Rock Quality Designation(RQD)

What is the ROCK QUALITY DESIGNATION(RQD)?

Rock quality designation(RQD) was introduced by Deere in 1964 as an index of assessing rock quality quantitatively. It is more sensitive as an index of the core quality than
the core recovery.

The RQD is a modified percent core recovery that incorporates only sound pieces of core that are 100 mm (4 in.) or greater in length along the core axis

RQD is found to be a practical parameter for core logging, but it is not sufficient on its own to provide an adequate description of rock mass (Bieniawski, 1984). The following methods are used for obtaining RQD.

DIRECT METHOD

For RQD determination, the International Society for Rock Mechanics (ISRM) recommends a core size of at least NX (54.7 mm) drilled with double-tube core barrel using a
diamond bit. Artificial fractures can be identified by close-fitting cores and unstained surfaces.

All of the artificial fractures should be ignored while counting the core length for RQD. A slow rate of drilling will also give better RQD.
The relationship between RQD and the engineering quality of the rock mass as proposed by Deere (1968), is seen in Table 1.
The correct procedure for measuring RQD is shown in Figure 1. RQD is perhaps the most common method for characterising the degree of jointing in borehole cores,
Although this parameter may also implicitly include other rock mass features like
weathering and “core loss” (Bieniawski, 1989).

Table 1. Correlation between RQD and Rock Quality