Assessing Geotechnical Variability in Geological Formations: Insights from In Situ CPT Data Analysis in Illinois

In soils, knowledge of the geologic units encountered at a site typically is insufficient to define the expected range of engineering properties due to the presence of large geotechnical spatial variability. This study describes an in-depth examination of cone penetration tests (CPT) in glacial soils from Illinois, advanced by the Illinois State Geological Survey (ISGS). The research examines the spatial variability of glacial geological units at multiple scales, both within a single site and regionally across multiple sites in Illinois. This analysis focuses on the Batestown unit, acknowledging its widespread presence as a commonly encountered geological soil, which is compared to the database of geotechnical properties of geological soils for three sites developed with the support of the Illinois Department of Transportation. The analytical method applied in this study evaluates the horizontal scale of fluctuation (δh) and the coefficient of inherent variability (COVw,h) from corrected cone tip resistance (qt), providing insights into both the correlation and dispersion of geological characteristics. Results suggest δh is more indicative of the spatial variability characteristics of a geological unit, while COVw,h is more site-dependent for the same unit. Furthermore, the study also integrates sequential simulations of unit side resistance in the Batestown unit to examine and quantify the effects of spatial variability on this deep foundation design parameter. The findings suggest that designs based solely on typically spaced CPT data can obscure the true underlying site resistance. Further research is necessary to quantify the reliability of the existing soundings in representing actual site conditions.