Research On Fracture Characteristics Of Escabrosa Limestone And Its Core Disking Mechanism

With the increasing complexity of deep ground stress,the failure modes of rock mass turn to be various.Mode I failure and mode II failure remain to be two important basic modes.Methods like laboratory tests,numerical tests and theoretical analysis are included in this research.Mode I fracture toughness and mode II fracture toughness of Escabrosa limestone were tested,including a stress analysis especially for the mode II fracture toughness test.A micro model of punch though test was constructed from a mesoscopic view and the relationship between confining pressure and mode II fracture toughness was established.The micro model also revealed the space-time revolution law of the micro fractures in the punch through tests.A quantitative relation was finally set up between mode ?fracture toughness,rock tensile strength and confining pressure.Under a lower confining pressure,the stress analysis of punch through tests shows that the maximum principal stress concentrates on the inside of the upper cut and the outside of the lower cut,and the stress concentration area seems like the shape of Chinese character "eight".The micro punch through model also obtains a same shaped fracture propagation characteristics.This finding is consistent with the real sample failure trace in laboratory punch through test.The numerical results from the micro model show that the mode ? fracture toughness increases linearly with the confining pressure under 40MPa.and tends to be stable when the confining pressure is larger.This conclusion agrees very well with those laboratory tests results,and.verifies the validity.of the micro model in simulating punch through tests.Besides,rather than the increasing of the shear cracks,the wing tension cracks decreases with higher confining pressure.Fracture toughness is a common rock property in rock mechanic research as the ability of resisting.fracture propagation.All the fracture failures in rock engineering are related to crack growth,propagation and coalesce,like rock core disking in drilling engineering,also a typical mark of high in-situ stress.Combining with the rock fracture mechanics,the core disking phenomenon was systematically studied in this thesis from a fracture propagation perspective.The Indiana limestone core disking laboratory test was simulated by discrete element method,and the relationship between core disking phenomenon and three principle stresses was revealed directly.The vertical drilled core disking phenomenon under three principal stresses will occur basically with the condition that the horizontal maximum principal stress should not be smaller than the vertical principal stress,and it should not be smaller than half of the compressive strength and five times of Brazilian tensile strength.The larger the difference value between horizontal stress and vertical stress,the thinner the core disk will be.The larger the difference value between two horizontal stresses,the more obvious saddle shaped the core disk will be.The core disking will initiate from the outside of the core root under most of-stress conditions except the thrust fault stress condition,under which the fracture initiate from the axis of the rock core below about half of the radius.These two different initiation points exactly correspond to two maximum axial stress points.