Subject IDs: SF01, SA03
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Shear strength evaluations of river embankment soils under low confinement pressureusing water absorption softening test
Yuichi Kubo1, Takeshi Kodaka 2, Kyu-Tae Lee 3, Masanori Ishihara4, and Yuto Nakayama2
1Geo-Labo Chubu, 804 Midorigaoka, Moriyama-ku, Nagoya 463-0009, Japan
2 Department of Civil Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
3 Osaka Main Office, CTi Engineering, 1-6-7 Osaka Doshomachi, Osaka 541-0045, Japan
4 Public Work Research Institute, 1-6 Minamihara, Tsukuba 305-8516, Japan

ABSTRACTIn this paper, we discuss a new testing method for determining the strength parameters of river embankment soils.
Our proposed water absorption softening test is a kind of triaxial test conducted under drained conditions that utilizes
low effective stress states to determine soil strength parameters. In order to evaluate our proposed method, samples
from three actual river embankment levees were extracted and prepared for use. From the results of a series of tests
conducted using these samples, it was confirmed that our new testing method is suitable for use in evaluating theseepage resistance of embankment soils and determining their strength parameters under the same low-level effective
stress states that could be assumed to exist during actual levee flood conditions.
Keywords: river embankment; triaxial test; strength parameters; water absorption softening test; seepage failure

1 INTRODUCTION
In recent years, levee collapses caused by
unexpectedly heavy floods have occurred frequently inJapan, making it increasingly clear that the ability to
precisely evaluate river embankment stability is a
crucial issue of levee management. The authors carriedout a series of triaxial tests on river embankment soils
under various test conditions (e.g. Kodaka et al., 2013,2015, 2017). Through these studies, they identified andproposed the use of effective stresses at the phasetransformation state as an effective way to determine
the strength parameters of tested samples.
Because soils reach maximum deviator stresslevels under relative high strain levels, those stresslevels are unsuited for determining the potential forriver embankment failures that might occur due toseepage force under either undrained or drained
conditions. In fact, since the deviator stress of dense
sandy soils obtained via consolidated undrained shear
triaxial tests increases continuously due to the positive
dilatancy restriction, this method is particularly
unsuitable for determining the physical behaviors ofsoils in actual river embankments. In contrast, a
strength parameter determination method that uses
effective stresses at phase transformation states is easy
to implement and more suitable for use whenevaluating the potential of river embankment seepage
failures under relatively low strain levels. However, amore precise testing method is required for determining

strength parameters under low levels of effective strain.
In the present paper, we report on a new water
absorption softening test that utilizes very low effective
stress states as well as the results of its use indetermining the strength parameters.
2 WATER ABSORPTION SOFTENING TESTOur proposed test is a kind of triaxial compression
test that is applied under drained conditions. First,
conventional drained shear testing is conducted until
the prescribed deviator stress is reached in order toinduce an anisotropic stress state in the soil specimenthat simulates the slope of an embankment. Next, thepore water pressure is gradually increased whileconstant deviator stress is maintained. In the case of asample consisting of fine rich soils, the rate of water
absorption must be limited in order to maintain auniform pore pressure distribution within the specimen.
In this study, the water absorption softening test isapplied to both clean silica sand samples and medium
grained soil samples with rich fine contents.
3 CLEAN SILICA SAND
3.1 Test Conditions
In this test, Mikawa silica sand No. 6 is used toevaluate the effect of specimen density on strengthparameters under low confining stress conditions. An
air pluviation (AP) method is used to produce theSubject IDs: SF01, SA03
TC/ATC # (optional): TC201

σ₃=100kPa60

σ₃=50kPa

σ₃=100kPa

40

20

20

0

0
0

Stress ratio at failure

40

2.0

1.0

30 40 50 60 80
Relative density Dr (%)

50
100
150
Mean effective stress (kPa)

Fig. 3. q/p’– Dr relations.

Fig. 1. Effective stress paths of clean silica sand by water absorption softening test.
50
degree)40

τ (kPa)τ (kPa)

40

20

20

0

0
0

40
σ (kPa)80

σ₃=100kPa

0.0
0

50
100
150
Mean effective stress (kPa)

σ₃=50kPa
3.0

040
σ (kPa)80

Fig. 2. Mohr’s stress circles of clean silica sand by water absorption softening test.

various relative specimen densities, Dr=30, 40, 50, 60
and 80%. After being placed in the triaxial chamber, thespecimens were fully saturated via the double-vacuum
method and the application of 200 kPa back pressure(i.e., B value of over 0.95).
In all of our tests, isotropic consolidation was
performed by applying predetermined effective
confining pressures (50 and 100 kPa). First, axial
compression was performed using a loading rate of
0.1%/min. under a fully drained condition and constantcell pressure until the prescribed deviator stresses (25and 50 kPa) were reached. Next, the pore waterpressure was gradually increased at a rate of 5 kPa/min.
while maintaining constant deviator stress mentioned
above until specimen failure was observed.
3.2 Test results
Figure 1 shows the effective stress paths obtainedby applying the water absorption softening test. The
dashed straight lines show the critical stress ratio
produced by the conventional triaxial test performed forcomparison purpose. The effective stress paths consistof two parts. One is a line that inclines at a rate of 1:3,
and the other is a nearly horizontal line. The inclined
line indicates the path of drained shear under constantconfining pressure in which stress moves from theisotropic to anisotropic state. The horizontal line shows
the process of mean effective stress decreasing due toincreasing pore water pressure under constant deviator
stress conditions.
The failure state is expressed as inflection on the left
side of the horizontal line. In the case of loose sand,

c
C

20

40

16

30

12

20

8

10

4

0

c (kPa)σ₃=50kPa

Deviator stress (kPa)

