Reliability and Validity Analysis of Correlations on Strength and Consolidation Parameters for Ankara Clay and Proposal for a New Correlation

Abstract

This study aims to assess the accuracy and consistency of existing correlations between undrained shear strength (cu) and modulus of volumetric compressibility (mv) with SPT N value, and compression index (Cc) with index parameters for Turkish clays, with a specific focus on Ankara clay using laboratory testing of over 2500 high-quality undisturbed samples from 42 sites. A comparison of correlations for cu using five different statistical variables indicates that no single method stands out as superior in terms of both accuracy and conservatism and that the quality of the tests in the database has a significant impact on the correlation performance. Even the most effective correlation for cu appear insufficient for direct application in geotechnical design processes, given its low consistency characterized by coefficient of variations (COV) ranging between 50% and more than 100%. The widely used mv estimates proposed by Stroud (1974) were found to be notably unconservative when applied to Turkish clays. Consequently, a new trendline, leveraging both SPT N value and Plasticity Index (PI), was introduced to address this discrepancy. Meanwhile, an examination of 283 high-quality undisturbed samples from Ankara Clay revealed existing correlations between initial void ratio (e0) and compression index (Cc) to be conservative, yet consistently reliable. This underscores their suitability for routine or preliminary design applications. Notably, a newly derived correlation was proposed, which demonstrates equal applicability to the well-established Bowles (1979) equation.