HOW TO CALCULATE CORE RECOVERY AND RQD OF ROCK?

The easiest way to characterize the amount of material recovered during rock coring is to calculate “core recoveryThe amount (i.e. length) of the recovered material is divided by the total length of the core run (presented as a percentage).

Rock Quality Designation (RQD) There is a modified core recovery percentage in which the length of all sonic rock core pieces longer than 100 mm in length is summarized and divided by the length of the core run. Core pieces that are not rigid and sound should not be included in the RQD evaluation, even if they are at least 100 mm in length. Strengthening requirements are intended to downgrade rock quality where rock has been altered and/or weakened by weathering. For RQD evaluation, the length should be measured along the center line of the core.


The figure shown below shows the correct procedure for core recovery and calculation of RQD.

Drawing on Core Recovery and RQD Calculations
Drawing on Core Recovery and RQD Calculations

The following table presents the correlation of RQD to rock quality.


Rock quality details based on RQD.
RQD value rock quality. description of
0-25% very poor
25-50% Poor
50-75% neutral
75-90% Good
90-100% Excellent

Important Articles:

  • The RQD is suitable for use with all core sizes except BQ and BX cores, with NX and NQ core sizes being optimal.
  • Core breaks caused by the drilling process must be fitted together and count as one piece of the sound core.
  • Drilling breaks are usually evidenced by rough fresh surfaces, although for laminated rocks (i.e., rocks with horizontally oriented fracture surfaces), it can be difficult to identify core breaks due to drilling. In this case, the RQD should be estimated conservatively; It is conservative not to calculate the length near horizontal breaks for shear strength characterization whereas for estimates of rock blasting requirements, it is conservative to calculate the length near horizontal breaks.

Reference

ASTM D 6302


Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.