Khemakhem
Khemakhem
Khemakhem
ABSTRACT: While deep foundations have been used extensively in clay to support both monotonic and cy-
clic lateral loading, significant uncertainties still now exist in predicting the field behaviour of such founda-
tions, especially for the cyclic loading. Cyclic loads result of waves and winds forces on offshore structures or
on bridge piers, mooring of boat on quays and variable overloads on many constructions and buildings.
Useful data on cyclic loading of piles in sand have already been obtained from centrifuge model tests and the
purpose of tl}e present study is to provide new data on the response of piles in clay under lateral static and cy-
clic loads. The clay is a normally consolidated kaolin whose undfained shear strength increases with depth.
Testing equipments and soil model preparation are presented in detail. The program of centrifuge tests is pre-
sented. Undrained tests on a rigid and flexible pile models (1/50) are performed. In the first test series, the ul-
timate static load was estimated. For cyclic loads, the pile top deflection was recorded using displacement
transducer installed on the model rigid pile. The effect of the number of cycles on the head displacement was
discussed.
1
rigid to flexible model pile. The table behind sum-
marize the p~oprieties of the prototype piles.
B(m) D(m)
r~l7.5kN/m
3
Average bulk unit weight
Final average water content w-50%
Compres,sion index Cc= 0,44
Vertical index of consolidation Cv -310 m 2 Is
Swelling index Cs = 0,44
2
was prepared with an initial average water content s 1are successively the settlements recorded at two
w = 90% and allowed tb rest for 24 hours:· Before successive time steps ! 1_1 and t 1 = tH + D.t. The val-
being placed in the container, the slurry was mixed ues of s 1_1 are then plotted versuss1 • The final set-
for about four hours and a drainage geotextil was tlement s., is the intersection of this curve and the
disposed at its bottom. Second, to· reduce the flight bisector. The final predict settlement and the last
time necessary to the final consolidation, this layer measured average settlement give the consolidation
was preliminary consolidated at lg under the value ratio:
of the overburden pressure enhanced at its center
U = s(t) x100(%) ( 1)
like in 50 g field normally consolidated clay. This s_
pressure is applied by stages. This process was re-
peated for each layer. When the preliminary consoli-
dation of a layer was achieved, the next layer was Table2. Asaoka method
placed above and then consolidated. The figure be-
low (Fig.4) presents the theoretical pressnre applied
for each layer The_ preliminary consolidation is fol-
I: \::L I ~, I ~, I ~: I ~: I ~: I ~, I : I
lowed by the in-flight consolidation. This process Generally, a value at least equal to U = 70% was
ensure the ultimate consolidation of the clay sample achieved before starting the pile loading. The final
The theoretical final height of each layer after the model sample thickness after the in-flight consolida-
preliminary consolidation at 1 g was about 133 mm., tion was about 395 mm. For each model container,
the final model sample thickness was about 400 mm. four tests were performed. The consolidation in
The pile was installed at 1g by driving it into a flight had to be repeated for each test: the centrifuge
drilled hole before turning on the centrifuge. was stopped to adjust the position of the serve-
actuator to conduct the next test. The duration of
consolidation was unpredictable. We have noted that
the consolidation in the first tests was faster and bet-
ter thmi the subsequent ones. If a maximum of four
hours was necessary to achieve U = 85 % for the
first and the second tests, the latest required about
six hours to reach 71% of consolidation.
3
Static loading tests were performed to estimate gas shown in Fig.6. The consolidation was achieved
the ultimate lateral load of the interaction soil-pile. in flight, thus the values of the undrained shear
The load is applied using dispiacement control strength were higher at 50 g.
mode at a rate of 4 mm Is . Lateral displacement was As mentioned, the re-consolidation in flight was
applied on pile until the load recoded did not io- unavoidable. For each test, a penetrometer test had
crease and became constant. This rate was chosen so to be done. The comparisons between the shear
that the conditions around the pile during the loading strength profile at 50g, for all test, show that the soil
are essentially undraine4. This rate was estimated resistance was almost comparable.
from T-bar tests performed in a centrifuge at the Undralned shear strength Su (kN)
University of Western Australia by Steward and
Randolph (1991). Authors suggested two approxi- 0
0 4
' " " " "
mate limits of loading's rates between the drained --At SOg
(Eq. 3) and undraioed conditions (Eq. 4). The rate ' -B-At lg
depends on the T-bar diameter B and the vertical 4
consolidatioll index Cv.
