The easiest way to characterize the amount of material recovered during rock coring is to calculate “core recovery” as the amount (i.e. length) of recovered material divided by the total length of the core run (presented as a percentage).
Rock Quality Designation (RQD) is a modified core recovery percentage in which the lengths of all sound rock core pieces over 100 mm in length are summed and divided by the length of the core run. Pieces of core that are not hard and sound should not be included in the RQD evaluation even if they are at least 100 mm in length. The purpose of the soundness requirement is to downgrade rock quality where the rock has been altered and/or weakened by weathering. For the RQD evaluation, lengths must be measured along the centerline of the core.
Figure shown below illustrates the correct procedure for calculating core recovery and RQD.
The following table presents the correlation of RQD to rock quality.
ROCK QUALITY DESCRIPTION BASED ON RQD.
Description of Rock Quality
The RQD is appropriate for use with all core sizes except for BQ and BX core with NX and NQ core size being optimal.
Core breaks caused by the drilling process should be fitted together and counted as a single piece of sound core.
Drilling breaks are usually evidenced by rough fresh surfaces, however for laminated rocks (i.e., rocks containing horizontally oriented fracture surfaces), it may be difficult to identify core breaks caused by drilling. In this case, the RQD should be estimated conservatively; for shear strength characterization it is conservative to not count the length near horizontal breaks whereas for estimates of rock blasting requirements, it is conservative to count the length near horizontal breaks.