| Cracks with different scales and the crack networks are commonly encountered in natural rock masses.These defects weaken the strength and deformation behaviors of jointed rock masses.However,as the distribution of the internal three-dimensional cracks inside the natural rock is generally different to be observed,previous scholars usually investigate the initiation and propagation process of cracks on specimens with pre-existing artificial flaws.This experimental method does not reflect the crack initiation and expansion state of the internal cracks in the rock.In addition,actually,the distribution of cracks inside the rock mass is three dimensional.So it is significant to investigate the propagation mechanism of three-dimensional cracks in practice engineering.In this paper,a series of uniaxial and triaxial compression tests were carried out on the intact and jointed rock to study the effects of dip angle of prefabricated cracks and confining pressure on cracking behaviors of the rock.The dip angles was set as 0°,30°,45°,60° and 90°,respectively.,Volatile camphor pellets were used to form prefabricated three-dimensional cracks in rock-like material samples.After the loading,CT scanning was performed on the samples,and the software Avizo was used to perform reconstruction of three-dimensional fractures.Thereby,the propagation mechanism of pre-fabricated three-dimensional internal cracks and failure modes of samples were studied.Subsequently,PFC3 D was used to numerically simulate the fracture failure process of the specimen,and compared with the physical test results.The main research results obtained are as follows:(1)The presence of built-in cracks has a significant effect on the peak strength and failure mode of the sample.With the increase of the crack inclination angle,the peak strength of the sample is greatly reduced,and the failure mode of the sample gradually changes from tensile and shear composite failure to shear failure,and then to cleavage failure.Three-dimensional CT scan reconstruction results show that the crack space is distributed on a curved surface.Three types of propagation modes for wing cracks that mainly occur at specimens with small dip angles(0°,30° and 45°)and shear cracks that usually occurs at specimens with large joint inclination(60°)were observed.(2)Based on the parallel bonding model,three-dimensional particle flow models for rock-like samples were established.By analyzing the sensitivity of the mesoparameters of the parallel bonding contact model,it was concluded that the macroscopic elastic modulus is positively related to the parallel bonding elastic modulus while is negatively correlated to the stiffness.In addition,the effect of the parallel bonding elastic modulus is more influential.The compressive strength is mainly determined by the normal strength of the parallel bond.With the increase of the parallel bond stiffness ratio,the compressive strength first increases and then decreases,with the parallel bond normal The increase in strength,cohesive force of parallel bonding and friction angle in parallel bonding will increase the compressive strength to an extreme value,but the compressive strength will decrease when the friction angle in parallel bonding continues to increase.(3)A three-dimensional particle flow model was established.By simulating the compression failure process of the three-dimensional built-in fracture specimen,the micro-mechanical responses such as micro-crack propagation,contact force chain,displacement field in the numerical model were analyzed.Numerical simulation results show that micro-cracks initiate at the crack tip,The propagation process corresponds to the stress-strain curve at each stage;the evolution process of the contact force chain and displacement field in the samples can be used to determine the types of stress distribution and failure modes in the samples. |
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