Variation of rock quality designation (RQD) with scanline orientation and length: a case study in Korea

Abstract Rock quality designation (RQD) has been widely used to evaluate the degree of fracturing of rock masses. Although it has been frequently pointed out that RQD is dependent on the orientation of measurement and on the scanline length, these effects have not been considered in site investigation for tunneling in Korea. Therefore, a series of practical examinations using in situ data collected from recent tunneling sites in Korea has been conducted. Variation of RQD with scanline orientation was observed to be as high as 24% in the sub-horizontal tunneling where the design was based on the results of RQD from vertical drill core logging. In addition, the range of possible variation of RQD with scanline length was analyzed with four different models, including two new models introduced in this case study for the first time. It is shown that at least 4 m of scanline length is required to maintain a reliable value for the RQD in the three models. The simulation from the so-called A–A–S model showed that it is possible to maintain the same reliability of the RQD in a situation where only a much smaller extent (i.e. 0.4 m) of rock exposure is available in the field. It is hoped that the results of this case study will provide the tunnel engineer with an enhanced understanding of the variation of RQD with measurement method.