Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

Effects of structure of sandy soil on various mechanical behavior
Effets de la structure du sol sableux sur divers comportements mécaniques
Takeshi Kodaka & Kaoru Fujita
Department of Civil Engineering, Meijo University, Japan, kodaka@meijo-u.ac.jp

Yuichi Kubo
Geolabo-Chubu, Japan

Keita Lee
Osaka Branch Office, Nippon Koei, Japan
ABSTRACT: In the present study, the effects of structure of sandy soil on various loading behavior, e.g. static undrained shear
strength, stress paths, liquefaction strength etc., are discussed. The sandy soil specimens with different soil structures were producedby air-pluviation or wet-tamping method with different initial water content. A series of triaxial tests both by monotony and cyclicloadings under undrained condition using the artificial structural sand specimen was performed. Furthermore, a series of simple shear
tests under monotonic loading was also conducted using the same specimens. Even though the specimens were fully saturated before
consolidation, the shear behavior remarkably changes with difference of initial water contents. In the case of monotonic loading of
triaxial tests, the highly structural soil characteristics, i.e. significant elastic response in the beginning of loading, the large maximum
deviator stress and the rapidly catastrophically failure after peak stress, were observed in large initial water content specimen. The
highly structural specimens with large initial water content showed the high liquefaction strength in the cyclic loading of triaxial test.
RÉSUMÉ : Dans la présente étude, les effets de la structure du sol sableux sur divers comportements de chargement, par ex. la résistance
au cisaillement statique non drainé, les chemins de contrainte, la résistance à la liquéfaction, etc., sont discutés. Les spécimens de sol
sablonneux avec différentes structures de sol ont été produits par la méthode de la pluie d'air ou du tassement humide avec différentes
teneurs en eau initiales. Une série d'essais triaxiaux à la fois par monotonie et chargements cycliques dans des conditions non drainées
en utilisant l'échantillon de sable structurel artificiel a été réalisée. De plus, une série d'essais de cisaillement simple sous chargementmonotone a également été menée en utilisant les mêmes spécimens. Dans le cas du chargement monotone des essais triaxiaux, lescaractéristiques hautement structurelles du sol, c'est-à-dire une réponse élastique significative au début du chargement, la grande
contrainte de déviation maximale et la rupture rapidement catastrophique après le pic de contrainte, ont été observées dans une grandeéprouvette initiale à teneur en eau. Les spécimens hautement structuraux avec une grande teneur en eau initiale ont montré la résistanceélevée à la liquéfaction dans le chargement cyclique de l'essai triaxial.
KEYWORDS: soil structure, triaxial test, simple shear test, liquefaction, sandy soil.

1 INTRODUCTION
In conventional and classical soil mechanics, without taking into
account the soil structure, the shear strength of soils is considered
to be determined by their void ratio. However, it is well known
that the difference of soil structure affects their shear behavior
despite of the same void ratio. For example, a series of triaxial
test was performed using by the gravel-mixed sand specimensthat have the same void ratio but considerably different shear
behavior by varying the initial water content during specimen
preparation (Kodaka et al. 2013). They also showed that the shear
behavior difference was able to be explained by changing the soil
structure though the numerical simulation using by SYS Camclay model. As it is possible to prepare sandy soil specimens thathave the same void ratio but considerably different soil structuresby the same manner mentioned above, this study demonstrates
the effects of soil structure on the mechanical behavior undermonotonic and cyclic loading in triaxial tests.
Ishihara (1993) showed that each type of undrained shear
behavior of the Toyoura sand specimens, made by various
specimen preparation methods, is different from the others. In his
study, the void ratios of all the specimens are different due to theinitial structure of the sand particles. However, since thespecimens in this study are reconstituted using well-graded sand,it is possible for the initial void ratios of all the specimens to bealmost the same. This point is quite different from the results of
Ishihara’s experiment.

