RQD [Rock quality designation]

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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.

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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:

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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.

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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.

RELATED ARTICLES:

EARTHEN DAM | TYPES OF AGGREGATES | MASS CONCRETE

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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