| Geometrical parameters of fractures developing in natural rocks, such as density, dominant orientations, length, fracture aperture, are long recognized by engineering scientists because they are not only important parameters to fluid dynamics of fracture systems but also main considering factors for large-scale projects during construction and stability analysis. In several decades many scholars have made wide and deep research to fracture geometrical parameters using tectonic geology, tectonophysical test simulation and numerical experimentation and emulator and obtained abundant results.In this thesis, as a candidate area for high level waste repository (HLW) in China, the rock mass fracture system of the Bantan unit granite of the Jiujing block the Beishan area in Gansu province, is chosen as the study object. The fracture system is studied in detail by field survey bedrock outcrop fractures surveying and combining completed regional geological survey results with acoustic borehole television measurements. Traditional probabilistic statistics and the fractal theory are used to analyze fracture geometric parameter including fracture length, azimuth and density . Statistics and calculation using the ArcGIS software is made to study fracture distribution characteristics in the granite rock mass.Quantitative description is performed to fracture geometrical parameters in order to provide necessary basic data for the selection of HLW. In the same time combining fracture density with fracture network spatial structure, using RQD and fractal dimension the quality of rock mass of the Bantan unit granite is assessed.The result is of this thesis are as follows:Firstly, fracture geometrical parameters are obtained by statistics on ArcGIS.Using numeral camera I investigate tectonic joints developed in the Bantan unit granite rock mass. Five specimens including 328 square meters of digital planar fracture images were finished. Then by geometrical transform and rectification disposal, manual judgments, interpretation, digitalization, fracture network digitalized map can be obtained. According to regional conditions for fracture development,combined with outcrop field fracture survey results and borehole fracture features, fractures are grouped, counted and calculated by Select Query Language (SQL.) in ArcGIS software.Fracture length distribution is inhomogeneous and anisotropic. Total fracture length of preferential orientation show rapid increasing. Fracture lengths change continuously. When the sampling area is bigger and the fracture number is large, fracture lengths accord with an exponential function. Fracture lengths mostly fall between 0.5 and 4 meters and their proportion reaches 95 percent. The probability density function of fracture lengths follows a power function. The proportion of long and short fractures is between 1 and 2.Fracture planar density reflects fracture developing degrees on a rock cross section. Its value is related to fracture number and size. Bigger density indicates well developed fractures. Two of 5 specimens are 4.642 and 4.113 in density, respectively, the others change near 3.Fracture permeability ratio is directly proportional to the cube of fracture aperture. So fracture aperture distribution is very important to study fracture permeability. By the statistics and analysis of outcrop fractures, it is found that the fracture aperture between 0.1 to 0.3 centimeter accounts for 78%, that between 0.3 and 0.5 centimeter is 12%, and greater than 1 centimeter is less than 5%. According to the description recommended by ISRM, the fractures of the Bantan unit granite of the beishan area mainly belong to the open-rupture type.Joint statistics is characterized by sizes effect. Different size of survey grids bring about distinct statistical results. This work chooses the sample of No.3 borehole-3 to simulate that fracture geometrical parameters are characterized by orientations and sizes when the of survey grid changes. Analysis result shows that the relatively increasing mode of fracture lengths fits the exponent function y=ce x whatever the measuring size is. With increasing of the survey size the length increasing trend becomes slow. The total characteristics of fracture density are from small-scale unstable change to large scale gradual steady. Survey orientations affect strongly calculated results.Secondly, spatial structure of fracture network is studied by the fractal theory. On the basis of box-counting dimension, a simple way to calculate fractal dimension is developed on the fracture network map of Beishan. The result shows that spatial structure of the fracture system is fit for fractal distribution within the range from 10 centimeters to 200 centimeters. At the same sampling site fractal dimensions of fractures in different group vary for different fracture developing degrees. While in the different survey places, because of varied survey grid sizes, fracture numbers, fracture lengths, fracture azimuths and fracture developing degrees fractal dimensions present different values. Fractal dimensions of two sites with relatively well developed fractures are big, separately 1.724 and 1.636. The other three of five average about 1.6.Many natures of rock mass fractures have fractal features to some degree, but their fractal properties are not often strict rather in a statistical sense. So, different measure areas correspond to obviously distinctive results. The research results are not comparable with each other. How to define the scale invariant section deserves deep investigation. This work seeks for a brief way to solve this problem.Fractal dimensions indicate complexity of fracture networks. A lot of impact factors have effects on their values. From the six aspects such as planar density, fracture number, fracture size, fracture predominant orientation, the ratio of long and short fracture, and area of survey grid, I think fore-three items affect obviously, in which density is related to linear correlation coefficient R up to 0.914.Finally, the rock mass engineering quality of the Bantan unit granite is appraised by RQD (Rock Quality Designation) and the fractal method.RQD (Rock Quality Designation) judges the good and bad rock mass qualities by intact degrees of drilling core and classifies rock mass. Using finished drilling data I analyze RQD of BSOl and BS03 and conclude that RQD of BSOl is between 75% and 100% accounts up 75.84%, and averages 74.4%. The RQD of BS03 is generally big. So, the engineering quality of the Bantan unit granite borehole belongs to the good class with intact core.Fractal dimensions not only describe fracture number of rock mass but alsoreflect homogeneous degree of fracture distribution and the intact and cracked conditions. So rock mass quality classified by fractal dimensions combined with RQD is more objective and efficient than single RQD index. Above research indicates that the fracture network is characterized by fractal distribution. Fractal dimensions of five sampling sites are in turn 1.636, 1.548, 1.596, 1.724, and 1.604, respectively, and fall into between 1.5 and 1.75. According to the rock mass quality classified by fractal dimensions, the Bantan unit granite belongs to rock mass with fracture well developed and normal quality.
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