Three-Dimensional Characterization and Visualization of Till in Boston, Massachusetts

In order to evaluate the benefit of using three-dimensional (3D) geostatistical interpolation and visualization methods over two-dimensional (2D) methods to characterize a site, we have assembled a database of densely spaced geotechnical data in downtown Boston. Our study area includes several high-rise buildings as well as a 300 m length segment of the Central Artery/Tunnel (CA/T) project. The stratigraphy of the site includes artificial fill, organic deposits, marine clay, till, and bedrock. The assembled geotechnical data include stratigraphy, N-values, sieve analysis, ground water depth, and sample descriptions. The till is the bearing material for most structures in the area; therefore, in this paper we present results on characterizing the till layer using corrected (N₁)₆₀. By using geostatistical interpolation to characterize the site, we could evaluate the confidence level of predictions. We showed that the confidence in prediction was greater when using 3D characterization than using 2D characterization. The 3D characterization maintained the integrity of the vertical variability of the till without needing to subset the data. Test boring data were grouped into three stages: 1) relevant test borings completed prior to 1988 (including 79 borings), 2) CA/T test borings (including 33 borings), and 3) the combined data set (including 112). The site was characterized separately for each stage providing an opportunity to evaluate the effect of boring density and spacing for site characterization. In 3D, we compared confidence volumes for different stages to show that incorporating prior test borings in the general vicinity of the CA/T alignment increased the confidence in the overall prediction. Using the CA/T test borings alone resulted in a predicted model with high variance and therefore low confidence. The interpolation of corrected (N₁)₆₀ values reflects the inherent variability in the test data and results in a prediction with low confidence levels.