Effect of Sample Preparation Method on Liquefaction of Sandy Soil

Effect of Sample Preparation Method on
Liquefaction of Sandy Soil
S. R. Pathak
Professor & Head, College of Engg. Pune, Shivajinager, Pune, India
e-mail: [email protected]
R. S. Dalvi
Assistant Professor, College of Engg. Pune, Shivajinager, Pune, India
e-mail: [email protected]
ABSTRACT
An experimental investigation was carried out on clean sandy soil to gauge its liquefaction
potential with different initial density (30%, 40%, 50%, 60% and 70%) and confining pressures
(60kPa, 120kPa, and 240kPa). Tests were performed using Triaxial apparatus. Four different
methods of sample preparation namely IS Code, wet tamping, moist placement and dry deposition
methods were employed to test forty-five samples revealing considerable differences in undrained
shearing responses produced under identical conditions of density and confining pressure. Samples
prepared by IS code method have a greater resistance to liquefaction while those by moist
placement and dry deposition have been found to be potentially liquefiable. Further, liquefaction
resistance increases with increase in initial relative density and confining pressure for IS code, wet
tamping method and moist placement method of sample preparation. Unstable zone is identified as
the region that lies between the confines of effective stress failure line and the peak pore pressure
line which is clearly seen in IS code and wet tamping methods, whereas in case of dry deposition
and moist placement method effective stress failure line and peak pore pressure line coincide each
other.
KEYWORDS: sandy soil, liquefaction, density, confining pressure, instability
INTRODUCTION
Liquefaction of loose sand layers occurs due to sudden increase of pore pressures and
corresponding decrease in the effective overburden pressure. Soil liquefaction and ground failures
are commonly associated with large earthquakes. The soil loses its strength and behaves like
liquid during dynamic loading. This natural phenomenon has been responsible for many damages
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in the world such as, 1964 earthquakes in Niigata, Japan and Alaska, Loma Prieta 1989, Kobe
1995 etc. Literature survey reveals that the main approaches for assessing liquefaction potential
are based on analytical models as well as some experimental investigations. Experimental work
has been carried out mainly using cyclic triaxial test in addition to few lab tests such as simple
triaxial test, shake table test, shear wave velocity test etc.
Majority of laboratory tests on granular soils, such as clean sand and gravels are performed
on reconstituted specimens because obtaining samples of these materials in their undisturbed state
or natural state is very difficult due to lack of ‘cohesion’. Various sample preparation methods
have been developed based on moisture condition of the soil (eg. dry, moist and wet), the method
of soil placement (e.g. pluviation, spooning or flowing ) and medium through which the soil is
placed (e.g. air or water). Various studies have reported that liquefaction resistance of soil is
greatly influenced by different sample preparation methods (Vaid et al.,1999, Ishihara 1993,
Wood and Yamamuro 1999, Della 2009). According to Vaid et al. (1999), Kuerbis and Vaid
(1988) water pluviation method is one of the popular methods of sample preparation. Pluviation
in water has been shown to resemble the alluvial deposition process because the fabric that ensues
upon water pluviation has been found to be similar to that of the naturally deposited alluvial and
hydraulic fill sands. Also, resistance to liquefaction is higher in water pluviation method than dry
deposition and moist placement method. Kramer and Seed (1988) also observed that static
liquefaction resistance increased with increasing relative density and confining pressure.
Since different methods of sample preparation exist (Papadimitriou et al. 2005,Wood et al.
1999 ), four basic methods of sample preparation have been employed in present work to study
their effect on liquefaction resistance of sandy soil. Each of the sample preparation methods
produce different initial structure of soil thereby changes liquefaction behavior of soil. A detailed
laboratory investigation has been presented in the subsequent sections.
EXPERIMENTAL WORK
Forty five undrained triaxial tests were carried out on clean sand. Four sample preparation
methods were used to prepare the sample namely IS code method, wet tamping method, moist
placement method and dry deposition method.
Material tested
All tests carried out in this study have been performed on clean sand, characterized by the
physical properties as summarized in Table.1. The clean sand is classified as uniformly graded
sand as per I.S. classification. The grain size distribution is as shown in Figure 1. The tests are
conducted for different initial relative densities 30%, 40%, 50%, 60% and 70% representing
loose, medium and dense conditions and for three confining pressures of 60 kPa, 120 kPa and 240
kPa.
