Correlation Between Undrained Shear Strength and Standard Penetration Test in Surface Clayey Soils of Basrah Province: Correlation Between Undrained Shear Strength and Standard Penetration Test in Surface Clayey Soils of Basrah Province

Keywords

Shear strength
SPT
soft soil
vane shear
Basrah

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.   

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