A new approach for evaluation of JRC was attempted, using borehole wall image scanned by Borehole Image Processing System. The in-situ work of scanning borehole wall image was executed on the CAES-01 hole excavated at Taejon, Korea between the interval of 200–232m in depth. The horizontal resolution was 720 pixels for the whole peripheral and the vertical was 4 pixels/mm. Joint profiles revealed on the borehole wall were captured and processed by image analyser to have an appropriate set of profile coordinates. Using macro commands built in image analyser programmed this image processing procedure. The coordinates of joint profile were converted geometrically to evaluate the orientation and the JRC value for a certain joint or joint set. A total of 124 discontinuities including 61 natural joints were detected in the image and one major joint set and another minor one were confirmed to exist from stereonet analysis. The JRC value of the major joint set was evaluated to be 11.2±2.8. For the 16 joints selected arbitrary, the JRCs obtained from image processing and real measurement on the joint surface in the rock core were compared for validation of this new method. As JRC is measured along a linear segment, the comparison was done six times on the same joint surface rotating 30 degrees for each time. So the 96 times of comparison was executed. The result shows 41.5% of full match, 41.5% of semi-match and 17.0% of mismatch, from which it could be determined that the image processing technique could be applied in evaluation of JRC.
Rock core recovered from borehole has been one of the most reliable and direct tools to see the inside of rock mass. But during the drilling work, some inevitable situations, such as loosing broken cores and infills and so on may occur.