# Core recovery parameters

Core recovery parameters describe the quality of core recovered from a borehole.

## Total core recovery

Total core recovery (TCR) is the borehole core recovery percentage.

TCR is defined as the quotient:

${\displaystyle TCR=\left({\frac {l_{\mathrm {sum~of~pieces} }}{l_{\mathrm {tot~core~run} }}}\right)\times 100}$%
${\displaystyle l_{\mathrm {sum~of~pieces} }}$ = Sum of length of core pieces
${\displaystyle l_{\mathrm {tot~core~run} }}$ = Total length of core run

## Solid core recovery

Solid core recovery (SCR) is the borehole core recovery percentage of solid, cylindrical, pieces of rock core.

SCR is defined as the quotient:

${\displaystyle SCR=\left({\frac {l_{\mathrm {sum~of~solid~core~pieces} }}{l_{\mathrm {tot~core~run} }}}\right)\times 100}$%
${\displaystyle l_{\mathrm {sum~of~solid~core~pieces} }}$ = Sum of length of solid, cylindrical, core pieces
${\displaystyle l_{\mathrm {tot~core~run} }}$ = Total length of core run

## Rock quality designation

Rock-quality designation (RQD) is a rough measure of the degree of jointing or fracture in a rock mass, measured as a percentage of the drill core in lengths of 10 cm or more. High-quality rock has an RQD of more than 75%, low quality of less than 50%. Rock quality designation (RQD) has several definitions. The most widely used definition was developed in 1964 by D. U. Deere. It is the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm in length measured along the centerline of the core. In this respect pieces of core that are not hard and sound should not be counted though they are 100 mm in length. RQD was originally introduced for use with core diameters of 54.7 mm (NX-size core). RQD has considerable value in estimating support of rock tunnels. RQD forms a basic element in some of the most used rock mass classification systems: Rock Mass Rating system (RMR) and Q-system.

RQD is defined as the quotient:

${\displaystyle RQD=\left({\frac {l_{\mathrm {sum~of~length~of~core~pieces~>100mm} }}{l_{\mathrm {tot~core~run} }}}\right)\times 100}$ %
${\displaystyle l_{\mathrm {sum~of~length~of~core~pieces~>10cm} }}$ = Sum of length of core pieces that are > 100 mm (4 inches) measured along the centerline
${\displaystyle l_{\mathrm {tot~core~run} }}$ = Total length of core run

### Classification table

From the RQD index the rock mass can be classified as follows:

RQD Rock mass quality
<25% Very poor
25-50% Poor
51-75% Fair
76-90% Good
91-100% Excellent

## Rock mass classification

Many rock mass classification systems use core recovery parameters as input parameter, such as Rock Mass Rating and Q-system.

## References

• Deere, D U (1964). "Technical description of rock cores", Rock Mechanics Engineering Geology, 1 (16-22).
• Deere, D U (1989). "Rock quality designation (RQD) after twenty years", U.S. Army Corps of Engineers Contract Report GL-89-1, Waterways Experiment Station, Vicksburg, MS (67).
• Deere, D U & Deere, D W (1988), "The RQD index in practice", Proc. Symp. Rock Class. Engineering Purposes, ASTM Special Technical Publications 984, Philadelphia, (91-101).
• Deere, D U, Hendron, A J, Patton, F D & Cording, E J (1967). "Design of surface and near surface constructions in rock", Proc. 8th U.S. Symp. Rock Mechanics, ed. Fairhurst, publ. AIME, New York, (237-302).