Nebraska Local Technical Assistance Program
Nebraska Department of Transportation: Research Reports
Date of this Version
1-2026
Document Type
Article
Citation
Song., C.R., Al-Nimri, B., Abu Al Sha'r, B., Pandey, D.R., Eun, J., and Kim., S. (January, 2026). Evaluation of Critical Shear Strength of Soils in Nebraska Based on Revised CPT. NDOT Research Report SPR-FY24(026).
Abstract
This study evaluated the “Fully Softened Shear Strength (FSSS)” of soils in Nebraska based on a revised Cone Penetration Test(CPT) method. FSSS is a unique strength condition that soils show substantially reduced strength during rainy seasons, eventually causing failure of slopes and other structures.
The revised CPT was adapted to inject water at targeted depths, inducing controlled moisture infiltration and capturing real-time soil softening over designated intervals of 30, 60, or 90 minutes. Through tip resistance (qc) and sleeve friction (fs) measurements under “in-situ” and “wet” conditions, the test effectively identified fully disturbed/low strength zones around the sleeve. Simultaneous sampling at test locations facilitated laboratory characterization of soil index properties. Subsequent direct shear testing under field-moisture conditions and 48 hour submerged conditions confirmed that high-plasticity soils exhibited similar strength loss. Conversely, lower-plasticity soils demonstrated limited magnitude of shear strength reductions.
The correlation proposed by Robertson (2009) was employed to estimate undisturbed strength (su) and disturbed strength (su(d)) for the revised CPT parameters, and these correlations were compared to laboratory direct shear strength test data. Marked decreases in peak shear strength similar to FSSS were observed at most sites. These outcomes well aligned with Skempton’s (1970) concept of fully softened shear strength, validating the measurement scheme developed in this study. Overall, the revised CPT procedure offers a reliable, efficient in-situ method to evaluate fully softened shear strength, facilitating improved assessments of slope stability and soil behavior.