Table 1: Properties of sand
emax
emin
0.90
0.62
γmax
kN/m3
16.65
γmin
kN/m3
14.20
Gs
2.76
D50
(mm)
0.30
D10
(mm)
0.17
D60
(mm)
0.32
D30
(mm)
0.24
Cu
Cc
1.82
1.02
Fines (% )
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100
90
80
70
60
50
40
30
20
10
0
0.01
1413
0.1
1
10
Figure 1: Grain size distribution curve
Sieve Sizes (mm)
Figure 1: Grain size Distribution curve
Test apparatus
The apparatus used to perform isotropically consolidated undrained triaxial compression
tests includes: Load frame (Motorized) 50kN capacity, triaxial cell stationary bushing, air water
constant pressure system (Capacity 7 Kg/cm2) ,oil water constant pressure system (Capacity
16Kg/cm2) and data acquisition system for recording load, pore pressure and displacement.
Method of sample preparation
Triaxial tests were performed on cylindrical specimens admeasuring 38 mm diameter by
76mm in height (H/D=2.0). Wet tamping and IS code [IS. 2720 (Part 12)-1981] methods were
used to prepare sand samples of initial relative density 30%,40%,50% and 70%. Dry deposition
and moist placement methods were used to prepare the samples of 30%, 40%, 50% and 60%.
IS code method is essentially similar to water sedimentation method put forth by Ishihara
(1993). The sample was prepared by continuous rapid flow of soil in the membrane of triaxial cell
filled with water. Wet tamping method used in the present work is different from the procedure
laid down by Ishihara (1993) in respect of amount of water added into the soil while preparing
the specimen. In this method, a known quantity of sand and water for the desired density was
mixed. The mixture was placed in five layers by tamping each layer with the help of a hammer.
Both these methods have already been discussed in detail in Pathak and Dalvi (2011).
In dry deposition method quantity of dry sand for particular density was placed in the split
mould in five equal layers (Figure 2). After each layer tamping was done with the help of
specially designed hammer. Calibration was done for number of blows required to achieve the
required specimen height. In moist placement method known quantity of sand corresponding to
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particular density and 5 % water content by weight were mixed properly to the desired density
and mould was filled in 4 to 5 layers and tamped to the desired density.
Figure 2: Photograph of dry deposition method
TEST PROCEDURE
After the specimens were prepared, porous stone and loading pad were placed and sealed
with O-rings. Negative pressure of 25 kPa was applied to the specimens to reduce disturbance
during removal of split mould and triaxial cell installation. When the cell was filled with water
the negative pressure was removed. The confining pressure of 50kPa was applied to the
specimens. In case of dry deposition and moist placement method saturation of the specimens
was accomplished by flushing with carbon dioxide for 2-3 minutes after which water was slowly
percolated through the specimen from the bottom. ‘B’ value of at least 0.97 was achieved for all
the specimens. The cell pressure was then slowly increased to provide the desired effective
confining pressure. Each sample was isotropically consolidated and loaded at the same axial
strain rate of 1.2mm/min. For each of the effective confining pressure values, readings of load,
deformation and pore pressure were recorded using data acquisition system during the tests.
The same test procedure was used for conducting tests on various samples prepared by all
sample preparation methods. The detailed test program is as given in Table 2.
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Table 2: Test Program
Sr.No.
1
Method of sample preparation
IS code method (IS)
2
Wet tamping method (WT)
3
Dry deposition method (DD)
4
Moist placement method (MP)
Relative density (%)
30
40
60
70
30
40
60
70
30
40
50
60
30
40
50
Confining Pressures (kPa)
60, 110, 220
60, 130, 200
60, 120, 240
70, 140, 240
60, 100, 210
60, 100, 210
60, 110, 210
60, 110, 210
60, 120, 240
60, 130, 200
60, 120, 240
60, 120, 240
60, 120, 240
60, 120, 240
60, 120, 240
RESULTS AND DISCUSSION
The laboratory investigation in this paper shows the effect of sample preparation method on
liquefaction behavior of clean sand. The investigation included total 45 tests on sandy soil at
various relative densities and confining pressures. The effect of relative density and confining
pressure on liquefaction behavior of the soil in each of the methods of sample preparation has
been discussed with the help of graphs in the following sections.
