2707
Uplift behavior of bored piles in tropical unsaturated sandy soil
Comportement en traction de pieux forés en sol tropical sablonneux non saturé
Carvalho de D.
State University of Campinas, Brazil
Rocha de Albuquerque
P
.J.
State University of Campinas, Brazil
ABSTRACT: Large deposits of high-porosity, unsaturated, sandy soils are found in the southern central region of Brazil, to several
meters of depth. This region regularly sees increases in uplift loads in structures such as wind energy and power transmission towers.
For the current study uplift load tests were conducted on three bored piles,10m depth and with respective diameters of 0.35m, 0.4m
and 0.5m. The piles were instrumented with strain gauges connected in full bridge at five depth levels. The load vs. displacement and
the load transfer curves along the depth were obtained. The data obtained from the field tests (CPT, SPT) and laboratory tests (c,
,
n
)
were used to predict the ultimate loads of the piles. The calculated ultimate load values were compared with those obtained in the
piles load tests. The tests were carried out on the campus at the University of São Paulo, in the city of São Carlos. The average
composition of the subsoil is sand (62%), clay (22%) and silt (16%), with an average porosity of 45%. The field tests produced
average values of N
SPT
=5, qc=1700kPa and fs=98kPa, respectively. The water table was encountered at a depth of 10m.
RÉSUMÉ : Dans la région centrale sud du Brésil on trouve de grands dépôts de sols sablonneux, non-saturés, jusqu'à plusieurs mètres
de profondeur. L’augmentation des tensions verticales de soutien de charge est fréquente dans cette région pour des structures comme
des tours d’éoliennes et des lignes de transmission d'énergie. Pour cette recherche ont été réalisés des essais d’arrachage sur trois
pieux forés à 10m de profondeur et avec des diamètres de 0,35m, 0,40m et 0,50m. Les pieux ont été instrumentés avec des jauges de
déformation connectées en ponts complets à cinq niveaux le long de l’encastrement. On a obtenu des courbes charge/déplacement et
le transfert de charge le long de l’encastrement. Les données des essais in situ (CPT ,SPT) et des essais en laboratoires (c,
,
n
) ont
été employées pour prévoir les charges ultimes des pieux. Les valeurs calculées ont été comparées avec les valeurs obtenues dans les
essais de charge. Les essais ont été réalisés dans le Campus da Université de São Paulo, dans la ville de São Carlos. La composition
moyenne du sous-sol est sable (62%), argile (22%) et limon (16%), avec porosité moyenne de 45%. Les essais in situ ont indiqué
des valeurs moyennes de N
SPT
=5, qc=1700kPa et fs=98kPa. Le niveau d’eau a été trouvé à 10m de profondeur.
KEYWORDS: uplift load test; unsaturated sandy soil; bored pile; instrumented pile.
1 INTRODUCTION
The southern central region of Brazil possesses vast areas of
sandy soil where the water table is often deeper than 10 meters.
This unsaturated soil condition, combined with the lateritic soil,
enables the use of bored piles, with no need to line the
boreholes in the vast majority of constructions. Where these
foundations are subjected to uplift forces, there is some debate
as to which method to use to calculate the ultimate load.
Consequently, this study, by way of the performance of three
uplift load tests on instrumented bored piles, analyzes the
mechanism for transferring load to the subsoil as well as
ascertaining the applicability of the methods for calculating
ultimate load that are available within the technical milieu. This
study was conducted on the campus of the University of São
Paulo, located in the city of São Carlos, in the state of São
Paulo, Brazil. Geographically, its location is defined by the
coordinates 22º 01' 22” South and 47º 53' 38” West.
2 GEOTECHNICAL CHARACTERISTICS
The region sits on rocks of the São Bento Group, composed of
sandstone of the Botucatu and Pirambóia formations and by
ridges of basalt rocks of the Serra Geral formation. On top of
these rocks appears sandstone of the Bauru Group, followed by
Cenozoic sediment.
Close to the piles used in the study, five CPT tests and five
SPT tests were carried out. Laboratory testing was carried out
on both disturbed and undisturbed soil samples collected via an
open hole down to the level of the water table, situated at a
depth of 10 meters.
The tests characterized the subsoil down to a depth of 10
meters as being made up of two layers, primarily composed of
fine lateritic sand separated by a line of boulders at a depth of
6.5 m. The soil in the first layer is characterized as Cenozoic
sediment and in the second layer as sandstone of the Bauru
Group. Table 1 shows the average values for the geotechnical
parameters of the subsoil in the chosen location.
Table 1. Average geotechnical parameters
Depth
1 m – 6 m
6 m – 12m
Formation
Sediment
Residual
n
(kN/m
3
)
16.3
18.9
w (%)
16
16
Gs
2.73
2.76
e
0.94
0.71
n ( % )
48
42
Sr ( % )
47
62
LL (%)
28
32
IP (%)
11
17
Sand (%)
62
61
Silt (%)
13
10
Clay (%)
25
19
c' (kPa)
6
20
'
30
23
SPT - N
72
4
7
CPT – qc (MPa)
1.07
2.36
CPT – fs (kPa)
45
150