Deviator stress (kPa)60

030 40 50 60 80
Relative density Dr (%)
Fig. 4.  and c – Dr relations.

(Dr=30%), the deviator stress gently drops after
reaching the failure state. In contrast, the stress paths ofdense sand (Dr=80%) drop catastrophically. This
indicates that the denser sand has a higher stress ratio at
failure (defined as the maximum inflection point of thestress path) that exceeds the critical stress ratio obtainedby the conventional triaxial test. Figure 2 shows the
Mohr’s stress circles defined by the effective stressstates at the maximum inflection points in Fig. 1.
Figure 3 shows the stress ratio relations at failure andrelative density of the specimens. Here, it can be seenthat the stress ratio at failure increases with increases in
the relative density. Furthermore, the stress ratio can beseen slightly high in the lower case of confiningpressure. As a result, we can infer that a dense soil
embankment would have higher resistance toflood-related seepage failure. This result can be also
seen from the strength parameters at various relative
densities that were determined by the stress state at
failure when using the water absorption softening test,
as shown as Fig. 4.
4 ACTUAL MEDIUM GRAINED LEVEE SOILS
4.1 Test Conditions
The water absorption softening test was carried out
using the three kinds of medium grained soils samples
extracted from actual Japanese levees: the Shibui River,in Miyagi Prefecture, the Kakehashi River at Ishikawa
Prefecture, and the Koyoshi River in Akita Prefecture.
The soil samples were collected in an undisturbed
state. Thin-walled tube sampling using a boringSubject IDs: SF01, SA03
TC/ATC # (optional): TC201(a) Shibui River(b) Kakehashi River(c) Developed driving sampler

80
60
40
20
0.01

0.1
Grain size(mm)

1

10

80
60
40
20
0
0.001

(a) Shibui River

Percentage by m ass passing(% )

100

100

100

0
0.001

Percentage by mass passing(%)

Percentage by m ass passing(% )Photo 1. Situations of field sampling of embankment soils at actual levees by the simple sampling method.

0.01

0.1
Grain size(mm)

1

10

80
60
40
20
0
0.001

(b) Kakehashi River

0.01

0.1
Grain size(mm)110(c) Koyoshi River

Fig. 5. Grain size distributions of river embankments soils used in this study
Table 1. Water absorption softening test conditions.

Samples

Kakehashi

ShibuiKoyoshi

Initialmeaneffective
stress/Cellpressure
(kPa)
50
100
100
50
100
100
50
100
100

Deviatorstressduringwater
absorption(kPa)

Initialwater
content(%)

Initial
void
ratio

Void
ratio atshearing

25
50
75
25
50
75
25
50
75

35.0
35.4
33.3
26.1
30.5
50
52.7
53.1

1.00
1.00
1.85
1.53
1.76
1.38
1.49
1.47

1.02
0.96
1.71
1.39
1.53
1.36
1.36
1.32

machine was employed to collect the Koyoshi Riverembankment sample, whereas the Shibui River andKakehashi River soil samples were collected via thesimple sampling method newly proposed by Kodaka etal. (2017). Photo 1 shows the simple sampling method
being used in the field along with the newly developed
driving sampler. The sampler has an inner tube madeby polyvinyl chloride pipe to avoid sample disturbance.
All specimens were fully saturated via the
double-vacuum method after being placed in the triaxialchamber. A series of water absorption softening tests aswell as conventional triaxial test were then conductedusing these soil samples. The confinement pressuresused were 50 and 100 kPa, and the constant deviator
stress levels used were 25, 50, and 75 kPa. Figure 5
shows the grain size distributions of the soils measured
after the test. Here, it can be seen that each soil has richfine-grained fraction contents. The test conditions usedin these measurements are listed in Table 1.

