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.
The RFQ is calculated by sampling the rock core from a borehole and the length of all sounding rock fragments that are at least 100 mm long is expressed and divided by the length of the core run.
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 of the pieces; 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 RQDs of different quality of rocks.
Table-1: Rocks quality and their RQD
|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%|
Roux core recovery and RQD test method:
For the calculation of roux core recovery and RQD, see the following figure:
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) x100
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.
RQD is used with all core sizes other than BQ and BX cores are perfect with NQ and 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.
Use of Rock Quality Designation (RQD) in construction:
- RQD 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.
Frequently Asked Questions:
What is RQD in Civil Engineering?
Rock quality designation (RQD) is a commonly used index for the description of rock mass fracture state.
How is RQD calculated?
The RQD is a modified core recovery percentage, in which all pieces of a sound core greater than 100 mm (4 in) are added and divided by the length of the core run over a longer period of time.
What does RQD 100 mean?
The core pieces that are not hard and sound should not be counted, although they are 100 mm in length.
What is RQD rock mechanics?
Rock-quality designation (RQD) is a rough measure of the degree of joining or fracture of a rock mass, measured as a percentage of drill core in length 10 cm or more.