Chapter 4 - Rock Quality Designation

Abstract

Deere introduced the rock quality designation (RQD) in 1964 as an index of assessing rock quality quantitatively. This chapter briefly discusses the RQD with direct and indirect method. It is more sensitive as an index of the core quality than the core recovery. The RQD is a modified percent core recovery that incorporates only sound pieces of core that are 100 mm (4 in.) or greater in length along the core axis. For RQD determination in direct methods, the International Society for Rock Mechanics (ISRM) recommends a core size of at least NX (54.7 mm) drilled with double-tube core barrel using a diamond bit. Artificial fractures can be identified by close fitting cores and unstained surfaces. RQD is the most common method for characterizing the degree of jointing in borehole cores; this parameter implicitly includes other rock mass features like weathering and “core loss”. There are two methods in indirect method, seismic method and volumetric joint count. The seismic survey method uses the variation of elastic properties of the strata that affect the velocity of the seismic waves traveling through them, thus providing useful information about the subsurface strata. This method is relatively cheap and rapid to apply and is helpful when studying a large volume of rock masses.