Tokimatsu et al. (1986a) pointed out that the liquefaction
strength of a clean sand was strongly correlating its elastic shear
modulus, and the elastic shear modulus can reflect soil fabric.
Tokimatsu et al. (1986b) also showed that the sample disturbanceaffected the dynamic properties of sand. Therefore, these study
suggest the sample disturbance of sand is relating to degradation
the soil structure.
A reconstituting structural sand containing some fine fraction
is used in this study. This point is different from the mentioned
above pioneers’ studies targeting to clean sand. However, we cansystematically discuss the effects of soil structure of sandy soilon its mechanical behavior by using the artificial structural sand.
2

TEST CONDITIONS

The test sample was a combination of Mikawa silica sand No. 4
and No. 6 and silt-rich Noma sand mixed in proportions of 3:1:3
by mass. Figure 1 shows the grain size distribution for the mixed
sample used in the tests. The figure also shows the results for
actual levee sand (sand collected at the Kitajima Levee on the
Chitose River), and the grain size of the mixed sample wasartificially adjusted to resemble this natural sand, including thefine fraction content.
Table 1 tabulates the specifications of each specimen used in
the triaxial test. The specimens used in the monotonic and cyclicloading were prepared so that their void ratios were almost thesame, as indicated in Table 1.

Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

Percentage by masspassing (%)

100
80
60
40

Mixed sample usedin this study
Kitajima levee sand

20
0
0.001

0.01

0.1

1

Grain size (mm)

10

100

Figure 1. Grain size distribution of the test sample.
Table 1. Specifications of each specimen used in the triaxial test.
Test
conditions

Monotonic
loading

Initial effective
confining pressure[kPa]

Void ratios after
consolidation

50

0.637

0

100

0.614

5

10

Cyclicloading(a) Initial water content of 0%, w0=0%

Initial watercontent, w0
[%]

150

0.608

50

0.679

100

0.645

150

0.651

50

0.659

100_no1

0.656

100_no2

0.645

150

0.638

0
5
10

(b) Initial water content of 5%, w0=5%

0.622
100

0.614
0.624

In this test, specimens with the same dry density (i.e., void
ratio) but different soil structures were prepared using the mixed
sample with exactly the same grain size and three different initialwater contents during preparation of 0%, 5%, and 10%.
Specifically, in the cases of 5% and 10% initial water content,
distilled water was added to the mixed sample in the dry state
until the prescribed water content was reached, and the sample
was stirred thoroughly until uniform. Specimens 50 mm indiameter and 100 mm in height were prepared by compacting themixed sample in five layers in a mold until it reached theprescribed density. Because the unsaturated specimens preparedin this way were self-supporting after demolding, they were
carefully covered with a rubber sleeve and then set up in thetriaxial testing apparatus. In the case of 0% initial water content(i.e., dry sand), a two-piece split mold covered with a rubber
sleeve was set up in the triaxial testing apparatus, and thespecimen was prepared using the sample in the dry state by the
air pluviation method. After applying a confining pressure by
negative pore pressure to the specimen covered with the rubber
membrane, the specimen was removed from the two-piece split
mold and the test was conducted. Although all of the specimens
were prepared in a dry or unsaturated state during sample
preparation, after being installed in the triaxial testing apparatus,
they were fully saturated using a double vacuum pressure method.
Figure 2 shows optical microscopy images of the surface forthe three types of specimens prepared by varying the initial watercontent. The specimens with initial water contents of 5% and
10% were self-supporting in the unsaturated state and thesurfaces were observed directly. However, in the case of 0%
initial water content, the specimen was prepared inside atransparent acrylic cylinder with an inner diameter of 50 mm by
the air pluviation method, using the same procedure as for theactual tested specimens, and was then observed through theacrylic cylinder. In Figure 2, grains of various sizes appear(c) Initial water content of 10%, w0=10%
Figure 2. Observed surface of specimens via optical microscopy.

tightly packed in the specimen with an initial water content of
0%. In the specimen with an initial water content of 5%, the fine
fraction is aggregated and dotted throughout in a particle-likemanner. In the specimen with an initial water content of 10%, asaggregation of the fine fraction progressed, aggregates
containing a large amount of water become attached to large sandparticles, and the gaps between the particles become larger. Thisshows that the soil structure of the specimen formed by the wet
tamping or air pluviation changes dramatically as a result ofvarying the initial water content. Figure 2 also shows that thehigher the initial water content, the larger the apparent gaps
between particles, and therefore, it can be inferred intuitively thatthe particles are difficult to pack together in this case.
Consequently, the higher the initial water content, the greater thenumber of times the specimen needed to be tamped down and thegreater the amount of compaction energy required during thetamping in order to achieve the same dry density.
We have already confirmed by observation of the otherspecimens after fully saturation that the soil structures observedin the unsaturated state, such as those in Figure 2, were preserved
after the specimens were fully saturated. Furthermore, to explain
the notable differences in mechanical behavior of the specimens

Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

200

150kPa100kPa

150
100
50
0
0

5

εa (%)

