Compressibility and stress history of very soft organic clays

The compression index, swelling index and preconsolidation stress values are essential for reliable settlement calculations. However, it is a challenge to obtain representative parameters for very soft, organic, high-plasticity clays, which are often found in the Jacarepaguá Lowlands, Rio de Janeiro, Brazil. In recent decades, several geotechnical correlations have been proposed for estimating the parameters to check oedometer test data. The compression index can be estimated based on simple laboratory index tests and the preconsolidation stress can be found by means of piezocone and vane tests. In this paper, data from 24 different investigation clusters located in the region are analysed. Undisturbed samples were collected from 20 different locations; 67 piezocone tests and 48 vane tests were carried out, which resulted in a comprehensive data bank. Then correlations were developed and compared with a number of equations to assess their reliability. Special emphasis was given to correlations between the compression index and soil water content, and also between the preconsolidation stress and the corrected cone resistance and shoulder pore pressure measured in the piezocone tests.