2731
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Model loading tests in large soil tank on group behavior of piles
Essais de chargement modèle
afin d’
étudier le comportement de groupe de pieux dans un grand
réservoir du sol
S. Goto, S. Aoyama, B. Liu & I. Towhata
The University of Tokyo
A. Takita
Nippon Steel & Sumitomo Metal Corporation
A. A. Renzo
Anddes Asociados S.A.C., Lima-Per
ú
ABSTRACT: Model pile loading tests in dry sand were conducted with applying confining pressure of 50-200kPa at the surface of
the model ground to investigate the behavior of a group pile. The group pile consisted of 9 cylindrical model piles of 40mm in
diameter, while two kinds of the pile spacing between pile centers were used; 2.5 times of the diameter of the pile and 5.0 times of
the diameter. For comparison, a single pile with a large diameter was also tested under the same condition. The test results were
discussed based from the following 4 points of view; the settlement at the yielding point of the total load, tip stress distribution by the
pile location in the group pile, pressure distribution in the soil measured by the tactile sensors and ground deformation after the
loading tests. All discussion suggested that the group pile of 2.5D spacing caused significant interactional effect between piles and
behaved in one block. In contrast, each pile in the group pile of 5.0D spacing behaved more individually.
R
É
SUM
É
: Des essais de chargement de modèle de pieu dans un sable sec ont été effectués en appliquant une pression de
confinement de 50-200 kPa à la surface du sol modèle pour
d’étudier le comportement d’un groupe de pieux. Le groupe de pieux
comprend neuf pieux modèles cylindriques de 40 mm de diamètre. Deux types d’espacement entre les pieux ont été utilisés dans le
groupe de pieux modèles : 2,5 fois le diamètre du pieu et 5,0 fois le diamètre entre les centres des pieux. Afin de comparer, un seul
pieu à grand diamètre a également été testé dans les mêmes conditions. Les résultats sont examinés selon les quatre points de vue
suivants : le tassement à la limite élastique du chargement total, la distribution de contraintes en fonction de la position du pieu au sein
du groupe, la distribution de la pression au fond du réservoir contenant le sol, mesurée par des capteurs tactiles, la déformation du sol
après les essais de chargement. L
’analyse suggère que le groupe de pieux de 2,5D d’espacement a
provoqué une forte intéraction entre
les pieux qui se sont comportés en tant qu’un seul bloc. En revanche, chaque pieu appartenant au groupe de pieux de 5D
d’espacement s’est comport
é de manière individuelle.
KEYWORDS: group pile, model test, interaction
1 INTRODUCTION
Group pile is the foundation that supports a footing with several
piles. The behavior of a group pile is totally different from that
of a single pile if the pile spacing becomes narrow enough
because of the pile-soil-pile interaction. To investigate the
effects of the interaction in the group pile, model tests were
conducted by previous researchers [Whitaker (1957), Vesic
(1967), and Itoh and Yamagata (1998)]. However, the bearing
mechanism of the group pile is not so clear yet as that of a
single pile. As a result, it is not yet understood clearly whether
or not the bearing capacity of the group pile is greater or less
than that of a single pile
To understand the bearing mechanism of a group pile, the
interaction should be studied more scientifically. It is thus
necessary to observe precisely the behavior of piles and the
surrounding ground in detail during group pile loading.
Additionally, a large-scale model is also required to make clear
the effects of interaction in a group pile.
Therefore, loading tests on a group pile and a single pile were
conducted in a large soil tank with several sensors; strain gauges
in the piles, colored sand layers and tactile sensors. To
investigate the effect of pile spacing on the interaction, two
kinds of models with different spacing were tested. A single pile
with a large diameter was also tested for comparison based on
the comparison between the group piles and the single pile, the
bearing mechanism of group pile was discussed.
2 TEST APPARATUS & TEST PROCEDURE
Group pile loading tests and single pile loading tests were
conducted in a large rigid soil tank as shown in Fig. 1. Its
internal dimension was 1600mm
×
1600mm (width)
×
1650mm
(height). At the top of soil tank, a loading actuator was installed.
Loading tests were able to be conducted everywhere in the soil
tank by moving the actuator. The maximum capacity of the
actuator was 500 kN and the loading was performed in a
displacement control manner. Details of the device were
described by Goto et al. (2012). Air bags were placed on the
surface of the model ground to generate confining pressure.
Thus, the present tests reproduced the in-situ situation around
and above pile tips.
Tactile sensors were installed on the bottom and sidewall of
the tank to measure how the earth pressure propagated in the
ground. The advantage of this filmy sensor is the ability to
measure the distribution of normal (effective) stress. The sensor
covered 440mm * 480mm area, containing 2016 sensing cells
spaced at 10mm interval in each direction as shown in Fig. 2. In
the experiments, stress distribution was measured in a wider
area by combining several tactile sensors.
Model ground measured 1200mm in height and was made of
air-dried Silica sand No.5; D
50
= 0.523mm, e
max
= 1.09 and e
min
= 0.66. It was constructed by spreading dry sand and manual
compaction at every 150 mm lift. The total amount of sand was
measured and the average relative density was calculated to be
around 90%. The several layers of colored sand were installed
Model loading tests in large soil tank on group behavior of piles
Essais de chargement modèle afin d’étudier le comportement de groupe de pieux dans un
grand réservoir du sol
Goto S., Aoyama S., Liu B., Towhata I.
The University of Tokyo
akita A.
Nippon Steel & Sumitomo Metal Corporation
Renzo A.A.
Anddes Asociados S.A.C., Lima-Per
ú
