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CPT Correlations for Estimating Undrained Shear Strength of Marine Clay Offshore East Africa
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Accurate estimates of undrained shear strength are important to develop geotechnical parameters used as input in slope stability analyses and to evaluate the foundation capacity and methods of offshore infrastructure installation.

In situ piezocone penetration test (CPT) measurements are commonly used in geotechnical practice to estimate undrained shear strength but they require empirically-derived cone factors (i.e., N values) that are representative of the clay materials, stress history, and geological conditions. The lack of empirically derived cone factors for a project site offshore of east Africa required that cone factors be independently developed. CPT correlations were developed for estimating undrained shear strength as measured in direct simple shear (DSS) tests for the marine clay found offshore east Africa in water depths greater than 800 meters. Four different cone factors were developed: Method 1, which is based on the net cone tip resistance, Method 2 which is based on the effective cone tip resistance, Method 3 which is based on excess cone pore water pressure, and Method 4 which is based on corrected CPT tip resistance. Statistical analyses indicated that despite the significant care in sampling and testing, the coefficient of variation of the cone factors ranges between 21% and 31%. Depth dependency of N values is a limitation for the use of Methods 2, 3, and 4. Methods 1 and 2 appear to be equivalent with respect to estimating DSS undrained shear strength, particularly at depths < 20 m.

Publication Summary

  • Geosyntec Authors: Ali Ebrahimi, Rodolfo Sancio
  • All Authors: Ali Ebrahimi, Rodolfo Sancio
  • Title: The Deep Foundations Institute (DFI)
  • Event or Publication: Publication
  • Practice Areas: Geotechnical and Geological Analysis, Modeling, and Engineering
  • Citation: John Barrett, P.E.; (Tennessee); Logan Brant, Ph.D., P.E.; Rodolfo Sancio, Ph.D., P.E., D.GE; and Ali Ebrahimi, P.E. (Texas) coauthored multiple papers for publication by the Deep Foundations Institute (DFI) for the 2021 International Symposium on Frontiers in Offshore Geotechnics.
  • Date: 2021
  • Publication Type: Journal Article