Study On Representative Elementary Volumes Of The Rock Mass At Different Scales And Geometrical Characteristics At The Right Dam Site Of Maji Hydropower Station

Because of the existence of a lot of random discontinuities in fractured rock masses,the mechanical,geometrical and hydraulic properties of rock masses become increasingly complicated,and these discontinuities control the rock mass deformation and stability.Hence,fractured rock masses have properties that are discontinuous,inhomogeneous,and anisotropic;furthermore,their properties vary at different scales.The Maji hydropower station dam site is located in the Southeast of Qinghai-Tibetan Plateau,the river valley is characterized by steep terrain,strong neotectonic movement,complex geological structure,developed fractures and complex engineering geological conditions.The predominant lithology of the project is mixed granite and mixed gneiss formed in the Himalayan period,as identified in the field outcrop and exploration tunnels.After multi-stage tectonic movement,the rock strength is high and the texture is hard.Therefore,analyzing the structural properties of rock masses at the dam site are very important when evaluating the design,construction and stability of rock engineering projects.In this paper,based on the fracture data located in the outcrop,borehole and tunnel,the information on lithological weathering degree of topography,geomorphology,geology,structure and stratum,relative coordinate trace length occurrence and trace type of initial end points of joints and fissures in the exploration tunnel are obtained.According to the structural characteristics of the rock mass,the occurrence of fractures and the lengths of traces,the improved Miller method is used to classify the statistical homogeneous zones of the fractured rock mass structure.Subsequently,taking the fractured rock mass in each homogeneous region as the research object,according to the distribution characteristics of occurrence,diameter,center point and density of the fractures,the three-dimensional fracture network models for different geological units at the dam site are established by using Monte-Carlo method and Baecher disc model theory.Based on the three-dimensional fracture network simulation technology and the fractured rock mass being located in the right bank dam site of Maji hydropower station,this paper discusses the evaluation method,size calculation and their relationship,anisotropy and geometrical characteristics of representative elementary volumes at different scales,according to different influencing aspects.The main research contents are as follows:1.Study on the representative elementary volume for the engineering rock massIn this paper,four exploration tunnels on the right bank of the dam site are taken as the research objects.Based on the Miller's methods,considering both occurrence and trace length parameters,similar boundary ranges of the rock mass structure between different tunnels and statistical homogeneous zones of different tunnels with the same properties are discussed respectively by using the likelihood ratio test method.Finally,principles for determining statistical homogeneous zone of the rock mass on the right bank of Maji hydropower station and the size of the representative elementary volume for the engineering rock mass are determined.2.Study on the representative elementary volume for the generalized rock sizeThe representative elementary volume for the generalized rock size is what we usually call the representative elementary volume?REV?.In this paper,based on the field investigation data,a 3D fracture network model of right bank rock mass at the dam site is established.100 random regions are selected to calculate the volumetric fracture intensity(P₃₂)in the model.Based on the distribution characteristics of the volume fracture intensity(P₃₂)obtained from each calculation region,the multisample Jonckheere-Terpstra test method which can simultaneously consider the statistical similarity between multiple samples is used to analyze the size effect of the fractured rock mass at the Maji dam site,and the sizes of the representative elementary volume for the size effect are determined.3.Difference analysis of the representative elementary volume at different scalesBased on the study of the representative elementary volumes of the engineering rock mass and the size effect,we find that these two representative elementary volumes have some similarities and differences under certain conditions.In this paper,the evaluation methods,the existing conditions and the relationship between these two scales are analyzed and discussed respectively.The size effects of these two representative elementary volumes are also studied.The results show that the representative elementary volume for the engineering rock mass is the maximum value of similar rock mass structure,while the representative elementary volume for the generalized rock size?REV?is the minimum value of similar rock mass structure.4.Anisotropic characteristics of REVIn this paper,based on the 3D fracture network model,the volume fracture intensity index(P₃₂)and multisample non-parametric statistical test method are used to analyze the REV characteristics in different sampling directions.The anisotropic characteristics of the representative elementary volume for the generalized rock size are studied and the relationship between the number of fracture groups and the anisotropic characteristics of REV is discussed.With the increase of the number of fracture groups,the anisotropic characteristics of REV become more obvious.5.Research on REV geometryIn this paper,the geometrical shape of the REV is studied by this method considering single fracture group.According to the concept of REV,the distribution rule of REV in any sampling direction of each fracture group is obtained by theoretical calculation.It is proposed that the maximum REV value in the fracture group be used to represent the REV of this fracture group.According to the principle of superposition,we combined the maximum value of the REV of each fracture set;the results showed that the geometric shape of the REV of entire rock masses is a complex,irregular geometry associated with fracture sets.6.A new concept and calculation method of the geometric shape of an REV are presented for entire rock masses.The size and direction of REV for each fracture group are determined according to the sampling direction and calculation parameters of each fracture group.These cubes?REV?are combined using the superposition principle and the center of the cube weight,this irregular geometry obtained is the REV for this structural geological unit.7.The equivalent structure rock mass model of the abutment rock mass is established according to the characteristics of multi-scale representative elementary volumes,and the distribution characteristics of the stress and displacement fields for the abutment rock mass slope are analyzed by the finite element software at the Maji dam site.