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Improved correlation between Texas cone penetrometer blow count and undrained shear strength of soft clays.
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2009-01-01
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Abstract:The objective of this project was to develop an improved correlation between Texas Cone Penetrometer (TCP)
blow count and undrained shear strength for soft, clay soils in the upper approximately 30 feet of the ground. Subsurface
explorations were carried out by Tolunay-Wong Engineers, Inc. (TWEI) at six sites in the Texas Gulf Coast region where
soft soils were expected to exist to obtain the data necessary to establish the improved correlation. A series of borings was
made at each site with TCP tests, conventional (“Dutch”) piezocone penetration tests, thin-walled tube sampling, and vane
shear tests. Laboratory testing was subsequently performed at The University of Texas at Austin on the thin-walled tube
samples obtained in the field. The results of these field and laboratory tests were used to develop average, lower- and
upper-bound undrained shear strength profiles for each of the sites (Varathungarajan, 2008). The data were used to
evaluate existing correlations between TCP blow count and undrained shear strength as well as to establish an improved
correlation. The following improved correlation was proposed:
su = 300 + 60N
where su is undrained shear strength in lbs. per square foot (psf) and N is the TCP blow count.
To evaluate the improved correlation, bearing capacity analyses were performed using the computer software
UTEXAS4 (Wright, 1999) to compute factors of safety for undrained strength profiles developed using the improved
correlation as well as for the undrained strength profiles developed by Varathungarajan (2008). Factors of safety computed
using undrained shear strengths based on the improved correlation generally showed good agreement with the factors of
safety determined using the strength profiles developed by Varathungarajan (2008).
The improved correlation is recommended over the existing correlations and is intended primarily for soft,
shallow (less than 30 feet) clays with undrained shear strengths of 1200 psf (approximately 0.47 atmospheres) or less (TCP
blow counts of 15 or less). Accordingly, this correlation should be used cautiously. Significant benefits can also be realized
by performing more extensive field and laboratory testing, rather than relying on simple, approximate TCP correlations.
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