| The geological disposal of high-level radioactive waste requires extremely strict surrounding environment.The first nuclear waste disposal site of China intends to use granite with good properties as the surrounding rock for permanent storage of nuclear waste.Underground research laboratory（URL）is a key facility for research on related issues,however,the existence of a large number of developed fracture discontinuities adversely affects the properties of the entire rock mass,thus posing great chal enges for the safe construction of URL and the safe disposal of nuclear waste.Therefore,it is of great theoretical and practical significance to study the relevant characteristics of fractures in granite and the permeability of fractured rock masses for URL site selection and evaluation.In this paper,the typical exposed granite outcrops in Beishan,Gansu Province,are taken as the research object.Based on the point cloud model constructed by digital close-range photography,abundant and accurate information of granite fractures are extracted,and their distribution law are analyzed in detail.Through the image processing technology,the geometric characteristics of fresh fracture aperture exposed by cutting surface weathered rock were studied.Then the connectivity and water conductivity of related fractures are explored by high-precision electrode resistivity method.Based on the above obtained fracture information,a three-dimensional fracture network model is established,and the fracture permeability and the permeability characteristics of the fractured rock mass are studied by the equivalent pipe network method.The research mainly achieved the following results:（1）Clustering analysis of the acquired occurrences by improved fast fuzzy clustering method can reasonably know the grouping of the orientation and the corresponding cluster center orientation.It is discussed that the information of fracture trace length and density can be obtained reasonably by arranging circular window with appropriate radius on the point cloud model.It is concluded that the fracture trace length mainly obeys lognormal distribution in the study area.（2）In view of the fact that most fractures in Xinchang area are mostly characterized by the steep dip angle and a little characterized by gentle dip angle,the improved Mil er method was used to divide the survey area into nine homogeneous areas.Combined with the distance between the homogeneous area and the main faults of the area,these homogeneous areas can be further divided into fault-affected areas and away from fault-affected areas.（3）In order to study the characteristics of the fracture aperture,the weathered surface fractures were cut and discarded.Based on the image processing technology,the geometric characteristics of the new aperture were obtained,and also the relationship between the fracture opening degree and the trace length was established.In order to further study the permeability characteristics of fractures under different opening degrees,the high-precision electrode resistivity method was used for field test research.It is concluded that the main reason that affects the permeability of fractures is whether the fractures have some degree of cementation and the connection of fractures.（4）Based on the three-dimensional fracture network model,a circular section variable diameter seepage channel equivalent pipe network model for studying fracture seepage is proposed.The geometric characteristics and related calculations of the model are given.In order to reflect the fracture permeability characteristics,the equivalent path permeability coefficient is proposed.It is concluded that in the same case,when there is a certain degree of cementation of a set of fractures and not,the mean value of the equivalent path permeability coefficient is1×10-7 and 1×10-3 respectively,the difference is 4 orders of magnitude.（5）On the basis of pipe network model,the permeability tensor of rock mass is calculated for related outcrop in homogeneous I,and it is concluded that the size of representative elementary volume is about 25m.By calculating 108 spatial directional equivalent permeability coefficients through spatial rotation matrix,the maximum principal permeability value of rock mass in this region is 5.159×10-7m/s,corresponding to the main direction of 200.95°∠68.47°.By fitting the spatial permeability el ipsoid,the principal permeability value of rock mass in this area is 4.863×10-7m/s,and the main direction is 208.65°∠76.53°.The two are close to each other and are within the range of borehole pressure test values near the area. |
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