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