Study On The Law Of Fracture Propagation By Linear Dense Multiple Drillings Sleeve Fracturing In Rock

The realization of directional fracturing of rock material is not only an important technical way to prevent a series of safety problems caused by hard roof of mine,but also a common scientific problem in rock mining,roadbed excavation and other geotechnical engineering fields.Based on the ides of combining the stress superposition effect between the drillings with the fracturing action by sleeve expansion,a solution of linear dense multiple drillings sleeve fracturing in rock was proposed.In this paper,a true triaxial experimental system is used to simulate the in-situ stress environment and provide the expansion pressure of the sleeve.By the means of indoor physical simulation experiment and theoretical analysis,the law of fracturing propagation and mechanical mechanism of linear dense multiple drillings sleeve fracturing in rock are systematically studied.Acoustic emission system and static strain gauge were used to monitor the acoustic emission signal inside the test block and static strain on the surface during the fracturing experiment.The results show that: before the failure of the test block,the acoustic emission signal is in a relatively quiet period,and the strain changes from compressive strain to tensile strain;when the test block ruptures,the acoustic emission and energy both have a surge,the number of acoustic emission events increases sharply,and the strain in the vertical direction of drilling line rises rapidly until the failure remains unchanged,as the expansion pressure continues to increase,the acoustic emission signal fluctuates frequently and the fracture opening continues to increase;the evolution characteristics of the distribution of acoustic emission events and surface strain indicate that the fracture initiation and propagation proceed from the middle to the two sides along the direction of the drilling core line,which verifies the effect of superposition of multiple drillings stress on the enhancement of tensile stress.The control variable method was used to carry out experiments on multiple drillings sleeve fracturing under different conditions.The results show that: the initiation and propagation of fractures under the action of different factors show different variation rules.By analyzing the variation characteristics of acoustic emission signal and static strain,it is found that the change of principal stress difference and the ratio of drilling spacing to diameter have obvious influence on the initiation pressure,penetration pressure,the time of formation of penetration fracture and the complexity of fracture,while the drillings arrangement direction is relatively small.Under different experimental conditions,the fracturing difficulty and the complexity of the fracture of test block are different,but in this study,all the test block were able to form an axial fracture surface basically consistent with the direction of the drillings center line at a part equivalent to the depth of the drilling after fracturing,which indicated that the multiple drillings sleeve fracturing had a better directional effect,the randomness of fracture propagation direction increased in the area near the boundary or not controlled by drilling.Based on elastic mechanics and fracture mechanics,the circumferential stress along the direction of drillings center line during multiple drillings sleeve expansion and fracturing stage and stress state at the crack tip during fracture propagation stage were analyzed by mechanics respectively,the conditions of fracture directional initiation along the direction of drillings center line during multiple drillings expansion and fracturing stage are deduced,the directional initiation of fractures can be realized only when the principle stress difference,drillings arrangement direction and the ratio of drilling spacing to diameter meet a certain coordination relationship,based on the theory of maximum circumferential stress,the formulas for calculating the crack propagation angle and radius in fracture propagation stage are obtained.This paper has 76 figures,11 tables and 101 references.