| The surrounding rock of high-level radioactive waste underground repository is thekey to prevent the leakage of nuclear waste,and the cracks in the underground rock can provide channels for nuclide migration.Therefore,it is of great significance to study the fracture seepage of surrounding rock of repository.Based on MTS815 rock mechanics test system and ST400 non-contact surface three-dimensional topography scanner,uniaxial and triaxial cyclic loading and unloading tests were carried out on complete granite samples,and fracture radiation seepage test and water force coupling cyclic loading and unloading seepage test were carried out on granite samples with horizontal through fractures in the middle,At the same time,the surface three-dimensional morphology of the upper and lower fracture surfaces of the sample was scanned before and after the test.The mechanical properties,deformation characteristics,fracture compression properties,seepage characteristics,fracture surface morphology distribution characteristics and evolution law of Beishan granite are analyzed,and the experimental results are verified by seepage model.Based on the above research,the main conclusions are as follows.（1）The two-dimensional and three-dimensional views of fracture surface are obtained,and the three-dimensional morphology parameters and fractal dimension of fracture surface are obtained.The change of fracture surface morphology before and after loading and unloading is revealed.The three-dimensional morphology of the fracture surface of different samples are different.The fluctuation state of the fracture surface can be directly judged by the two-dimensional view and three-dimensional view of the fracture surface.The three-dimensional morphology characteristic parameters and fractal dimension of the fracture surface quantitatively characterize the characteristics of the fracture surface,such as the height,fluctuation state and roughness.The results show that the height difference of the lower surface decreases by 0.3mm and 0.25mm,the root mean square height of the natural fracture surface decreases by 7%and 16%,respectively,and the fractal dimension of the two fracture surfaces decreases,and the fractal dimension of the profile is anisotropic.This reveals the evolution law of fracture surface morphology before and after loading and unloading,and lays a foundation for the subsequent study of the correlation between fracture seepage and fractal dimension.（2）The mechanical properties,deformation characteristics and strength characteristics of intact and fractured granite samples are obtained,and the compressive properties of different types of fracture surfaces of fractured granite samples are discussed.The results show that the deformation of the intact specimen is small under uniaxial and triaxial loading and unloading,and the axial strain curves are concave.The average axial strain and circumferential strain under uniaxial loading and unloading are 90%and 48%of those under triaxial loading and unloading.The confining pressure can restrain the deformation of the specimen,especially the transverse deformation;The unloading modulus of granite is always higher than the loading modulus under the same cycle,and both increase with the increase of cycle times.Under Triaxial Cyclic Loading and unloading,the area of the"plastic hysteresis loop"formed by the pre peak loading and unloading curve gradually increases with the increase of the number of cycles,and constantly moves to the positive direction of the strain,while the post peak loading and unloading strain curve presents a"willow leaf"loop,and the elastic modulus decreases with the increase of the number of cycles,with an average decrease of 2%.The area of"hysteresis loop"formed by the loading and unloading curve of the specimen with cracks and the deformation of the specimen are larger than that of the intact specimen,which is because the deformation during the first loading is mainly the compressive bonding of cracks.The average axial strain of the sample with artificial fracture and the sample with natural fracture are 0.67‰and 0.73‰respectively,which are 1.22 and 1.33 times of that of the intact sample under the same conditions.The axial strain and fracture compression strain of the latter are higher than the former,which indicates that the deformation of the sample with natural fracture is greater and the fracture compressibility is stronger,but the meshing degree between the upper and lower fracture surfaces is much lower.（3）The fracture radiation seepage characteristics of the fractured samples are obtained,and the evolution characteristics of fracture permeability with different axial stress under water force coupling are revealed.When the seepage fluid flows from the middle of the fracture to the surrounding without load,it will show significant anisotropy difference,which is related to the anisotropy of fracture surface morphology;The average permeability of natural and artificial fracture samples is 5.55×10-16 m2 and 2.40×10-16 m2,the radiation seepage of seepage fluid between natural fractures is easier than that under artificial fractures.Under the condition of water force coupling,the average initial permeability of natural and artificial fracture samples is 1.43×10-18 m2 and 3.03×10-19 m2.The permeability curves of the two groups of samples show"wavy"changes under 5 equal cyclic loading and unloading.The former shows a step-by-step decrease,with a large single decrease,while the latter is not obvious,with the permeability peak close,indicating that the natural fractures are closed step by step with the increase of loading and unloading times,while the artificial fractures are almost completely closed in the first cycle.The increase of axial load restrains the fracture seepage and reduces the fracture permeability（4）The relationship between the three-dimensional morphology characteristic parameters and fractal dimension of each fracture surface and fracture seepage is revealed,and the evolution law of fracture surface morphology is obtained.At the same time,the measured fracture permeability under loading and unloading is estimated by using the mixed model theory of pore and fine nail.The average values of surface characteristic parameters sq,Sal and SDR of artificial fracture surface,natural fracture surface and large-diameter natural fracture surface before loading and unloading all increase in turn,indicating that the more gentle the fluctuation of surface height of the three groups of fracture surfaces is,the lower the surface complexity is,and the easier the radiation seepage of seepage fluid between fractures is.The larger the fractal dimension is,the rougher the fracture surface is,and the more rugged and complex the height distribution is;On the contrary,the smoother and flatter it is.The fractal dimension of all fracture surface profiles decreases with the increase of single hole seepage in this direction,and the relationship between them is an exponential function,which can be expressed by D=a·ebQ+c.Loading and unloading will"sharpen"the surface peak of fracture surface,reduce its maximum height,and make the height distribution of fracture surface more uniform.This"sharpening"effect is more significant for natural fracture surface;After loading and unloading,the average fractal dimension of artificial fracture surface decreased from 2.17 to 2.14,and that of natural fracture surface decreased from 2.19to 2.15.Loading and unloading reduce the fractal dimension of fracture surface,weaken the surface roughness,and this weakening has more significant effect on the change of natural fracture surface roughness and fracture surface morphology.The theoretical value of fracture permeability of fractured rock samples is calculated by using the mixed model of pore and fine nail.The results show that the measured value of permeability is in good agreement with the theoretical value.The mixed model of pore and fine nail can be used to estimate the fracture seepage of underground fractured rock mass seepage Engineering. |
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