The RQD is defined as the cumulative length of core pieces longer than 10cm in a run divided by the total length of the core run. The total length of core must include all lost core sections. Any mechanical breaks cause by the drilling process or in extracting the core from the core barrel should be ignored. Core discing due to high stress should not be considered in the RQD calculation but noted separately in core logs. RQD should be measured as soon as possible after drilling. Sum of lengths of core sticks > 10 cm long x 100 Total length of core run |
Table 1. Rock Mass Classification Based on RQD RQD | Rock Quality Classification | <25% | Very Poor | 25-50% | Poor | 50-75% | Fair | 75-90% | Good | 90-100% | Excellent |
Reference: Deere et al, (1988) | 
