Abstract
The Standard Penetration Test (SPT) is one of the most widely applied in-situ techniques for evaluating soil strength parameters in both coarse- and fine-grained soils. In this study, an attempt is made to establish a reliable correlation for estimating the undrained shear strength (Su) of very soft clayey soils in Basrah Province, southern Iraq, using the corrected blow count (N₆₀) from SPT results. A regression-based approach was adopted to develop a local correlation between Su and N-values, and the obtained results were validated against widely used empirical correlations proposed by Terzaghi and Peck (1967), Stroud (1974), Decourt (1990), and Nassaji and Kalantari (2011). The analysis produced a coefficient of determination (R²) of 0.5893, suggesting that the proposed equation provides a reliable predictive tool for local soil conditions. The findings indicate that the developed correlation reduces estimation errors associated with generic formulas and offers a practical approach tailored to the geotechnical behavior of soft clayey soils in Basrah.