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TxDOT frequently uses Texas Cone Penetrometer (TCP) blow counts to estimate undrained shear strength. However, the current correlations between TCP resistance and undrained shear strength have been developed primarily for significantly stronger soils than are often encountered at shallow depths. Updated existing correlations would allow TxDOT to estimate better the undrained shear strength of soft soils for the design of embankments and retaining structures. Considering the limited data for these soils, the primary objective of this study was to characterize the undrained shear strength profiles for six sites with strengths generally less than 750 psf, such as those commonly found at depths up to approximately 30 feet. The undrained shear strength profiles developed in this study were used in further research by Garfield (2008) to develop and assess the reliability of new correlations between Texas Cone Penetrometer resistance and undrained shear strength of soft clays.
An analysis was performed comparing strengths measured in unconsolidated-undrained, consolidated-undrained, field vane shear, and piezocone penetration tests with respect to strengths from the average strength profiles. The degree of sample disturbance in unconsolidated-undrained tests was assessed based on values of axial strain at 75 percent of the principal stress difference at failure. While there was significant scatter in the data, unconsolidated-undrained tests gave strengths that tended to be significantly lower than strengths from the average profiles. The degree of sample disturbance in consolidated-undrained tests was assessed based on the volumetric strain during consolidation to the in-situ effective overburden stress. Consolidated-undrained tests on disturbed specimens gave strengths that were approximately 50 percent higher than what was believed to be the correct strength. Strengths measured in field vane shear tests were corrected with Bjerrum’s (1972) correction factor and found to generally agree well with strengths from the average strength profiles. However, vane tests in sandy clays tended to overestimate undrained strengths. Analyses also indicated that piezocone penetration tests can be used to establish reasonably accurate undrained shear strength profiles without the need for site specific correlations.

Center for Transportation Research The University of Texas at Austin
USA, Texas
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