RQD and its calculation

Rock Quality Designation (RQD), proposed by Deere (1967), is the percentage of sound core pieces 10 cm or longer in the total length of a core run. It is an index of the degree of jointing and fracturing of the rock mass.

From drill core

$$RQD = \frac{\sum \text{length of core pieces} \geq 10\ \text{cm}}{\text{total length of core run}} \times 100\ \%$$

Only natural, hard and sound pieces are counted; breaks caused by drilling or handling are ignored. Example: in a 150 cm run with pieces of 12, 25, 8, 30, 15 and 20 cm, the pieces counted are 12 + 25 + 30 + 15 + 20 = 102 cm, so RQD = 102/150 × 100 = 68 % (fair).

From outcrops (no core)

1. Volumetric joint count (Palmstrom):

$$RQD = 115 - 3.3\,J_v \quad (J_v \geq 4.5),\qquad RQD = 100\ \text{for}\ J_v < 4.5$$

where $J_v$ is the number of joints per cubic metre (the sum of the joint frequencies per metre of all sets). 2. Scanline (Priest and Hudson): count the discontinuities per metre, $\lambda$, along a scanline; then $RQD = 100\,e^{-0.1\lambda}(0.1\lambda + 1)$.

RQD classes

Importance

• It is a quick, cheap index of rock quality from boreholes.
• It is one of the parameters of the RMR system (rating 3 to 20) and of the Q-system ($RQD/J_n$ gives block size).
• It helps in choosing tunnel support, estimating rock mass strength and deformability, and deciding grouting and excavation methods.
• Limitation: it ignores joint orientation, condition and filling.