10

15

250

200
150
150kPa100

100kPa

50

50kPa

300

w0=5%

250

q (kPa)q (kPa)

250

300

w0=0%

q (kPa)

300

0
0

5

50kPa

εa (%)

200

150kPa50kPa

150

100kPa_2

100
50

10

w0=10%

100kPa_1

0
0

15

5

εa (%)

1015(a) Deviator stress - axial strain relationships

250

200

q (kPa)q (kPa)

250

300

w0=0%

150
100
50
0
0

50kPa
100kPa150kPa100

200

p' (kPa)

250

200
150
100
50

300

300

w0=5%

0
0

q (kPa)300

50kPa
100kPa150kPa100

200

w0=10%

200
100kPa_2
100kPa_1

150
100

50kPa

50
300

p' (kPa)

00

150kPa

100

200

300

p' (kPa)(b) Effective stress paths
Figure 3. Test results of triaxial test under monotonic loading condition.

in the test results discussed later, it is appropriate to consider thatthese soil structures were present even after the specimens were
fully saturated.
In this study, consolidated undrained shear tests wereconducted on the specimens. Monotonic and cyclic loading was
performed in the triaxial test. In the monotonic loading test, threedifferent initial effective confining pressures of 50, 100, and 150
kPa were used, as indicated in Table 1, and the specimens weresheared at an axial strain rate of 0.1%/min. In the cyclic loading
test, the initial effective confining pressure was 100 kPa, and thespecimens were loaded with three different deviator stressamplitudes at a frequency 0.1 Hz.
3

TRIAXIAL TEST RESULTS

Figure 3 shows the results of the triaxial test under monotonic
loading for the specimens prepared at the three different initialwater contents. The top row shows the relationship between
deviator stress q=1-3 and axial strain a, while the bottom row
shows the effective stress path diagrams, i.e. the relationshipbetween q and mean effective stress p’. The left-hand side, center,and right-hand side show the test results for specimens with 0%,5%, and 10% initial water content, in that order.
Focusing on the effective stress path in the case of an initialwater content of 0%, the specimens undergo rapid plastic
compression and reach the phase transformation state in the earlystage of shearing. Subsequently, deviator stress increases greatly
until the end of shearing due to the confinement of positive
dilatancy. At an initial water content of 5%, the specimens
exhibit plastic compression similar to the 0% specimens, butshow relatively high stiffness compared to the 0% specimens
until the peak deviator stress. However, the specimens abruptly
demonstrate softening behavior after the peak and show a final
state similar to static liquefaction toward the origin. At an initialwater content of 10%, the effective stress paths are similar to anelastic response, rising vertically in the early stage of shearing
compared to the 0% and 5% specimens, and the specimens reachextremely large deviator stresses. However, after reaching a peak,the specimens show rapid strain softening and undergo notablebrittle collapse.
As the initial water content increases from 0% to 5% to 10%,an elastic response in the early stage of shearing becomes clearly
apparent and brittle strain softening after the peak load also
becomes pronounced. This is considered to be because matrix
suction according the initial water content during the specimen
preparation causes the coarse and fine fractions to form a “strong”
structure. This kind of structure does not form in the specimens
with an initial water content of 0%, and those specimens
therefore demonstrate significant plastic compression from theearly stage of shearing, despite having the smallest void ratio.
However, after phase transformation, the specimens with an
initial water content of 0% exhibit significant heavy
overconsolidation characteristics due to the confinement of
positive dilatancy, and then deviator stress considerable
increases. This increase in deviator stress occurs as a result offully undrained conditions that do not typically materialize under
the natural condition, and it is important to note that it does notdirectly indicate that strength of the soil is large.
Based on the above, the more developed the soil structure in
the sandy soil, the more the soil is characterized by a large elastic
response and a high shear resistance in the small strain range, andrapid brittle failure occurs with the collapse of the soil structure.
Figure 4 shows the results of the triaxial test under cyclicloading. The top row shows the relationship between deviator
stress and axial strain, while the bottom row shows the effective
stress path diagrams. The left-hand side, center, and right-handside show the test results for specimens with 0%, 5%, and 10%
initial water content, in that order. These diagrams show only thetest results corresponding to a cyclic stress amplitude ratio of
0.15 at a maximum deviator stress of 30 kPa for each of thespecimens.
At an initial water content of 5% and 10%, the decrease ineffective stress in each cyclic loading is small and the axial strain
is also extremely small. However, as soon as the effective stressapproaches zero and liquefaction occurs, a large axial strain isProceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

