A Simplified CPT-Based Method for Estimating the Friction Angle of Partially Saturated Compacted Soils

Embankments and other compacted earth structures are widely used in civil engineering, and their mechanical behaviour depends on the properties of compacted soils, which are often affected by unsaturated conditions and matric suction. This study presents an empirical methodology aimed at the preliminary estimation of the peak friction angle of compacted unsaturated soils from cone penetration test (CPT) data, intended to support engineering assessments when laboratory testing and site investigation are limited. The approach was developed using CPT measurements collected at three levee test sites in northeastern Italy (Grizzaga, Panaro, and Tagliamento Rivers), combined with laboratory shear test results on undisturbed samples and supplemented by selected data from the literature. Three empirical equations were proposed and compared using different normalization schemes of cone tip resistance with respect to in situ stress conditions, while considering the influence of matric suction. For the soils examined in this study, the estimated peak friction angles were generally consistent with laboratory results, suggesting that the proposed relationships may provide a reasonable first approximation under similar soil conditions. The proposed procedure is intended as a preliminary engineering tool to complement, rather than replace, detailed laboratory testing in the geotechnical characterization of levees, embankments, and other compacted earth structures.