Correlation Between V\u3csub\u3eS\u3c/sub\u3e and RQD for Different Rock Types

Although there is no direct relationship between the shear wave velocity (VS) of in-situ rock and Rock Quality Designation (RQD), it is apparent that some relationship exists since both VS and RQD decrease in fractured and weathered rock. However, unweathered or unfractured soft rock will have comparatively low VS compared with unweathered or unfractured hard rock since VS is a function of rock hardness and density. Thus, any relationship between VS and RQD will be specific to a particular rock type, and, in many cases, to a particular area/region where the hardness and density of a specific rock type varies significantly with location. This paper examines the relationship between VS and RQD for three different rock types located in Virginia, South Carolina and Florida. The Virginia rock is strong metamorphic (gneiss), the South Carolina rock is strong igneous (granodiorite), and the Florida rock is moderately weak to strong sedimentary (limestone). High quality H- and P-size rock cores were taken in multiple borings at each site and RQD for each core was recorded. Rock thickness ranged from about 90 ft to over 300 ft. VS was measured in each boring at 1.64- ft intervals using P-S Suspension logging. Correlation between VS and RQD was computed for each site using all of the recorded data. Reasonable correlations were achieved at the two hard rock sites, although the relationships were predictably different. The relationships at the two hard rock sites being closer compared to that at the weaker rock site. At each of the three sites, only a small fraction of the borings made had VS measurements. In the remaining borings, the relationships derived for VS versus RQD could be used to obtain an approximate VS profile using the RQD measurements