Resonant Frequency Testing of New York City Rock Types

The elastic properties of rock are important in geotechnical design, including aspects of site characterization, foundation selection, and seismic wave attenuation and response of overlying soils. In this paper, the authors present results of laboratory-resonant frequency testing used to evaluate the low-strain dynamic elastic moduli of intact rock samples representing many of the rock types encountered in the New York City (NYC) metropolitan area. NYC lies on very old geologic formations, with bedrock more than a billion years old in some areas. Because of its age and the tectonic history, the regional bedrock is amazingly complex and varied, as demonstrated by laboratory tests performed on about 100 specimens of various rock types included in this study. The tests were performed using the resonant frequency method with impulse excitation, where the response of a rock specimen of known dimensions and mass is measured by a lightweight accelerometer on the surface of the specimen. This method requires an assumption that the rock's stiffness is isotropic, an assumption that simplifies the characterization of materials, which in actuality are often anisotropic and quite complex. Depending on the excitation mode, fundamental frequencies were measured in the transverse, longitudinal, and torsional directions. These frequencies are directly related to low-strain values of the Young's modulus (E), shear modulus (G), and shear wave velocity (Vs) of the intact rock. The paper compares these laboratory measurements to: (i) low-strain in situ field seismic testing results, (ii) large-strain laboratory results of unconfined compressive strength testing, and (iii) correlations in the literature. NYC region-specific preliminary correlations for the dynamic elastic stiffness properties of intact rock are presented.