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Centrifugal Study on Effect of Water Content of Clayey Backfill on the
Stability of MSEW
Huei-Tsyr Chen 1, Wen-Yi Hung 2, Bo-Wen Chen3 and Chung-Jung Lee 1
1 Associate
Professor, Department of Civil Engineering, National Central University, Taiwan, ROC
2
Graduate Student, Department of Civil Engineering, National Central University, Taiwan, ROC
3
Former Graduate Student, Department of Civil Engineering, National Central University, Taiwan, ROC
ABSTRACTS: Mechanically Stabilized Earth Wall (MSEW) requires the use of high quality granular soil as
reinforced backfill materials. However, in many cases the low quality in-situ clayey soil is frequently used instead
to cut down the cost, which obviously violates the design assumption. In addition, the clayey soil backfill may
block the draining path, resulting in increase in water content during heavy rainfall and thus affecting the stability of
MSEW. In this study, centrifugal tests of MSEW with clayey soil as backfill are performed. By varying the water
contents of the clayey soil backfill, the effect of water content on the stability of MSEW is investigated and the
countermeasure is proposed to maintain the stability at high water content of the backfill. The results indicate the
increase in water content of the clayey soil backfill will make MSEW become unstable and the stability can be
maintained by using 35% of the reinforcement spacing designed according to Mechanically Stabilized Earth Walls
and Reinforced Soil Slopes Design and Construction Guidelines (FHWA-NHI-00-043).
1. INTRODUCTION
2. TEST SYSTEM
Because of its flexibility and capability to absorb
deformations due to poor conditions in the foundations
and a higher resistance to seismic loading than rigid
concrete structures, many reinforced soil structures
have been constructed all over the world. The
Mechanically Stabilized Earth Wall (MSEW) is a type
of wall which is constructed by introducing reinforcing
into the backfill of wall via mechanical means such as
metal strips and rods, geotextile strips, grids and sheets,
or wire grids. Although it is required to use high
quality granular soil as reinforced backfill materials of
MSEW (Elias et al, 2001), the low quality in-situ
clayey soil is frequently used instead to cut down the
cost which obviously violates the design assumptions.
However, in-situ clayey soil backfill may block the
draining path, leading to increase in the water content
in MSEW and then affect the stability of MSEW.
There was a report of failures of three MSEW with
clayey soil as backfill after heavy rain in Taiwan.
(Huang, 1994)
In the past years many centrifuge studies (Porbaha
and Goodings, 1996; Zornberg, Sitar and Mitchell,
1996; Zhang, Lai, and Xu, 2000) have been performed
to investigate the behavior of MSEW with clayey soil
as backfill, but none of them studied the effect of water
content. Thus, in this study, centrifugal tests of
MSEW with clayey soil as backfill are performed. By
varying the water contents of the clayey soil backfill,
the effect of water content on the stability of MSEW is
investigated and the countermeasure is proposed to
maintain the stability at high water content of the
backfill.
The centrifuge model tests were conducted using the
100g-ton centrifuge (Acutronic 665-1) in the
Experimental Center of Civil Engineering of National
Central University shown in figure 1. The effective
radius is 3.0m and the maximum centrifugal
acceleration is 200g when the weight of the model is
550 kgf. Because of the setting of control system, the
acceleration of centrifuge will increase gradually to
10g automatically after starting.
Then, the
acceleration can be further increased through the
control panel according to the test requirements. The
soil container used is a rectangular box made of
aluminum plates, which has the internal dimensions of
223 mm wide, 820 mm long and 580mm deep.
Figure 1. The centrifuge in the Experimental Center of Civil
Engineering of National Central University
3. EXPERIMENTAL PROGRAM
dimension is put in an Ng field, it is equivalent to the
case of the same MSEW model built of material with a
unit weight of N where  is the unit weight of the
chosen material in 1g condition. In this study we
adopted this concept to design the MSEW models.
The Guidelines used is the Mechanically Stabilized
Earth Walls and Reinforced Soil Slopes Design and
Construction
Guidelines
(FHWA-NHI-00-043).
