Estimating overconsolidation ratio (OCR) in structured and unstructured cohesive soil with field vane tests referencing soil index properties

Abstract

Estimation of the preconsolidation stress and overconsolidation ratio (OCR) in uniform cohesive soils using a field vane is variably impacted by the combined effects of soil type and plasticity, geologic history, structured vs. unstructured behavior, and the presence of sand or organic matter. Published empirical correlations for cohesive soils consider the effects of soil type and plasticity, but significant variability can occur with changes in soil structure and organic matter content for specific instances. The adaption of the “stress history and normalized soil engineering properties” (SHANSEP) format improves the characterization of overconsolidated soils using field vane tests by applying a proposed empirical approach to identify structured soils from unstructured soils and updating the SHANSEP-based approach to separately evaluate structured and unstructured soils. Validation of the correlation coefficients for individual projects will be needed as the approach is applied to new geologic materials and with potentially different field vane equipment and laboratory testing procedures used to characterize the soils. This additional testing provides an opportunity to improve the correlations for specific conditions and reduce the variability in the OCR assessments.