Anisotropy of the rock quality designation (RQD) and its geostatistical evaluation
Résumé
Rock Quality Designation (RQD) measures the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm measured along the centerline of the core. It is an important attribute used in geotechnics via Rock Mass Rating (RMR) for example.
This presentation concerns the behavior of this attribute in a Chilean porphyry copper deposit by analyzing more than 60,000 samples 1.5-meter long.
The drill holes have different directions and the nature of RQD requires accounting for the sample direction if the fracture network is anisotropic, a concept different from the geostatistical anisotropy which measures the variability along a direction set by two samples. A directional analysis is conducted and shows different variograms associated with different sample direction classes; this leads to different maps obtained by kriging, calling into question the usual practices that do not account for the sample direction. In fact, the directionality of the measurements makes RQD non additive (and not krigeable) if the samples used in the kriging neighborhood belong to different directional classes.
A discussion follows; pointing out that RQD is subject to the same directional bias as the Fracture Frequency (FF) and should be corrected in the same way by a sinus of the angle between the sample and the fracture (Terzaghi correction). But such a correction is difficult, if not impossible; tests are presented.