Detailed Measurement of Nehrp Site Classifications Throughout Las Vegas Valley, Nevada, for Building Code Enforcement By Clark County

Abstract eng:
Rapid growth of the Las Vegas, Nevada urban area over the past 50 years resulted in a population of over 2 million exposed to earthquake risks. Federal agencies did not allocate funds to protect this new population and economy. Local municipalities thus sponsored the measurement of an “Earthquake Parcel Map” in 2007. With implementation of the IBC and its NEHRP provisions, local property owners, builders, developers, and engineers agreed it would be most costeffective for municipalities to provide consistent and efficient site measurements. Building departments of Clark County and the City of Henderson contracted with the Nevada System of Higher Ed. and Optim to make over 10,000 standardized Vs30 measurements using the SeisOpt® ReMi™ method across 1500 km2 of urban area within 3 years. The municipalities funded the Parcel Map for less than $20 per household. The Parcel Map shows boundaries between NEHRP soil classes are complex and impossible to predict without dense measurements. Urban planners, developers, and landowners use the Parcel Map to intelligently avoid over-strengthening buildings on stiffer sites, and properly assess the higher costs of developing softer sites. With a spacing of 300 m or less, the Parcel Map classifies every parcel on the NEHRP scale. While Vs30 average values are unable to capture all the variability in the shallow surface that affect site conditions, the parcel mapping exposed details of localized harder and softer locations. Existing geological maps do not reveal the random patches of Class C that are scattered within the general region classified as Class D area, nor the isolated patches of Class D within wider areas classified as Class C, as directly measured by the Vs30 Parcel Mapping. Identification of such anomalies is only possible through direct measurements of shallow shear-wave velocities at the parcel scale. The detailed Vs30 mapping also delineated the location and boundaries of a previously unknown buried alluvial fan surface. The detailed Parcel Map shows that current parametric approaches applied to create site-condition maps cannot account for distinctions between closely related units, and fail where relationships between the parameters and velocity vary spatially. Additionally, the 1D velocitydepth profiles obtained towards the Parcel Mapping provide information on velocities, soil thicknesses, interface depths, and resonant frequencies towards further building-code development and seismic hazard mapping.

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