Before starting the design process, the properties of
geosynthetic reinforcement are determined first. In
order to ensure the larger deformation at the designed
acceleration level, the geosynthetic reinforcement must
be weak enough and water-proof to assure that the
mechanical properties will not change both in dry and
in wet conditions to accord with the need of this
experiment. Shown in figure 3 is the geosynthetic
reinforcement used in this study which is manufactured
by Seven States Enterprise Company, LTD. It is a
kind of composite material which combines geotextile
with polymeric grids. It has the limit tension strength
of 1.62 kN/m at a limit extended strain of 9.1%. After
the strength of the reinforcement had been obtained,
the size of the model of MSEW is then chosen
considering the size of the container. The ensuing
design steps follow the adopted guidelines as stated
before except the weight of the clayey backfill being
multiplied by N since the model is to be placed in Ng
states. In this study the N is taken as 20. Table 1
depicts the dimensions of the design MSEW.
3.1 Preparation of soil sample
The soil used in the model is clayey soil of Linkou area,
and its fundamental characteristics are specific gravity
2.67, liquid limit 40% and plastic index 22.6. The
clayey soil is purged first to ensure its homogeneity and
then ventilated to the desired water content in the open
air. Figure 2 shows the variation of the water content
with days of ventilation. It can be seen that the water
content decreases as the day of ventilation becomes
longer. Also shown in the figure is an equation
derived from regression analysis which is used in this
study to predict the days of placement required for the
desired water content. However, it should be noted
that the actual water content of the model tested is
measured before each test.
Water content (%)
70
60
50
40
30
y = -4.0057x + 59.575
20
2
R = 0.9479
10
0
0
2
4
6
8
Days
Figure 2. Relationship between water content and day of
ventilation
3.2 Model construction and test procedures
In performing the centrifuge model test, an important
step is to derive the scaling law for the type of problem
investigated. For the MSEW, one of the important
scaling factors that must be determined is that for the
strength of reinforcement. Based on the requirement
that the model and the prototype must have the same
factor of safety against failure, Zonberg et. al. (1996)
proposed a reduction factor of 1/N for the MSEW in
Ng field where g is the gravitational acceleration, while
Lord, Jr. (1987) proposed the factor to be 1/N2. The
difference between Zonberg’s and Lord’s result lies in
whether the width dimension of the MSEW is taken
into consideration when computing the scaling factor.
Amid these conflicting results, we have performed
several tests to see what the scaling factor should be by
using the feedback analysis from the tests with concept
of modeling of models. These tests showed that the
strength scaling factor is different from those proposed
by the previous investigators. Thus, further study is
still needed to clarify this issue.
On the other hand, Sawicki (1998) considered the
centrifugal model as a small model placed in a high g
condition. That is, if the MSEW model of selected
Figure 3. Reinforcement material
Table 1. Geometry characteristics of the models
Physical quantity
Dimensions
Height of the wall
30cm
Length of the reinforcement
20cm
Spacing of the reinforcement
4cm
Thickness of the surcharge soil
2cm
The depth of the foundation
15cm
Before constructing the MSEW model, the inside
vertical boundaries of the container are sprayed with
silicon and overlain with a sheet of thin rubber to
reduce the boundary friction effects. The foundation
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1.455
82.6
82.4
82.2
3
Dry density(g/cm )
1.450
1.445
82.0
1.440
81.8
81.6
1.435
81.4
1.430
81.2
81.0
1.425
34
36
38
40
42
44
Average degree of relative compaction (%)
of wall model is prepared first by proctor compaction
method to ensure the global stability of the wall and
adequate bearing capacity.
After the foundation is constructed, several boards
are piled up vertically on top of the foundation to
provide lateral support for MSEW construction. The
MSEW is prepared by rolling compaction method.
The rolling concrete cylinder used is 210 mm long and
has a diameter of 150 mm with weight of 8.452kgf.
The compaction is made for a soil layer of 2cm thick
each time with concrete cylinder rolling forwards and
backwards 5 times to make the degree of relative
compaction greater than 80%.
The relationship
between the dry density and the water content of soil in
the models by rolling compaction method is shown in
figure 4. This process is repeated until the model wall
reaches the designed height. Deformation of the walls
during the test is measured using LVDTs whose
locations are shown in figure 5.
adopting such a scheme is to ensure that stresses
between soil and reinforcement are transferred evenly
at each stage and the significant behavior of the wall
such as the development of crack or sliding can be
recorded at the actual acceleration. In the course of
increasing acceleration, the deformation of the MSEW
is measured by LVDTs. When the acceleration
reaches 20g, it is maintained at that level for about 5
minutes until the readings of LVDTs become stable.