Effect of confining pressure
Figures 3 (i), (ii) and (iii) display the results of deviator stress vs % strain for different
sample preparation methods at confining pressures of 60kPa,120kPa and 240kPa typically for
40% relative density. It is evident from these plots that as the confining pressure increases
deviator stress also increases. This is in consonance with the findings of Kramer and Seed (1988),
Yamamuro (1997) and Della et al. (2011).
All the methods of sample preparation in the present work primarily lead to similar trends,
but the maximum value of deviator stress is found to be more in IS code method than the
corresponding values in other three methods. Peak value of deviator stress was found to increase
from 150kPa to 600kPa for confining pressure of 60 kPa (Figure 3(i)), 200kPa to 850kPa for
confining pressure of 120 kPa (Figure 3(ii)) and 200 to 800kPa for confining pressure of 240 kPa
(Figure 3(iii)).
In case of higher confining pressures (120 and 240 kPa) clear peak has been observed for IS
code method and wet tamping method at 6 to 8% of strain. In dry deposition and moist placement
method, no specific peak deviator stress has been observed. At higher % strain (20%) steady state
has been reached particularly for moist placement method as observed in Figure 3(ii) and (iii). At
confining pressure of 240 kPa peak value of deviator stress has been observed to be nearly same
Vol. 16 [2011], Bund. P
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(800 kPa) for both IS code &wet tamping methods whereas for moist placement and dry
deposition methods the value observed is of the order of 200kPa. As peak deviator stress is
higher in IS and wet tamping method at higher confining pressure, liquefaction resistance offered
by sand is higher than moist placement and dry deposition method.
Deviator Stress VS %Strain
700
σ’3=60 kPa
Deviator Stress (kPa)
600
500
WT
400
IS
300
MP
DD
200
100
0
0
5
10
15
Strain %
Figure 3(i): Deviator stress vs % strain for RD=40%. (σ’3=60 kPa )
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Deviator Stress (kPa)
Deviator Stress vs % Strain
900
800
700
σ’3=120 kPa
WT
600
500
400
300
IS
MP
DD
200
100
0
0
10
20
30
Strain %
Figure 3(ii): Deviator stress vs % strain for RD=40%. (σ’3=120 kPa )
Deviator Stress VS % Strain
900
Deviator Stress (kPa)
800
σ’3=240 kPa
700
600
WT
500
IS
MP
400
DD
300
200
100
0
0
5
10
15
20
Strain %
Figure 3(iii): Deviator stress vs % strain for RD=40%.(σ3=240 kPa)
Vol. 16 [2011], Bund. P
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In the present work out of all four sample preparation methods IS code method has offered
higher resistance to liquefaction while lower resistance to liquefaction has been observed in moist
placement method. This may be attributed to the amount of water in sample preparation in IS
code being much more than that in moist placement method due to which samples are generally
more compressible during consolidation than moist placement method. Thus the different method
of sample preparation produces different initial fabric under a particular confining pressure.
Figure 4 shows effect of confining pressure on the deviator stress at failure for loose state of
soil (RD=30%). It is noticed that as the confining pressure increased, peak value of deviator stress
at failure is also increased for wet tamping method, IS code method and moist placement method.
Thus as effective confining pressure increases, increase in deviator stress is an indication of more
liquefaction resistance offered by soil or increase in load carrying capacity of soil. Similar
observation was reported by Della (2009).
Such behavior has not been observed for dry deposition method. This could be attributed to
different soil fabric developed as dry sand is used at the time of sample preparation. Also at
higher confining pressure soil sample is more disturbed than lower confining pressure thus may
be incapable of carrying load.
1200
Deviator S tress at Failure(kP a)
MP Method
WT Method
1000
I.S. Code Method
800
DD Method
600
400
200
0
0
50
100
150
200
250
300
Effective Confining Pressure(kPa)
Figure 4: Deviator stress at failure vs effective confining pressure.(RD=30%)
Effect of relative density
The effect of relative density on liquefaction resistance of sand has been shown by plotting
effective stress path for different initial relative densities. Figure 5(i) shows a typical p’-q plot
for samples prepared by moist placement method at different initial relative densities 30%, 40%
and 50% for confining pressure of 240 kPa. The effective mean stress p’ is defined as
Vol. 16 [2011], Bund. P
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(σ1’+σ3’)/3, while q is given by the difference ((σ1’-σ3’). In case of moist placement method
contractive behavior is observed for relative densities of 30% and 40%.However, for relative
density of 50% dilative behavior has been observed. As the density changes from loose state
towards dense state the resistance to liquefaction has been increased. Figure 5(ii) portrays a p’-q
plot with IS code method of sample preparation typically for 240kPa.It is seen that none of the
samples indicate contractive nature, implying that identical samples with relative density 30%
when prepared with IS code method show the contrast behavior as compared with their
counterparts when the moist placement method is employed for sample preparation. Such dilative
nature in the water deposited state (under IS code method) may be ascribed to soil grains rolling
down into stable positions at lower densities: cf. Terzaghi and Peck (1967) and Been et al.
