Rock Quality Designation

Reference

RQD reduces a tray of drill core to one percentage that has steered rock engineering since the sixties: how much of the run came out in solid sticks.

The equation

RQD = (Σ pieces of at least 100 mm / run length) × 100
  • Count only sound core pieces 100 millimeters or longer
  • Measure along the core axis over each drill run
  • Mechanical breaks from handling are fitted back together first

Worked example

A 1.2 meter run yielding solid pieces totaling 0.78 meters scores RQD of 65 percent, class fair. The same rock recovered in short rubble would score near zero, flagging a crushed or heavily jointed zone.

When to use it

RQD feeds the Q system and RMR classifications that set tunnel support. It is direction dependent and blind to joint condition on its own, which is why the fuller classifications wrap other factors around it.

Related Calculators and Reference