During tests a CCD camera is also employed to
monitor the deformation of the model.
When the test is completed, the water content of the
soil is measured to see if there is significant change.
In addition, the undrained shear strength of the soil is
also determined using torvane shear test and uniaxial
compression test.
4. TEST RESULTS AND ANALYSIS
In this study 10 tests were performed which are
classified into two groups. Group 1 is designed to
investigate the effect of water content on the stability of
MSEW, while the purpose of Group 2 is to study the
effectiveness of reducing the reinforcement spacing in
maintaining the stability of MSEW at high water
content. To facilitate the following discussions, the
names of different parts of MSEW used in this paper
are defined as shown in figure 6.
Crest of
the facing
Top of the wall
Facing
MSE wall
46
Water content(%)
Figure 4. The relationship between the dry density and the
water content of soil by rolling compaction in 5 times.
Spacing
100mm 100mm 100mm
Foundation
Figure 6. Generic cross section of centrifugal model
48mm
48mm
625mm
295mm
200mm
916mm
Figure 5. Layout of the model
In performing the test, the model is accelerated to
10g at first. Then, it proceeds with an increase in
acceleration of 2g for each stage which is maintained
for about 30 seconds until 20g. The reason for
4.1 Effect of water content on the stability
Table 2 depicts the fundamental characteristics for each
test in group 1. In this group the reinforcement
spacing for all tests is 4cm; only the water content of
clayey soil backfill varies from 28.4% to 53.1%. The
one with 28.4% water content will be denoted as the
standard model. Test 2 and Test 3 have almost the
same characteristics, since they are used for
investigating the reproducibility of test. From Table 3,
it can be seen that the tests are reproducible.
Shown in figures 7, 8 and 9 are the deformations of
the model after the tests for clayey soil backfill for
water content 28.4%, 43.1% and 53.1%, respectively.
For water content of 28.4%, the deformation is a
type of forward tilting of wall face. As the water
content increases to 43.1%, the deformation becomes a
type of bulging in wall face. Finally the deformation
changes to overturning when the water content reaches
53.1% and it should be pointed out that this wall
actually collapses at an acceleration of 18g.
Figure 8 Deformation of MSEW for water content 43%.
Table 2. Fundamental characteristics of the models
MSE wall
Test
Water Undrained
content
shear
(%)
strength
(kPa)
Foundation
spacing
Sv
(cm)
Water
content
(%)
Undrained
shear
strength
(kPa)
1
28.4
44.5
4
28.9
58.3
2
35.1
33.1
4
28.1
61.6
3
36.1
31.2
4
29.2
56.3
4
43.0
23.8
4
30.0
56.3
5
53.1
11.6
4
31.8
47.7
Figure 9
Figures 10 and 11 depict the settlement and
horizontal displacement of the crest of the facing. It
can be seen that both the settlement and horizontal
displacement increases with increasing water content.
Also from these tests, it is observed that the width of
tension crack on top of the wall increases with
increasing water content.
Table 3. The results of repeatability test
Test2 Test3
35.1 36.1
33.1 31.2
28.1 29.9
61.6 56.3
4
4
2.6
2.9
1.6
1.8
0.8
0.9
5
4
Settlement(cm)
Water content in MSE wall (％)
Undrained shear strength in MSE wall (kPa)
Water content in foundation (％)
Undrained shear strength in foundation (kPa)
Reinforcement spacing (cm)
Maximum settlements of the crest of the facing
(cm)
Horizontal displacements of the crest of the
facing (cm)
Horizontal displacements of the crest of the
facing without the distances of sliding (cm)
Maximum settlements of the top of the wall (cm)
Maximum horizontal displacements of the facing
(cm)
Maximum horizontal displacements of the facing
without the distances of sliding (cm)
Deformation of MSEW for water content 53.1%
3
2
1
0
2.6
1.6
2.9
1.8
0.8
0.9
0
20
40
60
Water content(%)
Figure 10. Settlement of the crest of the facing for different
water contents
Displacement (cm)
5
4
3
2
1
0
Figure 7
0
Deformation of MSEW for water content 28.4%
20
40
60
Water content (%)
Figure 11 Horizontal displacement of the crest of the facing
for different water contents
4/6
Table 4. Fundamental characteristics of the models
Measurements
Water content in MSE wall (％)
Reinforcement spacing (cm)
Test5
Test6
53.1
52.6
4
2
Settlements of the crest of the facing (cm)
2.8
Horizontal displacements of the crest of
Collapse
the facing (cm)
at 18g
Maximum horizontal displacements of the
facing (cm)
Maximum settlements of the top of the
wall (cm)
Settlements of LVDT 1 (cm)
2.5
Maximum settlements of LVDT (cm)
5.2
soil backfill of 43% water content behaves almost the
same as the standard model.
