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Abstract The law of opening-mode crack initiation and the evolution of crack tip stress intensity factor were detected considering the combined action of seepage pressure and far-field stress. Accordingly，the damage fracture mechanical model of rock-like material and the evolution equation of the crack tip stress intensity factor were proposed for pre-cracking rocks under the combined actions of compression-shear field and seepage field. The crack tip stress intensity factor was advised to be used as a criterion for judging the degree of rock crack initiation. The results show that：(1) Open crack tip stress intensity factor is influenced by confining pressure，seepage pressure，crack tip radius of curvature，as well as crack inclination angle. (2) Little change is observed in the cracking angle，which maintains about 70.5° with the pre-crack angle varying. (3) The cracking strength is inverse proportion to the seepage pressure，crack length and the crack tip radius of curvature，while in direct proportion to the confining pressure. Also the crack angle is responsible for the crack strength. The case studies indicate that the uses of different crack initiation criteria all lead to an identical tendency that the crack initiation strength decreases with the increase of the seepage pressure. The laws that the crack initiation strength is inverse proportion to the seepage pressure and proportion to confining pressure are further verified by conducting experiments under various experiments. When the crack angle is 30°，the crack initiation strength is maximal，the crack angle of 60° next and 45° last. It is worthwhile to pay attention to the fact that increasing the seepage pressure would result in the decrease of the crack initiation strength，enlightening the prospect of easier ore extraction under high stress by hydraulically induced cracking.
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2012, Vol. 31 
