Estimating the overconsolidation ratio in uniform cohesive soil with cone penetrometer tests considering soil structure and index properties

Abstract

A basic empirical approach to estimate the preconsolidation stress and overconsolidation ratio, OCR, in uniform cohesive soils using cone penetration and piezocone tests uses a correlation factor, kc, that is equal to the preconsolidation stress divided by the net tip stress referenced to the soil plasticity index. An adaptation of the “stress history and normalized soil engineering properties” (SHANSEP) format extends this basic approach by organizing the data into normally consolidated and overconsolidated components. The SHANSEP-based approach is improved in this paper by applying an empirical method to identify structured and unstructured soil behavior and to develop a separate empirical correlation for each type of soil structure. Overconsolidated structured soils are shown to exhibit kc values less than the normally consolidated kc and the m exponent in the SHANSEP relationship is greater than 1. In unstructured overconsolidated soils, the kc value is greater than the normally consolidated kc value and the SHANSEP m exponent is less than 1. The proposed method to identify structured vs. unstructured behavior is an important improvement in the approach and helps illustrate why some mCPTu values are less than 1 and others are greater than 1. Site characterization efforts are significantly improved when structured soil behavior is identified and included in the assessment of OCR.