| The study on the mechanism of crack initiation and propagation in rock mass has always been a hot issue in the field of geotechnical engineering.In this paper,experiment and numerical simulation were used to study the failure mechanism on double jointed rock masses under the precast static and dynamic load,moreover,similarity model was built in the working face of a mine in Northwest China by means of the similar material,to acquire the failure mechanism of overlying strata,also the security in mining was evaluated.The main contents and conclusions are as follows:(1)According to the calculation on the propagation of double center symmetric precast intermittent cracks under biaxial compression,it was shown that the stress-strain curves can be divided into four stages,namely the initial compression section,stable crack extension,accelerated extension stage and post peak stage.With the confining pressure increasing,the compressive strength of samples increases linearly.Both elastic modulus and compressive strength are negatively correlated to the fracture length and fracture angle,moreover,elastic modulus has little relationship with the fracture spacing,while the compressive strength increases first and then decreases with the increase of crack spacing.(2)The inertial loading device accompanied with self supporting electric vibration test system was used to study the crack initiation and propagation in rock masses under vibration load.It was found that the angle between the vibration direction and edge crack inclination is small,which is approximately about 10 degrees,moreover,the edge crack which has the same inclination as the vibration direction extends most easily,and coalescence most easily happens between the edge crack and center crack that both have the same inclination as the vibration direction.What' s more,with the angle between vibration direction and crack inclination increasing,crack propagation becomes more and more difficult,and the crack which is perpendicular to the vibration direction most difficultly extends.Crack coalescence is usually along the shortest path between the crack tips,which conforms with the principle of minimum energy.Also the failure frequency is positively correlated with the crack spacing.(3)The similar material was developed using six common ingredients,which could be applied to a variety of practical projects.In addition,similarity simulation was carried out to study the failure mechanism of overlying strata in the working face of a coal mine in the Northwest.It was shown that there are obvious caving zone,water flowing fractured zone and separation space in the overlying strata of the working face.The height of the caving zone is about 52 m,the maximum height of the water flowing fractured zone is about 105 m,and the bottom surface of the separation layer is about 85 m from the top of the coal seam.The water flowing fractured zone above the separation space is located under the main aquifer,and will be gradually filled with water with the time passing by,at the risk of water swarming into the working face.Therefore,prevention of water separation layer should be strengthened in the mining process.(4)The failure mechanism of the overlying strata in the coal mine was numerically simulated by the particle flow software PFC.It was shown that micro cracks and coalescence is the primary cause of rock breaking.In the overlying strata of working face during initial mining,there are large tensile fractures and continuous extensions in the stratum which is thin and has low strength,resulting in strata subsidence caving.In the middle stratum which is thick and has high strength,mixed mode fractures of tensile and shear prevail,and develop into water flowing fractures and expand upward.While,there is no micro crack appears in the stratum above the separation space,and the stratum still has a good effect of bearing. |
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