Spt spt t procedures and practical applications -luciano decourt
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Transcript of Spt spt t procedures and practical applications -luciano decourt
Luciano Décourt
Civil Engineer – University Professor
2nd. International Conference
on Deep Foundations
Santa Cruz de la Sierra, Bolivia
May 12 - 15, 2015
SPT / SPT-T
Procedures and
practical applications
Heterogeneous behavior
Average values of NSPT
Never use single values.
The ideal is to compute average values for
each depth, and to Gibson type soils to
correlate average NSPT values with depth.
Loading Tests
Square footing (2,50m x 2,50m)
quc (extrapolated value)......................................1,181.31 kPa
quc (predicted on basis of average NSPT values)..1,190.35 kPa
Results of 3 SPT-T
Nav / Nmin = 1,86 = 86%
Nmax / Nav = 1,44 = 44%
Nmax / Nmin = 2,68 = 168%
Depth Effect
In a sand, the NSPT values are a function of the
relative density of the sand and of the
octahedrical stress.
Therefore, when the borings are carried out
before the escavation and these values of
NSPT are used for design purposes, the
obtained values are not representative of the
actual situation and, what is worse, they are
against the safety.
“The Mechanical Aging of Soils”
25th Terzaghi Lecture
John H. Schmertmann
L. Décourt (1989) observed that the Bazaraa (1967)
correlations, based on field data, if adjusted by an aging factor
of 0.4–0.6 would approximately match the SPT vs. relative
density correlations previously obtained by Gibbs and Holtz
(1957) and Marcuson and Bieganouski (1977).
The writer believes that the above aging explanation helps
greatly to explain a big mistery – namely why the Bazaraa
correlations did not fit well with the others.
SPT-T
Thank you
Muchas gracias