4.2 Test results
Figures 6, 7, and 8 show the test results for the
sampled soils extracted from the Shibui, Kakehashi,
and Koyoshi River embankments, respectively. Eachfigure shows the effective stress paths and therelationship between the stress ratio and axial strain as
observed by the water absorption softening test. In the
stress paths of each figure, the conventional triaxial test
results are also shown for comparison. The dashed lines
express the critical stress ratio assumed by the effective
stress path of the conventional triaxial test.
In the case of the Shibui River soil, as shown in Fig.
6, the effective stress paths produced by theconventional triaxial test express the shear behavior of
relative loose sand, which can be seen in the simple
decrease of mean effective stress due to plastic
compression. The stress ratios at failure that wereobserved by the water absorption softening test arearound the critical stress ratio. The Shibui River levee
ruptured in 2015 during heavy rains in the
Kanto-Tohoku region. Since the overflow was notobserved there, it was presumed that the embankment
collapsed due to piping of the body of the embankment.
Furthermore, from the results of this study, the Shibui
River embankment soil can be considered to be a
geomaterial that has relatively low seepage resistance.
In contrast, when water the absorption softening test
was applied to the Kakehashi River soil in a lower
deviator stress case, the stress ratio approached 3,
which is relative high, as shown in Fig. 7. Since theaxial strain at failure is around 1%, it is likely that the
Kakehashi River soil has the potential to fail suddenly
due to water absorption. On the other hand, since stressSubject IDs: SF01, SA03
TC/ATC # (optional): TC201

4.0
3.5
150
3.0
2.5
100
2.0
1.5
50
1.0
0.5
0
0.0
0
50 100 150 200
0Mean effective stress (kPa)

Deviator stress (kPa)ratios at failure are lower when higher deviator stress isapplied, the results also indicate that the seepage
resistance of Kakehashi River soil is high under low
confining pressure conditions.
In the case of Koyoshi River soil, as shown in Fig. 8,
the stress ratios at failure reach the value of 3 in spite ofthe deviator stress, which means the effective confining
stress reaches zero. From this result, it can be statedthat the Koyoshi River soil has high resistance toseepage failure.

Pattern 1

Pattern 2 Pattern 3

q=25kPa σ3=50kPa
q=50kPa σ3=100kPa
q=75kPa σ3=100kPa

Mean effective stress (kPa)
Fig. 9. Schematic diagrams of three typical patterns observed via
water absorption softening testing.

q/p'Deviator stress (kPa)200

5 CONCLUSION
5
10Axial strain(%)

15

Fig. 6. Shibui River soil test results.
4.0
3.5
3.0
150
2.5
2.0
100
1.5
1.0
50
0.5
0.0
0
0
50 100 150 200 0
Meen effective stress (kPa)q/p'Deviater stress (kPa)200

q=25kPa σ3=50kPa
q=50kPa σ3=100kPa
q=75kPa σ3=100kPa

5
10
Axial strain(%)

15

Fig. 7. Kakehashi River soil test results.

150
100
50
0
0

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
200
0

ACKNOWLEDGEMENTS

q/p'

Deviater stress (kPa)200

50 100 150
Mean effective stress (kPa)

Herein, we proposed a water absorption softening
test and showed how it can be used to evaluate theseepage resistance of the river embankment soils undervery low effective stress states. Based on the results oftests using clean silica sand, it was determined thathigher stress ratios at failure could be measured withincreases in the relative density. Large strengthparameter values were also obtained during these tests.
From the test results of actual river embankment
soils that contained rich fine contents, it was found that
effective stress paths followed three typical patterns
during the water absorption process. Of particular note,the results indicate that river embankments whose soils
are categorized by Pattern 3 during the water absorption
softening test should be evaluated as having low
resistance to seepage failure.

q=25kPa σ3=50kPa
q=50kPa σ3=100kPa
q=75kPa σ3=100kPa

5
10
Axial strain(%)

15

This research was partially supported by the
Ministry of Land, Infrastructure, Transport andTourism (MLIT), River and Sabo Research Grant
2012-2014, 2015-2017 (Kodaka). The authors would
also like to express their sincere thanks to Mr. Takayuki
Tanaka and the other graduate students of Meijo
University for their help with the experimental work.

Fig. 8. Koyoshi River soil test results.

REFERENCES

Figure 9 shows schematic diagrams in which threeeffective stress path patterns obtained by the water
absorption softening tests are displayed. Pattern 1shows that the stress ratio at failure reaches 3. Pattern 2shows that the stress ratio approaches 3 only in the caseof low confinement pressure. Pattern 3 shows that thestress ratios are almost the same as the critical stressratio of the conventional triaxial test. The embankment
soil categorized as Pattern 3 should be evaluated as low
resistance due to seepage, even if the soils are classified
as cohesive due to their rich fine contents.

Kodaka, T., Cui, Y., Lee, K.-T., Mori, S. and Kanematsu, Y.
(2013). Soil structure in gravel-mixed sand specimen and its
influence on mechanical behavior, Proc. of the 18th Int. Conf.
on Soil Mechanics and Geotechnical Engineering, Paris,
1691-1694.
Kodaka, T., Cui, Y., Lee, K.-T., Kobayashi, Y. and Wu, Y.
(2015). Evaluation of strength coefficients of sandy leveesoils under various triaxial test conditions, Proc. of the 15th
Asian Regional Conference on Soil Mechanics andGeotechnical Engineering, Fukuoka, JPN-088.
Kodaka, T., Lee, K.-T., and Ishihara, M. (2017). Simplified
sampling method for river embankment soils and strength
property evaluations of the sampled soils, Proc. of the 19thInt. Conf. on Soil Mechanics and Geotechnical Engineering,Seoul, 2395-2398.