Rock Quality Designation (RQD) after Twenty Years.

Twenty years of experience is now available in the use of the Rock Quality Designation RQD in practice. The RQD is an index of rock quality in which a modified core recovery percentage is obtained by counting only pieces of sound core 4-in. 100 mm or greater in length of NX size or larger core diameters. Experience now indicates both smaller NQ and larger core diameters are appropriate that slightly and moderately weathered core that can not be hand broken be included that length measurements be made along the center-line or axis of the core piece and that the requisite length of 4-in. 100 mm be retained. Problems with core breakage and loss occur in thinly bedded and schistose rocks, and, particularly, with weak argillaceous rock interbedded with harder sandstone or limestone, a problem that can be ameliorated by large diameter cores, shorter coring runs, and by use of the best drilling equipment and techniques. Correlations of RQD with certain engineering parameters are given, but the more recent classification system of Bieniawski or Barton et al, which include the RQD as a parameter, are preferred for estimating the design and construction parameters. For obtaining the RQD, the best drilling techniques and prompt core logging in the field by a qualified engineering geologist or geotechnical engineer should be used. The RQD is not a design parameter that stands alone, but must be used together with an appreciation of the detailed geology and the geotechnical aspects. FR