What is drilling rate index?
DRI is a measure of the difficulty or ease of drilling rock, and it was developed at the Norwegian University of Science and Technology in Trondheim, Norway. The DRI value is strongly related to the uniaxial compressive strength and indirect tensile strength. However, when the uniaxial compressive strength and tensile strength of rock are jointly considered, the correlation coefficient increases.
DRI classification
We can classify DRI as follows according to the NTNU, 1998.
| DRI Value | Term |
| 26 | Extremely low |
| 26-32 | Very low |
| 33-42 | Low |
| 43-57 | Medium |
| 58-69 | High |
| 70-82 | Very high |
| >82 | Extremely high |
The figure below presents some rock type with their DRI Value (Yarali S., et al., 2011).
