Can we interpolate RQD?

CIM Montreal 2003
Emmanuel Henry, Jeff Hussey, Véronique Falmagne,
Abstract The Rock Quality Designation (RQD) is a standard scalar measure of rock quality used in common rock mass classification systems (e.g. RMR, Q system). RQD is measured along diamond drillholes and interpolating it between holes is nontrivial. Despite the inherent limitations of drill hole data, associated to drill collar orientation measurements, and the local complexities of structural geology, it is often the only information available for mining and major civil engineering projects. Therefore maximizing the usefulness of this data is warranted.

RQD is by essence a parameter that is a function of the borehole orientation. Strictly speaking, a meaningful interpolation would necessitate the choice of a common reference borehole orientation to map the RQD. Even when all boreholes have the same orientation however, RQD remains a non-linear function of the fracture intensity and its direct interpolation can lead to unreasonable estimates. Better estimates are obtained by interpolating the linear density of fractures from which the theoretical RQD can be computed. However, the only fully valid approach when the borehole orientations differ is the complete modeling of the 3D fracture distribution. To do this modeling, unless strong simplifying assumptions are used, fractures must be exposed on a number of faces with different orientations and fracture families must be identified. Fractures along the boreholes must then be classified into the families. This is too demanding to be realistic in most applications. A simplification can be made in some favourable cases where the directional character of the RQD is relaxed.

Examples of such favourable cases are a uniform (random) distribution over the sphere, or the presence of three or more families with a large spread of the mean orientations and with low concentration parameters. In these cases, the linear intensity approach provides a good proxy to the full 3D approach. Simulations are presented to illustrate these findings and determine the conditions allowing substitution of the full 3D approach by its proxy linear intensity.
Keywords: RQD, Linear intensity, 3D fracture model, Rock mechanics, Theoretical RQD, Interpolation
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