Experimental And Model Study On Fracture Seepage Of Beishan Granite Under Temperature Condition

The development of nuclear industry will inevitably produce radioactive waste.T he safety of high-level radioactive waste（HLW）disposal is the key to the sustainable development for nuclear industry.Nowadays the deep geological disposal of HLW has been widely accepted worldwide.The deep geological disposal method separates HL W from biosphere for long time by engineering and natural barriers in 500 m～1000 m underground before the HLW decays to general radioactive level.During the construct ion of geological repository,the surrounding rock experienced cyclic excavation unloa ding and the redistribution of in-situ stress field.Moreover,the decay heat of HLW w ill increase the temperature of rock,which will induce thermal-stress in surrounding r ock.Engineering disturbance and thermal stress induce a large number of cracks in gr anite,to provide potential immigration pathway for nuclide.The key role of geologica l repository is tightness to nuclide.Thus to deepen the understanding of hydraulic pro perties under the coupled thermal-hydro-mechanical is of great importance for HLW d isposal engineering.In this paper,the Beishan granite in Gansu Province,China’s hig h-level radioactive waste disposal underground laboratory site,is taken as the research object,and the following research is carried out,（1）Experimental study of cyclic heating-cooling under different confining pressu re on hydraulic properties of granite.The result show,Confining pressure and tempe rature cycle have a significant effect on the hydraulic aperture.For the first heating-co oling cycle,the hydraulic aperture continuously decreased with increasing temperatur e,then slightly recovered in subsequent cooling process.The hydraulic aperture decre ased with increasing confining pressure,and the reduction magnitude of hydraulic ape rture is larger for granite fracture under lower temperature.For the second heating-coo ling process,the evolution of the hydraulic aperture with temperature is similar,while the hydraulic aperture and variation magnitude were significantly decreased and the c onfining pressure cause limited effect on the hydraulic aperture,which implies that th e fracture surface has subjected the permanent damage during the first heating-cooling process.A formula considering the influence of temperature and confining pressure w as established to characterize the hydraulic aperture of granite fracture.（2）The normal incremental cyclic loading-unloading-seepage tests were carried out on the single fracture samples of Beishan granite under different temperature cond itions（25°C,90°C,120°C）（the maximum loading amplitude was 12 MPa）.The c yclic loading and unloading tests under the same stress path and temperature conditio ns were carried out on the intact granite samples with good homogeneity.The evolutio n of fracture hydraulic gap width during normal cyclic loading and unloading,single f racture samples under stress-temperature conditions,intact samples and fracture defor mation characteristics（Young’s modulus,residual deformation）were analyzed.The results show that the evolution of fracture hydraulic gap width under normal incremen tal cyclic loading and unloading under different temperature conditions is similar.In t he loading stage,the fracture hydraulic gap width decreases.In the constant normal str ess stage,the fracture hydraulic gap width remains basically unchanged.In the stress unloading stage,the fracture hydraulic gap width recovery under normal temperature conditions is small.However,under the conditions of 90°C and 120°C,the fracture h ydraulic gap width recovers obviously during the unloading process.The residual def ormation of the fractured specimen decreases exponentially with the maximum loadin g stress during the normal incremental cyclic loading and unloading process.The tem perature has a significant effect on the residual deformation of the initial loading secti on of the fractured specimen,which is 90°C>120°C>25°C.As the loading stress i ncreases,the effect of temperature on the residual deformation of the fractured specim en decreases.The residual deformation of the complete sample is less affected by tem perature.At each loading stage,it fluctuates within the range of 0.03 mm,and the over all trend remains stable.（3）Based on the classical BB model,the stress-displacement evolution during th e crack loading process under different temperature conditions is fitted.This model ca n accurately predict the initial stiffness and the maximum normal closure of the crack during the crack loading process.The relationship between the maximum normal clos ure of cracks,the initial stiffness of cracks and temperature in the BB model is analyz ed.In the stage of 25°C-90°C,the maximum normal closure of cracks increases,and in the stage of 90°C-120°C,the maximum normal closure of cracks decreases.The relationship between the initial stiffness of the crack and the temperature is opposite t o the relationship between the normal closure of the crack and the temperature.The m aximum normal closure of the fracture calculated by the BB model is used as the initia l mechanical gap width.Combined with the fracture deformation during the loading pr ocess,the evolution of the fracture mechanical gap width during the loading process is obtained.Based on the normal compression-seepage test,the evolution of the fracture hydraulic gap width during the loading process can be obtained.The relationship bet ween the fracture hydraulic gap width and the mechanical gap width under different te mperature conditions（25°C,90°C,120°C）is established.The results show that the fracture hydraulic gap width shows a good linear relationship with the increase of the mechanical gap width,which can be expressed by linear relationship.The relationshi p between the hydraulic gap width and the increase of the mechanical gap width with t emperature is analyzed.By introducing the BB model to calculate the evolution of the mechanical gap width,the linear relationship between the mechanical gap width and t he hydraulic gap width is taken into the cubic law,and the stress deformation seepage calculation model under different temperature conditions is established.The model ca n calculate the change of fracture hydraulic aperture（permeability）caused by the ch ange of normal stress under three temperature conditions（25°C,90°C,120°C）.（4）A fracture deformation-seepage model considering the influence of temperatu re is established.Based on the BB model,the stress-deformation evolution of the fract ure loading process under different temperature conditions is analyzed.It is found that the BB model can accurately predict the initial stiffness and the maximum normal clo sure of the fracture during the fracture loading process.The relationship between the maximum normal closure of the fracture,the initial stiffness of the fracture and the te mperature in the BB model is discussed.The maximum normal closure of the fracture calculated by the BB model is the initial mechanical gap width.Combined with the fra cture deformation during the loading process,the evolution of the mechanical gap wid th during the loading process is obtained.Combined with the fracture hydraulic gap w idth data obtained from the seepage test,the linear function relationship between the mechanical gap width and the hydraulic gap width under the temperature condition is established,and the relationship between the parameters in the function and the tempe rature is discussed.Finally,the BB model and the relationship between hydraulic gap width and mechanical gap width are brought into the cubic law,and the stress deforma tion seepage model considering the influence of temperature is established.The model is verified.The results show that the model can well characterize the permeability ev olution during fracture loading under temperature conditions.（5）The point cloud data before and after the seepage of the fracture surface are obtained by using the three-dimensional laser scanning test,and the three-dimensional model of the fracture surface is reconstructed by programming.Based on the digitization of fracture surface,R_S,Z₂ and three-dimensional fractal dimension D are used to quantitatively characterize the fracture roughness.The results show that the roughness of fracture surface decreases after variable temperature-stress fracture seepage.In addition,the K-means clustering algorithm is used to identify the mineral distribution of the fracture surface,and the proportion of three types of minerals（quartz,feldspar,mica）is calculated.The results show that after the temperature rise and fall cycle seepage test,biotite and feldspar increase significantly,and quartz decreases.