RQD [Rock quality designation]

Rock quality designation (RQD) is the quality of a rock core taken from a borehole.

RQD refers to fracture measured by joining or percentage of the rock mass, where RQD of 75% or more exhibits good quality hard rock and 50% lower quality rock.

Pieces of the rocks whose length is greater than 100mm are not considered for calculation of RQD.

Those pieces of rock that are hard and of good quality are considered; weathered rocks do not satisfy the soundness requirements.

The length of the core pieces is measured along the centerline; the RFQ test provides an assessment of the damage caused by rock sound and weathering.

Rock core samples for RQD calculations:

The following table shows the values of RQD of different quality of rocks.

Rock Quality RQD (%)
Very poor (completely weathering rocks) <25%
Poor (cracked rocks) 25 to 50%
Fair (moderately pitted rocks) 51 to 75%
Good (hard rock) 76 to 90%
Very good (fresh rocks) 91 to 100%

Determination and Applications of Rock Quality Designation (RQD):

For the calculation of roux core recovery and RQD, see the following process:

Rock core recovery calculation:

Core recovery is calculated by the following formula:

Core recovery (CR) = (total length of recovered rock / total core run length) x100

There, the total length of the rock = 250 + 200 + 250 + 190 + 60 + 80 + 120 = 1150 mm

Total core run length = 1200mm

Therefore, core recovery (CR) = (1150/1200) x100 = 96%

Rock quality designation RQD = (SUM {sound piece length> 100 mm} / total core run length) x 100

SUM {length of sound pieces> 100 mm} = 250 + 190 + 200 = 640 mm

Therefore, RQD = (640/1200) x100 = 53% which are rocks of proper quality, i.e. the rocks are of medium scale.

Necessary Measures:

RQD is used with all core sizes other than BQ and BX cores are perfect with NQ core sizes.

The core brakes initiated by the drilling process must be fitted collectively and calculated as a single piece of sound core.

Drilling brakes are typically eliminated by rough fresh surfaces & for the laminated rocks, it would be difficult to classify core breakage due to drilling.

In this case, RQD should be conservatively estimated:

For classification of shear strength, do not calculate the size of the near-horizontal fissure.

Whereas, for estimates of rock blasting requirements, it is customary to estimate the length of the horizontal gap approximately.



Uses of Rock Quality Designation (RQD):

  • Rock quality designation is used to estimate the quality of rocks such as degree and depth of weathering, rock weakness and areas of cracking.
  • This information is used to conclude the depth of the foundation, to find the carrying capacity of the rocks, also to find the settlement of the foundation and sliding possibilities.
  • RFQ helps to achieve favourable tunnelling conditions and to estimate the areas with poor quality rocks that cannot support engineering structures.
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Hello, I'm Rahul Patil founder of Constructionor.com, I had studied B.E. Civil. This blog provides authentic information regarding civil structures, equipment, materials, tests & much more.

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