Rock Quality Designation, RQD

Deere et al (1967) developed the Rock Quality Designation, RQD, in response to the need for a quick and objective technique for estimating rockmass quality from diamond drill core logs during the initial exploratory phase of construction. The method is simple and efficient to implement in mining environments and can be assigned to the drillers themselves or to the geologists analyzing core for grade assessment. Information gained at this early stage of exploration is extremely valuable for the geomechanics engineer involved in the mine planning process.

RQD is calculated as the ratio of the sum of the lengths of all pieces of core greater than 10 cm to the total length of the core run. Core discing due to high stress should not be considered in the calculation of RQD but should be noted in the drill log (give an estimate of discing frequency). Discing does not contribute to RQD but does indicate potential risk of brittle overstress problems during excavation.

A great deal of work has been done to correlate RQD with joint frequency, rockmass stiffness, and other properties (Deere and Miller, 1966; Deere and Deere, 1988; Cording and Deere, 1972; Coon and Merritt, 1970; Bieniawski, 1979). Philosophically, RQD provides a crude estimate of the percentage of the rockmass which can be expected to behave in a fashion similar to a laboratory sample (typically 10 cm long). A rockmass with a low RQD (< 50) has few intact blocks larger than 10 cm. In such a rockmass, joints and fractures dominate the rock's response to stress and gravity. The strength and stiffness of the rock, as determined in the laboratory, has little relevance here. On the other hand, rockmasses with RQD > 95 % possess strength and stiffness much closer to the values obtained in the lab. Joints may still dominate behaviour in low stress environments but may have little or no influence at depth (provided joints are clean and tight).

RQD does not accurately reflect conditions in rockmasses with joint spacings greater than 0.3m. Rockmasses with block sizes in this range can be problematic and therefore require additional parameters for adequate classification. While RQD forms the starting point for most assessment procedures, more comprehensive classification schemes are discussed in the following sections. Also consider the directional nature of RQD. For example, a drill hole parallel to a set of major laminations in highly anisotropic rock will yield a relatively high RQD. Core taken from a hole perpendicular to the lamination set will give a much lower value.

When practical, drill core from two or more boreholes at different angles should be considered for complete assessment of RQD. RQD is intended to give a measure of in situ and undisturbed rockmass conditions. Therefore all core breaks due to drilling, handling and discing must be ignored in the calculation of RQD. In addition, minor cracks in the core which are not related to established jointing should also be ignored. Failing to do so may result in an overconservative or unrepresentatively low measure of RQD. In hard rock mining applications, RQD typically measures between 50 and 100. Values lower than this represent special conditions or an unusually poor rockmass.

Exceptions include RQD measured perpendicular to schistosity or foliation. Such a measurement may be much lower than the RQD of the surrounding rock. Blast damage to a rockmass can also be reflected in a reduced RQD (Løset, 1992).

Ideally the RQD should be measured as soon as possible after the core has been removed from the core barrel. RQD should be a part of the preliminary logging procedure. If a clearly marked rule is laid out along side of the core box, a geologist or technician can become very efficient at estimating RQD with a minimum of additional time expenditure. The window size for RQD calculation and recording can vary between 2 m and 10 m (e.g. record a separate measure foreach successive 2 m of core recovered) depending on the resolution required for the project. The window should be reduced in zones of geological transition or where the measured RQD is observed to change significantly over short distances.

In addition, RQD can be remeasured some time after recovery to determine if the rock is susceptible to rapid disintegration upon exposure.

Alternate methods of estimating RQD

Unfortunately, the engineer is often required to estimate RQD without timely access to drill core or without historical logs of RQD. Palmström (1982) proposed a fallback method of estimating RQD from exposed joint traces on excavation walls or outcrops. The Volumetric Joint Count, J , is the sum of the number of V joints per metre for each of the major joint sets present. Alternatively, the inverse of the representative true spacings for each set can be used, as shown in Figure 2.14.4. Note that true spacings must be used and not the apparent spacings produced by oblique intersection with the rock wall. This measure is valid for rockmasses with 3 or more well developed joint sets. This estimated RQD will represent a maximum value. That is, no random joints or fractures are considered and damage due to blasting and stress are also ignored. Palmström (1995) gives alternate relationships for one and two dominant joint sets, while Priest (1993) and Priest and Hudson (1976, 1981) present RQD relationships using scanlines.

Another simple method for estimating RQD is illustrated in Figure 2.14.5. A two metre graded rule can be placed on an exposed rock face. Calculate RQD as described for the drill core, considering any well developed joint which intersects the ruler as a core break. When estimating RQD for an undisturbed rockmass, care must be taken to consider only in situ joints and not induced tensile cracks and blast related fractures. Disregard any fractures which are less than 30 cm in length and consider disregarding larger fractures which are clearly induced (have a "sugary" surface). This will give a "best case" or upper-bound value of RQD.

This technique can also be used to determine the degree of degradation due to blasting and excavation. By considering all joints and fractures (induced) in the measurement of RQDW ( wall ), an estimate of post-excavation rockmass quality can be obtained (ignore fractures less than 30 cm in length). This may be a more relevant value for local support design. Note the additional subscript W attached to this measurement. Maintain this notation to differentiate the value from true RQD (joints only). Palmström's RQD (1982) and this crude RQDW measurement serve to provide an upper and lower bound respectively for local rock quality.

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