Study On The Long-term Stability Of Beishan Granitic Host Rock For High-level Radioactive Waste Repository In The Beishan Area In Gansu Province

Safe disposal of high-level radioactive waste(HLW)is a challenging task in the scientific and technological communities.Unlike conventional underground engineering,a high-level radioactive waste repository should be safe for tens of thousands of years or more long-term during the life cycle of the project.Based on the previous studies and requirements of the research and development for the HLW repository,this thesis takes the Beishan granite from Gansu Province as the research object,and uses the methods of theoretical analysis,mechanical tests and numerical simulation.The long-term stability of host rock for a high-level radioactive waste repository at the candidate Beishan area was studied.The following summarizes the major conclusions obtained from this study.(1)In-situ stress measurements were conducted successfully in the Beishan area in vertical boreholes at depths ranging between 30 and 700 m below the ground surface.The distribution characteristics of the regional in-situ stress field were analyzed.The results show that in-situ stresses increase with depth and the maximum horizontal stress is generally larger than the vertical stress,indicating that within the range of tested depth,the regional stress field is dominated by tectonic horizontal stress.In addition,the maximum principal stress is less than 25 MPa within the measurement depth range.The ratio of maximum horizontal stress to vertical stress KHv displays a remarkable nonlinear behavior with increasing depth.On the other hand,the depth has a small influence on the ratio of maximum to minimum horizontal stress ratio KHh,which is on average in the order of 1.5.The stress state is very favorable for the stability of underground excavations due to the relatively low stress magnitudes when compared with the strength of Beishan grainite and small stress difference between the maximum and minimum principal stresses.(2)The conventional triaxial compression tests and cyclic loading tests under different confining pressures were performed.The complete stress-strain curvescoupled with acoustic emission characteristics of the rock samples were obtained.The damage evolution mechanism of the Beishan granite was revealed.According to the volumetric strain measurement results,the plastic strain locus of the specimens was captured and subsequently a dilation angle model considering confining stress and plastic deformation was established.(3)The influence of damage evolution on the permeability coefficient of Beishan granite was investigated using the improved low permeability hydraulic-mechancial coupling test method.The results indicate that the general trend of permeability coefficient is closely associated with the damage evolution process of the rock,which can be divided into three stages,i.e.,the transient stage,the steady stage,and the accelerated stage.It is also found that the increase of the permeability coefficient of Beishan granite falls slightly behind the damage evolution process.(4)The creep test under different stress ratios was conducted.Combined with the information obtained from the three-dimensional acoustic emission monitoring,the influence of stress level on the creep characteristics of Beishan granite was revealed.The results show that the increase of stress level can increase the velocity of rock fracture and hence reduce the failure time under the same confining stress condition.Meanwhile,when the confining stress is kept constant,the deformation of Beishan granite at failure is almost constant,and is not affected by the applied stress level.(5)Based on the distribution characteristics of in-situ stress in the Beishan area and mechanics properties of Beishan granite,the long-term stability analysis of the host rock during excavation of the deep disposal vault was conducted.The long-term evolution characteristics of the excavation damage zone were obtained.The study shows that the disposal vaults and tunnels can maintain long-term stability over tens of thousands of years under the deep engineering geological environment conditions in the Beishan area.