Unified Correlation between SPT-N and Shear Wave Velocity for all Soil Types

The shear wave velocity (Vs) of sediments plays a key role in seismic wave amplification and is required in site response analysis. Such information is usually lacking during field exploration, but standard penetration test blow count (N) is mostly obtained. Therefore, several studies have established empirical correlations between N and Vs for engineering use. These empirical correlations, however, vary significantly in terms of model form and parameters. A unified empirical correlation is developed in this study through the use of the Engineering Geological Database for the Taiwan Strong Motion Instrumentation Program, which contains grain-size distribution, void ratio, water content, specific gravity, unit weight, liquid limit, and plasticity index (PI) in addition to the Vs and N of strata. The influence of overburden pressure, fines content (FC), PI, and soil types on small-strain behavior (i.e., Vs) and large-strain behavior (i.e., N) is discussed. A unified correlation between Vs and N that is dependent on overburden pressure, FC, and PI is proposed through the conditional prediction approach. Introduction A key property required to effectively estimate the seismic response of a site is small-strain shear modulus Go, which is often computed by measuring shear wave velocity Vs and mass density ρ as follows: Go= ρVs. (1) Geophysical investigations are typically performed to measure the Vs profile. However, these measurements are not always common because of the additional cost of the field investigation. Correlations between Vs and standard penetration test (SPT) blow count (N), conditioned on the geologic setting and site stratigraphy, are potentially useful for the above situation. Numerous relations between N and Vs for Taiwan regions have been established in the previous study (Kuo et al., 2011). These empirical correlations, however, vary significantly in terms of model form and are limited to a specific soil type (e.g., sand or clay). The development of these correlations also lacks theoretical support. Therefore, with a theoretical basis, this study aims to establish a unified correlation between N and Vs that can be applied to sand, silt, and clay. The Engineering Geological Database for the Taiwan Strong Motion Instrumentation Program 1 Assistant Professor, Department of Civil Engineering, National Chung Hsing University, Taiwan, tsaicc@nchu.edu. tw Post-Doctoral Researcher, Pacific Earthquake Engineering Research Center (PEER), University of California, Berkeley, USA, kishidapple@gmail.com (EGDT) (Kuo et al., 2011) is utilized in the analysis. The factors that can change the small-strain properties Go are reviewed. Based on a previous study on laboratory test data, a model form that describes small-strain properties (i.e., Vs) and large-strain measurements (i.e., N) is proposed. The influence of effective overburden pressure (σo’), fines content (FC), plasticity index (PI), and over consolidation ratio (OCR) on small-strain properties and large-strain measurements is discussed based on the regression analysis results. Lastly, a unified correlation between Vs and N that is dependent of these parameters is proposed through the conditional prediction approach. Theoretical Form of Vs and N Fundamental Functional Form The most common functional form for the relations of Go proposed in literature is