DISCUSSION ON CORRECTION METHODS OF ESTIMATING MIDPOINT DENSITY OF JOINT TRACES

The midpoint density of joint traces at outcrop is an important index for identifying the distribution characteristics of joint. However,the calculated results of midpoint density generally contain censoring,size and orientation biases,because of the factors including the limited size of outcrop or survey window,the difference in trace-length and the variation of the included angle between the outcrop and the joint. Therefore,we firstly present the conventional methods of correcting the censoring,size and orientation biases,and then propose a new method of correcting the above biases by importing the correction coefficient of orientation bias into the end-point estimator. We systematically test the proposed method by generating a 3D discrete joint network model and calculating the midpoint densities of joint traces from survey windows with different orientations,sizes and shapes. Finally,we use the proposed method to estimate the midpoint density of joint traces at a real outcrop,located at the entrance of the Beishan Exploration Tunnel(BET) in Gansu Province,China. The calculation example and the case study indicate that the proposed method is effective in correcting the censoring,size and orientation biases. The proposed method provides a reliable tool to estimate the density of joint randomly distributed in rock mass.