International Journal of Geosciences, 2012, 3, 211-221
http://dx.doi.org/10.4236/ijg.2012.31024 Published Online February 2012 (http://www.SciRP.org/journal/ijg)
Simulation Vacuum Preloading Method by
Tri-Axial Apparatus
Ngo Trung Duong1, Wanchai Teparaksa1, Hiroyuki Tanaka2
1Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand
2Faculty of Engineering, Hokkaido University, Sapporo, Japan
Email: ngotrungduong@yahoo.com, wanchai.te@chula.ac.th, tanaka@eng.hokudai.ac.jp
Received October 25, 2011; revised December 14, 2011; accepted January 13, 2012
ABSTRACT
It is very important to control any risk of instability of embankment during vacuum construction, the simulation vacuum
preloading method using tri-axial apparatus is proposed to predict the behavior of soft soil improvement in the labora-
tory, as well as to make this method become familiar and easier in the future. The tri-axial apparatus is used instead of
the large-scale one, which has been performed by Bergado (1998) and Indaratna (2008). The tri-axial test on small size
specimen can be carried out in one week compared to the large-scale apparatus takes one month for big specimen. In
addition, the lateral deformation as well as the shear strength increase with time can determine accurately.
Keywords: Soft Soil; Soil Improvement; Vacuum Preloading; Degree of Consolidation
1. Introduction
Nowadays the vacuum preloading consolidation method
becomes the popular method to improve soft soil. This
method is an effective method of improving soft soil
conditions as introduced by Kjellman (1952) [1] in early
1952. With the merging of new materials and technolo-
gies, this method has been further improved in recent
years.
The modeling vacuum method to improve soft soil in
the laboratory has been performed by Indaratna 2008 [2]
using the large-scale apparatus and follows one-dime-
nsional consolidation theory (Tezaghi). The results ob-
tained from this modeling partially evaluated the behav-
ior of soft soil reinforced by vacuum preloading method
in the laboratory. Using the large specimen 45 cm × 90
cm in diameter and height respectively in the large-scale
apparatus, the time used for this test was more than one
month.
The horizontal deformation εr during the tested time,
which is the typical deformation of soft soil improvement
by vacuum, and also the increasing of shear strength
could not be measured. So far, the controlling surcharge
processing during vacuum construction has not been
discussed sufficiently.
The new method is proposed using tri-axial apparatus
to simulate the comprehensive behavior of soil impro-
vement by vacuum preloading method in the laboratory
to support the engineering task quickly. In addition, it is
desired to make the method become familiar in the fu-
ture.
The finite element method (FEM) is used to analyze
two cases of drainage condition at the boundary and cen-
ter of the axisymmetric soil cell.
The study aspects to solve these matters are as follows:
1) Simulation the behavior of soft soil improved by
vacuum method by tri-axial apparatus under axisymetric
consolidation condition;
2) The lateral deformation and vertical settlement are
concerned during soil improvement by vacuum prel-
oading method;
3) Evaluating the degree of consolidation during soil
improvement;
4) Combination surcharge and vacuum preloading to
estimate the increasing shear strength).
2. Tri-Axial Apparatus
Tri-axial apparatus can clearly evaluate the failure mech-
anism as well as the capacity of increasing shear strength
of soil in the laboratory. From the tri-axial test, the result
parameters are used to predict the behavior of soil in the
field during construction.
Under vacuum pressure condition alone, the soil mass
at depth is subjected the isotropic stress status (K = 1).
With flexible functions of the tri-axial machine as shown
in Figure 1, the isotropic condition of the soil mass can
generate the same vacuum condition by controlling the
lateral earth pressure (K).
During vacuum condition, the surcharge loading can
C
opyright © 2012 SciRes. IJG
N. T. DUONG ET AL.
212
drainage layer
(filter paper)
rubber membrane
porous disc
Volume gauge and back pressure system
filter paper
overlaps
porous disc
horizontal
(radial)
drainage
Volume gauge and cell pressure system
PWP
measurement
ac
b
sample
steel plate
a1
porous disc
steel plate
Figure 1. Scheme tri-axial apparatus.
be generating as axial force by the loading rod at top of
the machine. The deformations of soil specimen in verti-
cal and horizontal direction are measured during testing
to evaluate the behavior of soil specimen.
The specimen is covered by rubber membrane and
placed in the water tank; therefore the friction between
circular soil specimen and the cell is eliminate, which is
different to the oedometer apparatus.
The steel plates at the ends of specimen are as the im-
pervious layer, only radial drainage is induced during
consolidation. The filter paper covers around the bound-
ary of specimen and overlaps the porous discs at the both
ends of specimen as the drainage layer. For the large-
scale oedometer apparatus, the drainage was established
at the center of specimen. The FEM has been used to
define the different between the drainage path conditions
of specimen, and defines the correction factor for this
simulation. The boundary conditions were illustrated in
the Figure 2.
The assumptions were used for simulation of vacuum
preloading method are as follow:
1) Soil mass as subjected vacuum pressure follows ax-
isymmetric consolidation.
2) Under vacuum pressure only, soil mass will be sub-
jected the Isotropic stress state, it mean that the coeffi-
cient of horizontal earth pressure (K) equal to one, while
for the surcharge only (K) value can calculate from
Equation (1).
3) For the soil mass, the vacuum pressure is distributed
along to the specimen is uniform.
H
De
H
De
(a) Drainage at center (b) Drainage at outer boundary
F
K1sinφ
igure 2. The drainage condition of axisymetric cell.
(1)
where φ: the friction angle
3. Finite Element Method of Analysis for
3.1
drain
Nakanodo, 1974; Hansbo, 1981). Most researchers ac-
of soil.
