Three Gorges Underground Power House Rock Fractures Orientation Correction

Most of the engineering geological incidents appearing in the rock are related to the structural surface, and fractures in the rock is an important factor to the properties of rock engineering .The structure not only undermines the integrity of rock, but also directly affect the rock’s mechanical properties and stress distribution. Therefore, to grasp and describe the fractures in the rock network correctly is the foundation to establish a reliable rock mechanics model.The number of fractures in Three Gorges Underground Power House is so large that it’s impossible to understand the geometry of each fracture. We can just determine the fracture density, the average size, the overall direction in a certain range as a whole and perceive these parametrers as random variables, then build the quantitative model which is called the stochastic model. In this way, we can establish stochastic fracture network model when tread geometry parameters of fracture as random variable.According to fractures on the upstream observation window in the Three Gorges Underground Power House, then statistic the orientation and the trace length of each fracture. The orientation of fracture can be observed directly on observation window, however, if we determine the orientation and distribution features of the fractures from the statistical data on observation window directly , there would be some errors exist. This is because the fracture which is vertical to observation window tend to intersect with obveration window and appear more easily, and there would be more when we statistic fractures. On the contrary the number of those parallel to the obveration window will decrease as little appeared when we statiatic them. Therefore, the statistical error analysis correction of fractures orientation is an important process. The fractures were grouped according to their orientation after correcting the fracture, and the average orientation and length and density distribution function can be calculated seperately. The fractures on the observation window can be divided into three groups. The average orientation are 77.7°∠82°, 355°∠73.1°, 76.1°∠30.3°, and the direction of the distribution are subject to Fisher. According to making fractures trace length histogram and testing their Pearson conduct, we can obtain fractures trace length in lognormal distribution, and the mean fracture of three groups are 6.29 m, 6.63 m, 7.71 m, and the corresponding standard deviation of three groups are 3.74 meters, 3.93 meters, 4.87 meters respectively. Then we can assume that the orientation and length of fractures in the model of rock fracture are subject to Fisher distribution and log-normal distribution respectively. According to the number and the mean fracture trace length, standard deviation, the value of one-dimensional density of each group ,then use Monte-Carlo theory to simulate fractures of the mapped fractures on two-dimensional outcrops and use inverse method to optimize the geometric parameters such three-dimensional size and the density of fracture. When the each group the number of fractures and the mean, standard deviation, and the measured one-dimensional density of the fractures trace length are very consistent to surface of the observation window of the statistic value of these parameters, we consider that the three-dimensional fracture network model of underground powerhouse is reasonable, and the model have some guidance to the seepage and stability of the whole region.