(1988).For wet tamping method contractive nature has been seen for loose state of soil
(RD=30%) with lower value of confining pressure (60kPa).In dry deposition method contractive
behavior has been observed for 30%, 40% and 50% relative densities (Figure 5(iii)). Thus
samples prepared by IS code method showed dilative behavior for all relative densities for all
confining pressures. For moist placement and dry deposition method as relative density increases
from 30% to 60% soil changes its behavior from contractive to dilative.
Moist Placement Method
400
σ3’=240kPa
350
q (kPa)
300
RD=30%
250
RD=40%
200
RD=50%
150
100
50
0
0
100
200
300
p'(kPa)
Figure 5(i): p′-q plot for moist placement method.
Vol. 16 [2011], Bund. P
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IS Method
q (kPa)
900
800
σ3’=240kPa
700
600
RD=30%
500
400
RD=40%
300
200
RD=70%
RD=60%
100
0
0
200
400
600
800
p' (kPa)
Figure 5(ii): p′-q plot for IS code method
Dry Deposition Method
300
250
σ3’=240kPa
q (kPa)
200
RD=30%
150
RD=40%
RD=60%
100
50
0
0
100
200
300
p' (kPa)
Figure 5(iii): p′-q plot for dry deposition method.
The values of deviator stress at failure for effective confining pressure 240kPa have been
plotted for initial relative densities 30, 40, 50, 60 and 70% in Figure 6.It is observed that as the
relative density increased, peak value of deviator stress at failure also increased for all sample
preparation methods. Similarly, maximum value of deviator stress was reached in case of I.S code
method. Samples prepared by dry deposition and moist placement method show nearly the same
Vol. 16 [2011], Bund. P
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values of peak deviator stress. Thus as the relative densities increase, increased deviator stress
values signify more resistance to liquefaction offered by the soil for all sample preparation
methods. Kramer and Seed (1988) observed similar behavior for sample prepared using moist
placement method.
Deviator Stress at Failure (kPa)
1200
MP Method
WT Method
1000
I.S Code Method
800
DD Method
600
σ3’=240 kPa
400
200
0
0
20
40
60
80
Relative Density (%)
Figure 6: Deviator stress vs relative density
Effect of sample preparation method
The effect of sample preparation methods on effective stress path typically for 30% relative
density and 120 kPa confining pressure has been shown in Figure 7. It is observed that sample
prepared by IS code method and wet tamping method is more dense which shows dilative
behavior because of amount of water at the time of sample preparation and rolling of soil grains
into stable position. While sample prepared by moist placement method and dry deposition
method shows contractive behavior. This could be because soil structure developed in these two
methods has been comparatively loose than wet tamping and IS code method due to which higher
pore pressure has been developed in the sample prepared.
These differences in the behavior noted among the four methods of deposition, can be
explained by the fact that the molecules of water contained in the structures prepared by moist
placement method constitute some macrospores easily compressible at the time of the shearing of
the sample and at the same time prevent the grain-grain adhesion from which the tendency of
sample is to contract. This trend accelerates the instability of the samples which show a very
weak resistance and even provokes the phenomena of liquefaction of the sand for lower densities
and low confinement leading to the collapse of the sample.
