation mechanism of core discing. Therefore, the effects of in situ stress combination condition, core diameter, and drilling depth on the maximum tensile stress after core stub unloading are discussed by PFC2D discrete element numerical simulation method, and the formation mechanism of core discing is preliminarily explored based on strain energy and dissipated energy. Research shows that in situ stress combination is the most dominant factor of core discing. When the hydrostatic stress increases from 40 MPa to 80 MPa, the maximum tensile stress at the core stub increases by 98.42%. When the horizontal stress is 60 MPa and the axial stress increases from 40 MPa to 100 MPa, the maximum tensile stress at the core stub increases by 53.33%. However, when the axial stress is 60 MPa and the horizontal stress increases from 40 MPa to 80 MPa, the maximum tensile stress at the core stub first increases and then decreases, and there is an inflection point when the horizontal stress is 75 MPa. When the core diameter increases from 8.00 mm to 13.00 mm, the maximum tensile stress at the core stub increases by 1.58 times. When the drilling depth increases from 3.00 cm to 4.00 cm, the maximum tensile stress at the core stub increases by 4.01 times; that is, small diameter cores and cores with large drilling depth are more prone to core discing. The total energy of the system increases with the increase of in situ stress. When the hydrostatic pressure is 80 MPa, the dissipated energy of the system is 2.80 times greater than that when the hydrostatic pressure is 40 MPa. More energy must be dissipated when the core discing occurs under higher in situ stress conditions. It is preliminarily proved that the formation mechanism of core discing is that the tensile cracks extend and penetrate at the core stub, and the core is pulled off to form a core disc, which is reciprocating to produce core discing. The research results are expected to provide a certain reference for understanding the formation mechanism of core discing and provide useful ideas for the related research of discing phenomenon.