What Is RQD | Advantages of Rock Quality Designation | Limitations of Rock Quality Designation (RQD)

Introduction of RQD

In Civil engineering, different parameters are used to characterize the Quality of the rocks. Evaluation of the Mechanical Properties of the rocks and Characteristics of rocks is one of the challenging parts of rock engineering. Rock mass has wide applications in the construction of structures, excavation and tunnels and maintaining the stable slopes.

Hence, there must be a perfect parameter which is used to evaluate the Quality and the Properties of the rock mass. RQD is widely used as a crude indicator to evaluate the quality of Rock mass and its properties. 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. RQD (Rock quality designation ) signifies the degree of jointing or fracture in a rock mass measured in percentage, where RQD of 75% or more shows good quality hard rock and less than 50% showed low quality weathered rocks.

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.

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.

The empirical methods which are based on Rock Quality Designation provide a very convenient way for estimation of the deformation modulus and unconfined compressive strength of rock masses. 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

1. Rock quality designation(RQD) is widely used to check the quality of rocks.
2. 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.
3. Rock quality designation(RQD) is also used to evaluate the zones of rock weakness and degree of jointing for fracture in a rock mass.
4. It is used to determine the depth of foundation and Bearing capacity of the rock mass.
5. Rock quality designation(RQD) is also used to identify the weak rock zones which is very important in the construction of heavy structures.
6. The RQD has many applications in mining, engineering geology as well as geotechnical engineering.
7. Rock quality designation is also used to obtain the tunnelling conditions.

Limitations of Rock Quality Designation(RQD)

1. The Rock quality designation has variable values depending upon the direction of the run.
2. Rock quality designation(RQD) does not give in the information of the core pieces which are less than 10 cm.
3. Rock Quality Designation gives wrong values where joints contain thin clay fillings or weathered material.
4. Does not take direct account of joint orientation.

FAQs on Rock Quality Designation (RQD) in Civil Engineering

What is RQD in civil engineering?

RQD stands for Rock Quality Designation. It is a parameter used to measure the degree of jointing or fracture in rock masses. RQD provides an assessment of the quality of rock obtained from boreholes.

How is RQD measured?

RQD is measured as the percentage of the drill core length in pieces 10 cm or longer, relative to the total length of the core run. The formula for RQD is: RQD=(Sum of the length of core pieces greater than 10 cmTotal length of the core run)×100\text{RQD} = \left( \frac{\text{Sum of the length of core pieces greater than 10 cm}}{\text{Total length of the core run}} \right) \times 100RQD=(Total length of the core runSum of the length of core pieces greater than 10 cm​)×100

Who developed the RQD index?

The Rock Quality Designation index was developed by Deere in 1964.

What does an RQD value indicate about rock quality?

RQD values indicate different levels of rock quality:

1. Less than 25%: Very poor (completely weathered rock)
2. 25% to 50%: Poor (weathered rock)
3. 51% to 75%: Fair (moderately weathered rock)
4. 76% to 90%: Good (hard rock)
5. 91% to 100%: Very good (fresh rock)

How is the core recovery calculation performed?

Core recovery is calculated using the formula provided for RQD. It involves measuring the length of core pieces that are 10 cm or longer and dividing by the total core run length, then multiplying by 100 to get the percentage.

What are the advantages of using RQD?

1. RQD provides a quick assessment of rock quality.
2. It helps identify zones of rock weakness and the degree of jointing or fracture.
3. RQD is used to evaluate the depth of foundation and bearing capacity of rock masses.
4. It is crucial for identifying weak rock zones, essential in constructing heavy structures.
5. RQD has applications in mining, engineering geology, geotechnical engineering, and tunneling conditions.

What are the limitations of RQD?

1. RQD values can vary depending on the direction of the core run.
2. It does not provide information on core pieces less than 10 cm.
3. RQD can give incorrect values where joints contain thin clay fillings or weathered material.
4. It does not directly account for joint orientation.

What core size is recommended for determining RQD?

The International Society for Rock Mechanics (ISRM) recommends using a core size of at least NX (54.7 mm) drilled with a double-tube core barrel and a diamond bit.

How is RQD used in rock mechanics and engineering?

RQD is used to estimate the deformation modulus and unconfined compressive strength of jointed rock masses. It is a key factor in assessing the deformability and strength of rock masses.

Can RQD be used in both field and laboratory settings?

Yes, RQD can be determined both in the field and laboratory settings to assess rock mass strength and other properties.