2
v< 0• Cv for drained conditions
B
(3 )
...
:[6
N
.c
c•
v>
2
° Cv forundrained conditions
B
(4)
10
"
"
The limit" rate for. undrained conditions was esti- 16
z
•
"-
GOD r---------------------------,
n 4 12 1& w u 28 n
Time fsecondesl
~ ® « « ,
z 500
:t: 400
Figure 5. Beginning of cyClic load sequence in model scale 'll -e- Test 1 Rigid pile
..9 300
jj -s- Test 2 Rigid pile
c 200 -*""Test 3 Rigid pile
3 . TEST RESULTS 2
'0
-+-Test 4 Flexible pile
~ 100 Computed result
All results presented herein are in prototype scale. Flexible pile --+---Test 5 Flexible pile
ol---~--~----~--~~~~~
3.1 Undrained shear strength profile 0,0 0,2 0,4 0,6 0,8 1,0 1,2
Normalized Horizontal Displacement Ytop/8
Results reveal as expected that the soil strength
Figure 8, Static lateral load-displacement relationship
increase lineill-ly with the depth both at 1 g and at 50
4
It is reasonable to attribute this phenomenon to
The relationship between lateral load and lateral the degradation of clay's strength under cyclic load-
displacement at the loading point is drawn as shown ing, which contribute to increase the displacement.
in Fig. 9 , the horizontal displacement was nonnal- Cyclic simple shear loading tests were carried on
ized with respect to the pile diameter B. As ex- clay (Purzin & a/, 1995), it was seen that the mean
pected, the applied load increased with applied dis- effective stresses decreases and so the clay was no
placement For small displacement, the lateral load longer as resistant as before cyclic loading. A slight
H can be considered to increase linearly with the top increase of the pore pressure may cause the degrada-
deflection of the pile Yrop . This relation can be ex- tion of the soil.
pressed by the following equation: The last issue to be discussed in this section con-
(5) cerns the comparison between the behavior of the
H=kyTop . head pile under cyclic loading in the sand (Rosquoet,
2004) and in the clay. We noted a fair agreement be-
Where k is a coefficient of the soil-pile interaction. tween them. The top displacement of the pile em-
We noted that equal stiffuess rigidity piles have ap- bedded in the sand was fitted by a logarithmic curve:
proximately the same coefficient k. In addition, the
coefficient k for rigid piles was higher than flexible _,)',_ = 1+b ln(N) (6)
ones for all tests. We concluded that the lateral load y,
for small displacements depends on the stiffuess ri-
gidity of the pile considered. Centrifuge tests were Where _b is a coefficient depending on the amplitude
conducted by Katuzume & Miyajima (1994), it was
and the density of the soil. and N is the number of
found that the horizontal load increased almost line-
arly with the increase of the square root of pile rigid- cycles applied.
ity. A similar fitting is possible to piles embedded in
For higher displacement, the results emphasize clay, but more tests are needed to define parameters
the highly non linearity nature of the soil- pile- inter- which could affect the coefficient b. These parame-
action. The ultimate load H u was deduced from the ters could be the amplitude, the consolidation degree
load-displacement .A value between 400 and 500 kN or both.
was approximately found for all piles regardless the 250
stiffness rigiaity.
