Introduction of RQD
In Civil engineering, different parameters are used to characterize the Quality of the rocks.
In this article, you will get to know What is RQD and the method of Core Recovery calculation.
What Is RQD?
The full form of RQD is Rock Quality Designation which is a parameter used to measure the degree of jointing or fracture in the rock mass.
In simple words, the Rock quality designation is nothing but the measure of the quality of the rock which is obtained from the borehole.
It is generally measured as the percentage of the drill cover in the length of 10cm or more.
The Rock quality designation index was developed by Deere in the year 1964.
The pieces of Rock are considered which is of hard and good quality. The length of core pieces of Rock mass is measured along the centerline of the pieces
While measuring the index there should not be inconsistency in the measurement.
Core Recovery Calculation
Core Recovery calculation is done by using the following formula
Rock quality designation is generally calculated by taking the sample of rock mass which is obtained from a borehole.
RQD =[(Sum of the length of core greater than 10 cm) X 100] divided by the total length of the core run
The Rock quality designation (RQD) is generally expressed in the Percentage (%).
In the rock quality designation, the surface is drilled at a certain depth and with the help of the core property of the rock mass is determined.
For determination of Rock quality designation using core boring, the International Society for Rock Mechanics (ISRM) has recommended a core size of at least N X (size 54.7 mm) drilled with double-tube core barrel using a diamond bit.
Condition of the Rock as Per There Rock Quality Designation.
|Condition of the Rock
RQD in %
|Very poor completely (weathered rock)
Less than 25%
|Poor (weathered rock)
25 to 50%
|Fair moderately (weathered rock)
51 to 75%
|Good (hard rock)
76 to 90%
|very good( fresh rocks)
91 to 100%
The determination of rock mass strength using the technique of RQD can be performed in the field or the laboratory.
Determination of the deformation modulus and unconfined compressive strength of jointed rock masses is a crucial and challenging part in rock mechanics and rock engineering.
In many cases, The Rock quality designation (RQD) is only available data about the discontinuity in the routine site investigation.
RQD is the only factor which affects the deformability and the strength of jointed rock mass.
Advantages of Rock Quality Designation
- Rock quality designation(RQD) is widely used to check the quality of rocks.
- This test helps to provide an assessment of the soundness of the rocks and the damages which occurred in the rocks due to the action of weathering.
- Rock quality designation(RQD) is also used to evaluate the zones of rock weakness and degree of jointing for fracture in a rock mass.
- It is used to determine the depth of foundation and Bearing capacity of the rock mass.
- Rock quality designation(RQD) is also used to identify the weak rock zones which is very important in the construction of heavy structures.
- The RQD has many applications in mining, engineering geology as well as geotechnical engineering.
- Rock quality designation is also used to obtain the tunnelling conditions.
Limitations of Rock Quality Designation(RQD)
- The Rock quality designation has variable values depending upon the direction of the run.
- Rock quality designation(RQD) does not give in the information of the core pieces which are less than 10 cm.
- Rock Quality Designation gives wrong values where joints contain thin clay fillings or weathered material.
- Does not take direct account of joint orientation.