Rock Quality Designation (RQD) is used vastly as an engineering measure for classification of discontinuous rock masses. In the RQD definition by Deere (1963), a threshold of 10 cm was proposed, which makes rock masses classification a crisp process. In this research, a fuzzy approach is applied covering thresholds in a range to introduce a Fuzzy Rock Quality Designation (FRQD). The results obtained show that FRQD is more consistence with engineering judgments.
The rock quality designation (RQD) was developed by Deere (1963) to provide a quantitative estimate of rock mass quality from drill core logs. It is defined as "the percentage of intact core pieces longer than 10 cm in the total length of core. For at least two main reasons, the use of RQD for classifying discontinuous rock masses is widespread: It is a measure that is reasonably easy and quick to apply as only certain core pieces (longer than 10 cm) are included. Also, due to the recognition of its definition worldwide, it allows in-site comparison to be made. However, in years, it is concluded that the RQD measure encountered with several limitations and drawbacks and that this measure should be applied with care. These limitations influence the engineering judgments where RQD is used in classification systems such as RMR and Q, numerical modeling and other engineering assessments on rocks . One of the drawbacks is that for example, there is no difference between the quality of rock mass with length pieces of 11 cm and 1m as for both RQD is 100%. Another drawback is that for rock masses in which engineering is taking place, RQD values tend to be either high or low. This lack of certain values of RQD must be due to the way in which RQD is expressed . Clearly, as RQD is defined in terms of the percentage of intact pieces of rock greater than the so-called threshold value (10 cm), then this is the only parameter that can affect the value of RQD. This is well known that changing the threshold value changes the RQD and for a given set of discontinuity spacings . It is possible to determine a wide range of RQD values through judicious choices of the threshold value . Selection of the threshold value in RQD assessments was carried out by Harriso . In his work, a technique for determining the RQD threshold value appropriate for any particular rock mass is presented. Furthermore, he showed that how the use of a so-called optimal threshold value enhances the utility of RQD, in that it allows RQD to become a much more discriminatory measure of anisotropy and inhomogeneity in a rock mass. In the present piece of work, a fuzzy approach is applied covering a range of threshold values, which are selected through judicious choices of threshold values.