Preconsolidation stress ( σ P') is the maximum effective stress that a soil has suffered throughout its life. From a geotechnical point of view, pre-consolidation stress has great importance because it separates elastic and reversible deformations from inelastic and only partially irreversible deformations and marks the starting point of high compressibility. In this study we calculated the preconsolidation stress for 72 undisturbed soil samples from the Yellow River Delta, using the oedometer test and applying the method proposed by Casagrande. The results showed that the over-consolidation ratio (OCR-the ratio of pre-consolidation stress to current natural overburden stress) values of the soil from soil surface to 6m depth varied from 1.72 to 15.34 and the maximum pre-consolidation stress was above 200 kPa. In consequence, the soils of the Yellow River Delta are highly over-consolidated within the upper 6 m, the OCR decreasing gradually with depth from 6 to 12 m. For samples from deeper than 12 m the soils were found to be under-consolidated, which was proved by standard penetration tests (SPTs). The main causes of this type of consolidation are the mineral composition and the designability of the soil body.
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