Rock Quality Designation (RQD): A Simple Guide

Introduction:

Rock Quality Designation (RQD) is a widely used method to assess the quality of rock masses. It was developed by Deere et al. in 1967 to provide a quick and objective way to evaluate rock quality during the early stages of construction or mining projects. RQD is particularly useful for geologists and engineers who need to understand the strength and stability of rock formations.

What is RQD?

RQD is calculated by measuring the lengths of intact rock pieces in a drill core that are longer than 10 cm. These lengths are added together and divided by the total length of the core run. The result is expressed as a percentage. For example, if a 1-meter core contains 80 cm of intact rock pieces longer than 10 cm, the RQD is 80%.

Key Points to Remember:

1. Core Discing: If the core breaks into discs due to high stress, these breaks should not be counted in the RQD calculation. However, they should be noted in the drill log as they indicate potential stress-related issues during excavation.
2. Rockmass Behavior: RQD gives a rough idea of how the rock will behave. A low RQD (below 50%) means the rock is heavily fractured, and its strength will be much lower than what is measured in the lab. A high RQD (above 95%) suggests the rock is strong and stable, similar to lab samples.
3. Limitations: RQD works best for rocks with joint spacings less than 0.3 meters. For larger block sizes, additional parameters are needed for accurate classification.

Directional Nature of RQD:

RQD can vary depending on the direction of drilling. For example, drilling parallel to rock layers (laminations) may give a high RQD, while drilling perpendicular to the layers may result in a lower RQD. To get a complete picture, it’s best to drill cores at different angles.

How to Measure RQD:

1. Measure RQD as soon as possible after retrieving the core to avoid errors caused by handling or exposure.
2. Use a clearly marked ruler placed alongside the core box for accurate measurements.
3. Calculate RQD over intervals of 2 to 10 meters, depending on the project’s needs. Shorter intervals are better in areas where rock quality changes quickly.

Alternative Methods for Estimating RQD:

Sometimes, drill cores are not available, and RQD must be estimated from exposed rock surfaces. Two common methods are:

1. Volumetric Joint Count (Palmström, 1982): Count the number of joints per meter on a rock face and use this to estimate RQD.
2. Graded Rule Method: Place a 2-meter ruler on a rock face and count the joints that intersect the ruler. Only count natural joints, not cracks caused by blasting or stress.

Applications of RQD:

• Mining and Construction: RQD helps engineers design safe and efficient excavation plans.
• Blast Damage Assessment: By measuring RQD before and after blasting, engineers can assess the damage caused by explosions.
• Rockmass Degradation: RQD can also be used to check if the rock is breaking down over time after exposure.

Conclusion:

RQD is a simple yet powerful tool for assessing rock quality. While it has some limitations, it provides valuable insights into rockmass behavior, especially when combined with other classification methods.

Reference Source:

• Deere, D.U., et al. (1967). "Rock Quality Designation (RQD) After Twenty Years."
• Palmström, A. (1982). "The Volumetric Joint Count as a Measure of Rock Mass Quality."
• Priest, S.D., and Hudson, J.A. (1976). "Discontinuity Spacings in Rock."
• Bieniawski, Z.T. (1979). "The Geomechanics Classification in Rock Engineering Applications."

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