The lateral load- displacement relationship was
,/
/;I~ ~
also computed using the Pilate LCPC software
which is based on the elastic continuum analysis. 'C
Calculated P-y curyes, stiffuess rigidity of piles and ~150
boundary conditions were introduced first into Pi- ;;
iJ
' III/I
r~/;
late. The pile tip was supposed free. The results ob- C'100
tained for each load increment are presented in Fig. 2
9. It is a fair agreement with the measured curves. ·c
0 50
Moreover, it can be seen that the difference still ex- :J:
ists between the responses for rigid and flexible V
piles. The coefficient of the interaction soil-pile of 0
rigid pile remains higher than the flexible one for 0,00 0,05 0,10 0,15 0,20
small displacement. Finally, the ultimate load was Horizontal displacement (m)
the same for the highest displacement as experimen- Figure 9. Cyclic lateral load -displacement relationship
tal results.
14 r------------------- ------------.
3.3 Pile he"ad displacement under cyclic lateral 12
load 10
5
4 CONCLUSION Ilyas, T.; Leung, C.F., Chow, Y.K. & Budi, S.S. 2004. Centri-
fuge model study· of laterally loaded pile groups in clay.
Journal of Geotechnical and Geoenvironmental engineering,
The behavior of the· pile embedded in normally 2004, Vol. 130(3), pp. 274-283
consolidated clay under static and cyclic lateral load-
ing was undertaken in this study. The displacement Kitazume, M. & Miyajima, S. 1994. Lateral resistance of a long
of the head pile was investigated. Results were re- pile in soft clay. Centrifuge 1994, pp. 485 -490
producible despite the stopping of the centrifuge and
the reconsolidation of the sample clay for each test. Magnan JP & Deroy JM. 1980. Analyse graphique des courbes
de consolidation oedom6trique. Bulletin de Liaison des
Careful supervision was needed to avert the clay's FontS et Chauss6es, sep-oct. 1980, pp. 53-56
swelling. In fact, it was necessary to aspirate the wa-
ter above the ground surface as soon as the centri- Magnan JP & Deroy JM. 1980. Analyse graphique des tasse-
fuge was stopped. In sum, the following conclusions ments observes sous les ouvrages. Bulletin de Liaison des
were obtained: Pants et Chaussees, sep-oct. 1980, pp. 45-52
1) The lateral load-displacement relationship de-
Reese. L & Welch.R. 1975. Lateral loading of deep founda-
pends on the stiffuess rigidity of Piles. This result tions in stiff clay. Journal of geotechnical engineering divi-
was affirmed by calculation (Pilate). sion, Proceedings of the American Society of Civil Engi-
2) The ultimate lateral load deduced from the neering, Vol.101, No. GT7, July, 1975, pp. 633-649
load -displacement curve at the loading point was
reproducible regardless the stiffuess rigidity of pile. Rosquoet, F. 2004. Pieu sous charge lattirale cyclique. These de
Doctorat, Ecole Centrale de Nantes, Universite de Nantes,
2) Cyclic loading could cause the degradation of
France, 305p. (in French).
the normally consolidated clay, thus the pile head
displacement didn't reach stabilization. Purzin A, Frydman S & Talesnick, M. 1995. Nonnalized non-
For a through investigation, tests with more sig- degrading behavior of soft clay under cyclic simple shear
nificant number of cycles are needed to define loading. Journal of Geotechnical Engineering, 1995, Vol.
probably a stabilization threshold of the residual dis- 121(12), pp. 836-843
placement. In addition, an accurate measure of the Tassios, .T & E. Levendis E. 1974. Efforts tept'ititifs horizon-
pore pressure nearer to the pile is particularly rela- taux sur pieux verticaux, Annates de l'institut teclmique du
vant even if the conditions are supposed undrained b8.timent et des travaux publics .No. 315, 1974, pp. 45-71
according to Steward & Randolph (1991). The pur-
pose of these measurements is to focus on the causes
of the degradation of the soil around the pile under
cycles. Lastly, it is necessary to test clay with differ-
ent consolidation degree to compare the effect of cy-
clic loading on the clay and on the pile head dis-
placement under the same cyclic loading.
References