50

50

w0=0%

q (kPa)q (kPa)

0

0
-25

-25
-50
-20

-10

0

10

0
-25

-50
-20

20

w0=10%

25

25

25

q (kPa)

50

w0=5%

-10

0

10

-50
-20

20

-10

εa(%)

εa(%)

0

10

20

εa(%)

(a) Deviator stress - axial strain relationships

50

w0=0%

0

-25

0

50

-50
0

100

0

-25

-25

-50
0

w0=10%

25

25

q (kPa)q (kPa)

25

50

w0=5%

q (kPa)

50

50

-50
0

100

50

p' (kPa)p' (kPa)100

p' (kPa)(b) Effective stress paths
Figure 4. Test results of triaxial test under cyclic loading condition.

Stress amplitude ratio

0.25

w0=0%
w0=5%
w0=10%

0.20
0.15
0.10
0.05
0.00
1

disturbance of soil is closely related to soil structure in sandy
soils. If we clarified the formation mechanism of the soil
structure and artificially reproduced the soil structure by
regarding disturbance as a deterioration in the soil structure, itwould be possible to obtain the high quality test results fromreconstituted specimens using disturbed samples collected using
an inexpensive method.
4

10

100

1000

Number of cycles
Figure 5. Liquefaction strength curves

observed. This is the same as the tendency for brittle failure inmonotonic loading. At an initial water content of 0%, thedecrease in effective stress in each cyclic loading is large fromthe early stage of shearing, and a relatively large axial strain is
produced from the loading onset.
Figure 5 shows the liquefaction strength curves including thetest results at other stress amplitude ratios. Due to the differences
in initial behavior during cyclic loading, the liquefaction strengthof specimens prepared with an initial water content of 0%, i.e.
dry sand, is clearly low compared to specimens prepared with an
initial water content of 5% and 10%. In other words, specimensthat form soil structures have higher liquefaction strengths.
In the past, the difference in liquefaction strength between
naturally deposited sand that was undisturbed and sand that had
been reconstituted at the in situ density was discussed based on
the level of disturbance (Tokimatsu et al. 1986b, Kiyota et al.
2009). As a result, the common understanding is that it is difficult
to accurately predict liquefaction strength in field without
performing liquefaction tests using high-quality undisturbed
frozen samples. This test results in this study suggests that

SIMPLE SHEAR TEST RESULTS

Table 2 indicates the specifications of specimens used in thesimple shear test. As in the triaxial test, specimens were preparedwith three different initial water contents. However, thespecimens in the simple shear test were cylinders of 60 mm indiameter and 30 mm in height, and they were prepared in threelayers. After the specimens had been fully saturated using adouble vacuum method, they were consolidated isotropically to
an initial effective stress of 100 kPa. Then, a simple shear test
under undrained condition was performed by loading
monotonically the upper pedestal horizontally at a constantdisplacement rate with the lower pedestal still in place.
Table 2. Specifications of each specimen used in the simple shear test.
Test
conditions
Monotonic
loading

Initial watercontent, w0[%]

Initial effective
confining pressure
[kPa]

0
5
10

Void ratios after
consolidation
0.637

100

0.636
0.636

Figure 6 shows the effective stress paths and shear stress–shear
strain relationships obtained from the simple shear test. The kindof strain softening observed in the triaxial test results is not found,but for all of the specimens regardless of initial water content,

150

150

100

100

τ (kPa)τ (kPa)Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

w=10%

50

w=5%

5

γ (%)

10

w=5%

w=0%

w=0%

0
0

w=10%

50

0
0

15

50

100

150

p' (kPa)(a) Shear stress - shear strain relationships(b) Effective stress paths

Figure 6. Simple shear test results.

200

Simple shere test (SS)
Triaxial test (TX)

Deviatric stress (kPa)

150

w=10% (TX)
w=0%(TX)
w=10% (SS)
100 w=5%(SS)
w=0%(SS)
w=5%(TX)

50
0
0

5

10

15

Deviatric strain (%)

Simple shere test (SS)
Triaxial test (TX)

150

w=10% (TX)
w=0%(TX)

100

0
0

20

50

Deviatric stress (kPa)

Deviatric stress (kPa)200

Simple shere test (SS)
Triaxial test (TX)

150
100 w=0% (TX)5000w=0% (SS)

50

100

150

200

w=5% (TX)
w=5% (SS)