Drainage Boundary Condition Analysis
. Thetheory of Asixmetric Consolidation
The unit cell theory representing a single circular
surrounded by a soil annulus in an axisymmetric condi-
tion has been used (e.g. Barron, 1948; Yoshikuni and
Copyright © 2012 SciRes. record the behavior data and deformation of soil spe-
cimen automatically. The soil is consolidated completely
when the effective stress increase to the target and the
excess pore water dissipates completely. However, from
data from FEM and test the times of primary consolida-
tion reach to as the excess pore water pressure dissipates
about 95%.
Where:
CP: Cell pressure
Pore water pressure
σv': Vertical effective stress target
σh': Horizontal effective stress target
K: Coefficient of horizontal earth pressure
The shearing steps are carried out to define the undra-
ind shear strength of soil improved. Depend on the goals
these step could be performed at the time after vacuum
loading completely or with the increase of degree of
consolidation reach to 40%, 70%, 100% combine with
surcharge to estimate the capacity of soil embankment each
construction stage respectively. This step is performed
drained progress stage, the consolid
b
BP: Back pressure
PWP:
75mm
150mm
'1
'3 = '1
Pre-consolidated,
OCR=1.25, k0=0.5
'1+'va
'3 +'va
Vacuum condition,
'va (kPa), K
Figure 20. Soil specimen under vacuum c ondition.
Table 5. Parameter in vacuum proceed by tri-axial appar-
atus.
CP BP PWP
v' h' (K)
Step
(kPa)
Step loading 220200 200 20 20 1
Recompression 240200 200 80 40 0.50
Vacuum supplying
(50kPa) (Undrained)290200 250 80 40 0.50
Vacuum applied 290200 200 130 90 0.692
Vacuum supplying
(100kPa) (Undrained)340200 300 80 40 0.50
Vacuum applied 340200 200 180 1400.778
to control the surcharge preloading at the site to avoid the
soas the surcharge is over the soil capacity as
we of embankment. The capacity of
somulation (as empirical me-
th
il failed
ell as instability stag
il can be predicted by for
od Tanaka):
uuS(s )*0.8*p
v
σ
(14)
w
4.3. Test Results and Analysis
The Figures 21 and 22 are shown the relationship be-
tween case (NB) in FEM and lab test, the vacuum 50 kPa
was applied only and combination to surcharge respec-
tively. In the both cases the DOC in 100% were induced,
the largest different of U is 3.5% between FEM and lab
test occurring at Th = 0.3.
The final deformation of specimen nearly same value
at end of vacuum stage and vacuum combine surcharge
as shown in Figure 23. The largest different in volume
strain 0.3% occur at 350min as DOC at 95%.
The volume strain in 4.92% and 7.12% were found
both /e )
ted isotropic stress; this ratio is nearly one during
vacuum only and res archwas estabhe
Thelts agretrongwith behavi of-
provement by vacuum preg theory.
T24 is shown the increasing shearen
of soil specimn bepplica-
tio aeragre olida-
t a00%pely.
is shown in the Figure 25, there are
here
su/σv': determine with the lateral earth ratio K by the
conventional tri-axial test in Figure 1 8 .
p: the loading is applied (vacuum pressure or surch-
arge loading).
The undrain shear strength of soil specimen from lab
test result is analysis to check the capacity of soil im-
proved.
in
cases FEM and Lab Test. The strain ratio (er v
illustrated the inward lateral deformation of specimen
subjec
duce
e s
s su
ly
arge lis
soil
d.
imse resuor
loadin
he Figure r stgth
e
n vacuum preloa
fore im
ding
provem
t sev
ent and
l de
after a
e of cons
ion of 40%, 70%
The stress path
nd 1 resctive
Copyright © 2012 SciRes. IJG
N. T. DUONG ET AL.
220
Figure 21. Case n = 20, vacuum only; Va = 50 kPa, U = 10 0%.
Figure 22. Case n = 20, vacuum combine surchage; Va = 50
kPa, U = 100%.
Figure 24. Increasing undraine d shear strength.
Figure 25. Stress path.
some sections during simulate vacuum preloading me-
thod, pre-consolidation (AB), vacuum application (BC),
surcharge loading (CDEF). Under vacuum condition, the
stress path of soil moves from B to C far from the failure
line, while it changes from D to E close to the failure line
when surcharge was applied. The behavior of soil speci-
men under vacuum preloading method simulated by
Tri-axial apparatus is well matched the before studies.
5. Conclusions
The study has been developed based on the combination
of finite element analysis and the results of laboratory
experiments to simulate a new appropriate method. This
method can be widely applied for soft ground improve-
ment by vacuum preloading method. The results om the
etric unit cell used to model the behav-
Figure 23. Case n = 20, vacuum combine surchage; Va = 50
kPa, U = 100%.
fr
study can be summarized as follows:
1) The axisym
Copyright © 2012 SciRes. IJG
N. T. DUONG ET AL.
Copyright © 2012 SciRes. IJG
221
ior of soil treatment by vacuum preloading as none
boundary conditions are considered, the behavior of soil
is more close to real soil state in the field.
2) Two cases of drainages boundary showed that time
for consolidation in cases outside drainage of unit cell is
faster than at the center by ThNC/ThNB ratio. However, the
deformations of the specimens in all cases are in same
shape and value with the same applied condition.
3) Results of experiments by tri-axial apparatus en-
tirely agree with FEM model. It is suggesting that the
theories are given full compliance, highly compelling to
predict the behavior of soil improvement by vacuum
preloading method.
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