535
The seismic SPT test in a tropical soil and the G
0
/N ratio
L'essai SPT sismique pour le sol tropicaux et la relation G
0
/N
Giacheti H.L., Pedrini R.A.A.
Universidade Estadual Paulista, Departamento de Engenharia Civil e Ambiental, Bauru – SP – Brazil
B. P. Rocha
Universidade de São Paulo, Departamento de Geotecnia, São Carlos – SP – Brazil
ABSTRACT: The seismic SPT, a test which associates the up-hole technique to the SPT, is briefly described. The maximum shear
modulus (G
0
) can be determined together with the N value with this hybrid test. Seismic (Cross-hole, Down-hole and SCPT) and SPT
test data for a Brazilian tropical sandy soil are presented and discussed emphasizing the advantage of using the interrelationship
between the small strain stiffness (G
0
) and an ultimate strength (N value) to identify different soil behavior. A seismic SPT test was
carried out in this research site and the G
0
/N ratio is discussed as an interesting index to help characterize tropical soils, similar to
what has been suggested for the Go/q
c
ratio determined in a single test.
RÉSUMÉ : Le SPT sismique, qui associe le up-hole au SPT est brièvement décrit. Le module de cisaillement maximale (G
0
) peut être
déterminé avec la valeur N de ce test hybride. Des données sismiques (Cross-hole, Down-hole and SCPT) et SPT pour un sol sableux
tropical du Brésil sont présentées et discutées soulignant l'avantage d'utiliser la corrélation entre (G
0
) et une résistance à la rupture
(valeur N) afin d'identifier le comportement de différents sols. Un essai SPT sismique a été réalisé dans le site expérimental et la
relation G
0
/N est discutée comme un indice intéressant pour aider à caractériser les sols tropicaux, de la même façon que ce qui a été
proposé pour le rapport G
0
/q
c
mesuré dans un essai unique.
KEYWORDS: In situ testing, SPT, seismic, up-hole, tropical soil, G
0
/N ratio.
1 INTRODUCTION
Site characterization can be defined as the process of identifying
the geometry of relatively homogeneous zones and developing
index, strength and stiffness properties for the soils within these
zones. Some in situ testing can be used as an alternative to the
traditional approach of drilling, sampling and laboratory tests.
Combining stratigraphic logging with a specific measurement in
a in situ test is a modern approach for site characterization.
Some authors have shown that it is possible to incorporate
the measurement of shear wave velocities using the SPT blow
by the up-hole technique. This hybrid test is known as the
seismic SPT (S-SPT), which combines stratigraphic logging,
estimative of geotechnical parameters and determining small
strain stiffness (Go) in one single test similarly to the SCPT.
This paper briefly describes a system to carry out the S-SPT
test and the approach to interpret the seismic data. It also
discusses the applicability of the interrelationship between (Go)
and N value to identify unusual soil behavior based on the tests
carried out in a research sites located in the city of Bauru, inland
of São Paulo State, Brazil emphasizing the advantage of using
the S-SPT test for this approach.
2 BACKGROUND
2.1
Tropical Soils
Tropical soils are formed predominantly by chemical alteration
of the rock, and they are considered a non-textbook type
geomaterial because their peculiar behaviors that cannot be
explained by the principles of classical soil mechanics.
The term
tropical soil
includes both lateritic and saprolitic
soils. Saprolitic soils are necessarily residual and retain the
macro fabric of the parent rock. Lateritic soils can be either
residual or transported and are distinguished by the occurrence
of the laterization process, which is an enriching of a soil with
iron and aluminum and their associated oxides, bonding a
highly porous structure. Saprolitic soil has structural or
chemical bonding retained from the parent rock. The
contribution of this cementation to the soil stiffness depends on
the strain level the soil will experience. Differences between the
mechanical behaviors of the mature (lateritic) and young
(saprolitic) soils have been reported for both natural and
compacted conditions.
2.2
Go/q
c
Ratio
The pore pressure measurements cannot always be considered
useful to allow an adequate classification of tropical soil based
on CPTU data. The small strain stiffness (Go) and cone tip
resistance (qc) ratio has been suggested as an additional
information for classifying different soil types, especially to
identify soils with unusual compressibility. Schnaid et al (2004)
suggested that the ratio Go/qc provides a measure of the ratio of
the elastic stiffness to ultimate strength and may therefore be
expected to increase with sand age and cementation, primarily
because the effect of these on Go are stronger than on qc. They
proposed a chart and boundaries by correlating Go/qc
versus
normalized type resistance (qc
1
). This chart can be used to
evaluate the possible effects of stress history, degree of
cementation and ageing for a given profile. Three lines divide
upper and lower bounds for cemented and uncemented sands.
Giacheti & De Mio (2008) presented SCPT test results from
three tropical research sites in the State of São Paulo, Brazil and
plotted all the data in the Schnaid et al (2004) chart as shown in
Figure 1. The authors pointed out that the SCPT test allows
calculating Go/qc ratio simplifying interpretation and reducing
site variability. The SCPT data interpretation indicated that the
bonded structure of tropical soils gives Go/qc ratios that are
systematically higher than those measured in cohesionless soils.
Rocha B.P.