50

100

150

200

p' (kPa)p' (kPa)(b)-1 Effective stress paths (w0=0%)

150

200

Simple shere test (SS)
Triaxial test (TX)

100

0
0

100

200

(b) Effective stress paths

150

50

w=5%(SS)
w=0%(SS)p' (kPa)(a) Deviatoric stress - deviatoric strain relationships

200

w=10% (SS)

w=5%(TX)
50(b)-2 Effective stress paths (w0=5%)

Deviatric stress (kPa)

Deviatric stress (kPa)200

Simple shere test (SS)
Triaxial test (TX)

150
100

w=10% (TX)

w=10% (SS)

50
0
0

50

100

150

200

p' (kPa)(b)-3 Effective stress paths (w0=10%)

Figure 7. Results of triaxial test and simple shear test

the behavior before phase transformation resembles that in thetriaxial test. Specifically, the higher the initial water content of
the specimen, the more the effective stress path rises vertically inthe early stage of shearing, and these specimens can beconsidered to be highly structured.
Because the shear modes differ greatly in the triaxial andsimple shear tests, the results of both tests are required tocompare using the same parameters. That is, the results arereorganized as shown in Figure 7 by calculating both the second
invariant of the deviatoric stress tensor, here called “deviatoricstress” for convenience, for vertical axis and the second invariantof the deviatoric strain tensor, likewise called “deviatoric strain”,
for horizontal axis. Looking at the effective stress paths in Figure
7, in addition to the phase transformation lines roughly
coinciding, the phase transformation points also coincide to asurprising extent in the specimens with initial water contents of

0% and 5%, despite the different shear modes in the triaxial andsimple shear tests. In the specimens with an initial water content
of 10%, the fact that the effective stress path rises vertically andthe specimen exhibits elastic behavior in the early stage ofshearing is the same, but the maximum shear stress is far larger
in the triaxial test. This test result for specimens with an initialwater content of 10% is consistent with the finding that theremarkable strain softening of sensitive clays with notable soil
structures is a phenomenon observed only in triaxial tests(Kodaka et al. 2011). Additionally, at all initial water contents,
the effective stress path after phase transformation differs
considerably in both tests. Specifically, in the triaxial test, adegree of strain softening is apparent in specimens possessing
notable soil structures with initial water contents of 5% and 10%,
while a degree of strain hardening is apparent in specimens
possessing poor soil structures with an initial water content ofProceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021

0%. Because the shear modes differ greatly, it is thought thatmajor differences appear in the shear stress, which is a response
value, due to a loss in homogeneity in the specimens, particularlyin the latter half of shearing. At present, it is not possible to draw
a simple conclusion as to which test is appropriate.
5 CONCLUSIONS
Specimens with different soil structures were prepared byvarying the initial water content during their preparation, andtriaxial and simple shear tests were conducted. The results werecompared, focusing on the effective stress paths. In the triaxial
test, it was shown that the shear behavior differed greatly, under
not only for monotonic loading but also for cyclic loading.
Because the matrix suction that occurs between soil particles
varies depending on the water content during specimen
preparation, it is thought that as the water content increased, thefine fraction coagulated around the coarse particles and formed asoil structure. This was confirmed by optical microscopy.
Additionally, it was inferred from the results of the cyclic loading
test that this soil structure and the sample disturbance are closely
related, and this suggests that it is possible to prepare artificial
quasi undisturbed samples by understanding the soil structures ofnatural sand deposits. Further, it was shown that despite the shear
modes differing greatly in the triaxial test and the simple shear
test, the paths up until phase transformation are relatively
consistent. The phenomenon of high maximum shear stress andsubsequent large strain softening observed in the specimens withan initial water content of 10%, which had superior soil structures,
was observed in the triaxial test only.
6 ACKNOWLEDGEMENTS
This research was partially supported by JSPS KAKENHI Grant
Number JP16H04412. The authors would also like to express
their sincere thanks to a former graduate student of Meijo
University, Mr. Shota Mitarai for his help with the experimental
work.
7

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disturbance on small strain characteristics and liquefaction
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Kodaka T., Itabashi K., Miyoshi N., Fukuzawa H. and Yoshida S. 2011.
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Kodaka T., Cui Y., Mori S., Kanematsu Y. and Lee K. 2013. Soil
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Tokimatsu K., Yamazaki T. and Yoshimi Y. 1986a. Soil liquefaction
evaluations by elastic shear moduli. Soils and Foundations 26(1),25-35.
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