Laboratory Scale Effects: Comparing Results from HQ- and NQ-Sized Rock Core

ABSTRACT: An extensive field exploration was performed in support of a proposed modification to an existing concrete dam, including many borings, geophysics, and downhole testing. Typical Reclamation practice is to perform laboratory testing on HQ-sized (61-mm-diameter) core. To facilitate dilatometer testing, NQ-sized (48-mm-diameter core) holes were required to use industry-standard equipment. In order to constrain uncertainty regarding scale effects and better characterize the foundation conditions at the right abutment, a HQ/NQ doublet was drilled with the holes less than 30 m from each other. Laboratory testing was performed on samples from both holes at similar depth intervals to develop realistic ranges of engineering parameters to describe the foundation. This paper provides a comparison of uniaxial and triaxial compressive strength and direct shear test results on HQ- and NQ-sized rock core obtained from close spatial proximity to investigate scale effects in typical laboratory samples. The testing found that the larger (HQ-sized) core was stronger and stiffer for all tests performed, contrary to the typically accepted scale effect laws. These findings indicate that spatial heterogeneity, even over relatively small distances in the same geologic formation, can be significant and may be more important than small changes in the diameter of rock core or volumes of material tested.