Vol. 16 [2011], Bund. P
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Effective Stress path (RD=30%)
σ3’=120 kPa
350
300
q (kPa)
250
WT
200
IS
150
DD
MP
100
50
0
0
50
100
150
200
p' (kPa)
Figure 7: Effect of sample preparation method
(Relative density =30% and confining pressure 120kPa)
COMPARISON WITH PREVIOUS
RESEARCH STUDIES
Figures 8 (i) and (ii) show deviator stress vs % strain and excess pore pressure vs % strain
respectively for relative density 40% and for confining pressure 240kPa. On the same graph
results of Seed et al. (1988) and Lee (1965) have also been plotted. Seed et al. (1988) used moist
placement method with 6-8% water content. While Lee (1965) conducted undrained test on soil
sample of relative density 38%.It can be seen from both these figures that though the trend of
variation is same especially for moist placement and dry deposition method, the values from the
present research work are higher than those of the previous researchers. This could be because of
the smaller sample size (38mm dia.) used in present work as compared with Seed et al. (1988)
and higher effective size (D50)of soil used (0.3mm) than that used by earlier researchers.
Vol. 16 [2011], Bund. P
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Deviator Stress VS % Strain (RD=40% )
σ3’=240 kPa
Deviator Stress (kPa)
900
800
700
WT
600
IS
500
MP
400
DD
300
Seed at al (1988)
200
Lee (1965)
100
0
0
5
10
15
20
25
% Strain
Figure 8(i): Deviator Stress vs % Strain (RD=40%)
Excess Pore Pressure Vs % Strain (40%)
Excess Pore Pressure (kPa)
200
150
σ3’=240 kPa
100
WT
50
IS
0
MP
0
10
20
-50
30
DD
Seed et al (1988)
MP
-100
-150
Strain %
Figure 8(ii): Excess Pore Pressure vs % Strain.(RD=40%)
Vol. 16 [2011], Bund. P
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UNSTABLE ZONE
Figures 9 (i) and (ii) illustrate effective stress paths for the moist placement method and wet
tamping method, respectively, for all confining pressures typically with the relative density of
40%. In the present work the Mohr Circle at failure for each of the specimens was plotted and
from the maximum shear stress value for each of Mohr Circle the p′-q values were deduced,
which were then plotted on the p′-q diagram. The straight line joining these points with the origin
is thus appropriately termed as the Effective Stress Failure Line, delineated in Figs. 9 (i) and (ii)
as Kf line. Corresponding to each peak pore pressure points recorded during the tests, p′-q values
obtained are plotted to draw the line passing through origin, which represents the peak pore
pressure line. Thus the zone between the effective stress failure line and peak pore pressure line
can be regarded as Unstable. Unstable zone in the present study is different than the instability
zone proposed by Chu et al. (2003). It is seen from the figures that the unstable zone has been
clearly identified in wet tamping method while in case of moist placement method peak pore
pressure line and effective stress failure line are identical. This may be because large pore
pressure has been developed in moist placement and dry deposition method (due to initial fabric
of sample) thereby initiating liquefaction and ultimately failure of soil. However, in IS code and
wet tamping method comparatively lesser pore pressure was developed the rearrangement of the
soil particle could have taken place in unstable zone and thus dilation of soil was observed instead
of liquefaction. It is evident from these observations that the zone between Kf line and peak pore
pressure line termed as unstable zone can be effectively used in classifying the liquefaction
behavior of soil. It could be stated that the soil samples for which Kf line and peak pore pressure
line coincide each other are contractive in nature and those wherein this unstable zone is clearly
identifiable show dilative behavior when tested in triaxial test. This behavior moreover depends
upon method of sample preparation.
Figure 9(i): p′-q plot for moist placement method (RD=40%)
Vol. 16 [2011], Bund. P
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Figure 9(ii): p′-q plot for wet tamping method (RD=40%)
CONCLUSION
Consolidated-undrained triaxial tests conducted on clean sand in the present experimental
work show dilative behavior for all relative densities under the IS code method of sample
preparation as used in this study. However, samples prepared by moist placement and dry
deposition method for 30% and 40% relative densities have shown contractive behavior. For all
the samples tested in this work deviator stress is found to increase with % strain irrespective of
method of sample preparation. However, peak value of deviator stress is higher in IS code
method as compared with that of other sample preparation methods. Thus, method of sample
preparation plays crucial role in assessing liquefaction behavior of clean sand when tested in
undrained triaxial test. It is noticed that liquefaction resistance increases with relative density and
confining pressure for all methods of sample preparation. The steady state was achieved and
maintained over the greater strain range (20%) for dry deposition and moist placement method.
Similarly, higher pore pressure has been developed in dry deposition method and moist placement
method. Unstable zone could be clearly identified in case of IS code and wet tamping method
indicating dilative behavior whereas it was not identifiable in case of dry deposition and moist
placement method, thus showing contractive behavior.
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