Distinguishing Between Data Uncertainty and Natural Variability in Virtual Geotechnical Databases: Fid-Bau Portal

Distinguishing Between Data Uncertainty and Natural Variability in Virtual Geotechnical Databases

Rogers, J. David / Chung, Jae-Won

Subsurface geological and geotechnical databases using geographic information systems have come to be known as virtual geotechnical databases (VGDB's). VGDB's can be employed to estimate the relative positions of stratigraphic horizons or bedrock surfaces, groundwater elevations, and seismic site response in most metropolitan areas. In the St. Louis metro area, most of the geotechnical borings do not pierce the Paleozoic-age bedrock, causing considerable uncertainty in estimating the depth of unconsolidated surficial materials, which exert marked impact on seismic site response, amplifying or attenuating seismic energy. The uncertainty associated with groundwater levels commonly results from limited sampling, measuring methods, and different water depths recorded at a given site. Subroutines for estimating the positions of bedrock surface and groundwater table were developed to circumvent prediction errors associated with interpolation methods, which are over-utilized and inappropriate for most uneven, eroded surfaces. The scarcity and uneven distribution of shear wave velocity (Vs30) measurements across the St. Louis area resulted in poor correlations between the codified NEHRP soil site classes and the measured Vs30 values. In these situations the surficial geologic units were employed to ascertain the appropriate site class assignments in areas bereft of Vs measurements.