Table 5. Fundamental characteristics of the Group 2 models
Test
7
8
9
10
4
MSE wall
Foundation
Water Undrained Reinforcement Water Undrained
content
shear
spacing
content
shear
(%)
strength
Sv (cm)
(%)
strength
(kPa)
(kPa)
43.1
22.7
1.5
30.1
59.5
42.4
25.1
2
31.2
57.8
44.0
20.9
2.5
28.0
61.6
42.8
24.2
3
31.9
47.4
43.0
23.8
4
31.8
52.0
5
Settlement (cm)
4.2 Improvement by varying spacing on the stability
As stated in previous section, the MSEW with clayey
soil backfill of water content 53.1% (Test 5) collapsed
at 18g which is 2g less than the desired acceleration
level. In order to see if such a situation can be
improved by reducing reinforcement spacing, Test 6 is
then constructed under the same conditions as Test 5
except that the reinforcement spacing is reduced to 2cm.
From Table 4 and figure 12, it can be seen that
although lager deformation can be observed for the
MSEW of Test 6, it does not collapse as Test 5. This
indicates the reduction of reinforcement spacing can
effectively improve the stability of MSEW at high
water content. However, in reality the water content
may not be as this high; therefore, a second group of
test was designed with clayey soil water content of
43% and the reinforcement spacing varying from 4cm
to 1.5cm as described in Table 5.
3
2
1
0
0
1
3.7
2
3
4
5
Spacing (cm)
4.5
Figure 13. Settlements of the crest of the facing for different
reinforcement spacing
2.7
Displacement (cm)
(a) Test 5
4
(b) Test 6
4
3.5
3
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
Spacing (cm)
Figure 12. Deformation of Wall: (a) test5 and (b) test6.
Figures 13 and 14 show the settlement and
horizontal displacement of the crest of the facing for
the reinforcement spacing considered. On these two
figures, the horizontal arrows indicate the deformation
for the models having 28% water content and 4 cm
spacing (standard model), while the vertical arrows
indicates the reinforcement spacing required for the
MSEW model with 43% water content to achieve the
same deformation as that for standard model. It can
be seen that using 35% of the reinforcement spacing as
determined from the guideline, the MSEW with clayey
Figure 14. Horizontal displacements of the crest of the facing
for different reinforcement spacing
5. CONCLUSIONS
In this study, centrifugal tests of MSEW with clayey
soil as backfill were performed. The effect of water
content of backfill on the stability of MSEW was
investigated and the effectiveness of reducing
reinforcement spacing in maintaining the stability of
MSEW at high water content was also studied. From
this study, the following conclusions can be drawn.
(1) The increase in water content of the clayey soil
backfill will lead to decrease in stability of MSEW.
(2) If the low quality in-situ clayey soil is used as
reinforced backfill materials, the reinforcement spacing
should be reduced in order to maintain the stability of
MSEW at high water content. In this study the use of
35% of the reinforcement spacing determined from the
guidelines is proposed for clayey soil of Linkou area.
6. ACKNOWLEDGEMENTS
The writers appreciate some of the fundamental issues
regarding this study raised by Dr. Nelson N. S. Chou,
the general manager of Genesis Group/Taiwan and
would like to express our sincerest thanks to Mr. C. H.
Wang, the general manager of Seven States Enterprise
Company, LTD. for his generosity in providing the
reinforcement materials used in this study. We are
also grateful for the help from members of centrifuge
modeling group of the Experimental Center of Civil
Engineering of National Central University.
REFERENCES
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Publication No. FHWA-NHI-00-043.
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Walls, Proceedings of Recent Case Histories of Permanent
Geosynthetic-Reinforced Soil Retaining Walls, pp.219-222.
Lord Jr., A.E. （1987 ）: Geosynthetic/soil studies using a
geotechnical centrifuge, Geotextiles and Geomembranes, Vol.
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Porbaha, A., and Goodings, D. J. (1996): Centrifuge modeling
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