analysis of the improvement effect of expansive soil through the soil-water characteristic curve
TRANSCRIPT
Journal of Highway and Transportation Research and Development Vol. 6 ,No. 4(2012 )036
Analysis of the Improvement Effect of Expansive Soil through the
Soil-water Characteristic Curve *
YANG Heping(i%fIJ¥-) * * ,LIN Liping(;f;fc88�) ,XIAO Jie( �?tt) ,ZHAN WentaoOl)(�)
( Research Institute of Special Soil Engineering, Changsha University of Science and Technology, Changsha Hunan 410004, China)
Abstract: This study aims to establish the relationship between the theory of unsaturated soil and the corresponding engineering
problem by using the soil-water characteristic curve to determine the improvement effect of expansive soil. The contents of two
improvers, i. e. , lime and fiy ash, were added into Baise expansive soil for free expansion ratio testing. Based on the optimum
moisture contents determined by using the modified Proctor compaction method, testing specimens of each improved soil sample
were obtained. By dewetting the soil-water characteristic curve testing in the range of 5 kPa to 1 000 kPa by using a pressure
plate apparatus, the expansion properties and soil-water characteristic curves of the improved expansive soils were determined.
The effect of different improvers and mixing contents on the soil-water characteristic curves, which were fitted by the variance
gamma model, was analyzed based on the theory of unsaturated soil and on the national determination criterion for expansive
soils. Saturated gravity water content, air entry value, residual water content, and curve slope were found to have potential for
use in evaluating the chemical improvement effect on expansive soils. Further analysis shows that the improvement effect can be
determined if three of the four indexes of expansive soil meet the requirements. However, saturated gravity water content is in
dispensable, but any two of the remaining three indexes can be considered.
Key words: road engineering; unsaturated soil; soil-water characteristic curve; expansive soil; chemical improvement; evalua
tion of effect
1 Introduction
Considerable attention has been accorded to the is
sue of how expansive soil can be rationally utilized to re
duce its destructiveness to infrastructure and the environ
ment. The common method for treating expansive soil
continues to be chemical improvement, with lime, fly
ash, cement, etc. , used as modifiers. Improvement
effects are evaluated by using the swell-shrink in
dex[I-4].
Unsaturated soil remams a research hotspot in the
field of geotechnical engineering. The soil-water charac
teristic curve ( SWCC) of unsaturated soil has signifi
cance in explaining and predicting the engineering char
acteristics of unsaturated soil, including seepage flow,
strength, and volume change [5 -7]. However, most re
searchers focus on theory. Thus, only few findings have
been implemented in actual projects. reference [8] out
lines the application of SWCC in determining the lower
limit of suitable soil water in crop non-sufficient irriga-
Manuscript received March 12, 2012
tion. SWCC was applied to landslide forecasting in refer
ence [9]. To establish the relationship between unsatu
rated soil behavior and corresponding engineering prob
lems, SWCC is used to discuss the improvement effects
on expansive soil (as sub grade filler) . In this paper, va
rious modifiers at different concentrations were added to
expansive soil. The optimum water contents and maxi
mum dry density of testing specimens were determined by
using the modified compaction method. Dewetting SWCC
tests were conducted by using a pressure plate apparatus
to analyze variations in the four indexes of SWCc. Ac
cording to the free expansion ratio, the evaluation indexes
and the values of improvement effects on expansive soil
were established based on the unsaturated soil theory.
2 Basic experiment
2.1 Main technique theroy
The SWCC has recently been thought to have poten
tial for use in analyzing the soil improvement effect.
However, when the determination of whether expansive
, Supported by the National Nataral Science Foundation of China e No. 50978035 ) ; and the Road and Transpont R & D Project for
Western Regions of China Commissioned by MOC e No. 200831800089 )
• • E-mail address:[email protected]
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YANG Heping,et al : Analysis of the Improvement Effect of Expansive Soil through the Soil- water Characteristic Curve 37
soil doped with lime achieves modification cannot be
evaluated through the analysis of SWCC, the contents of
improved soil can be identified with the use of an existing
assessment criterion, which is the free expansion ratio,
Based on this ratio, the change in the main parameters of
the SWCC of the improved soil induced by the increased
concentrations of ameliorants can be analyzed, and the
optimal materials and conditions for the test can be indi
rectly determined theoretically, Existing research high
lights that initial water content, dry density, and com
paction energy are the primary influencing factors of the
linear shape of the SWCC [!OJ, In engineering practice,
the requirements of subgrade compaction are satisfied by
controlling optimum moisture content and maximum dry
density. The modified SWCC test can be performed by
modification, but also agrees with actual working condi
tions. In this study, relevant experiments on improved
soils involving the compactions, SWCC, and free expan
sion ratio tests were conducted to determine the optimum
water content and maximum dry density.
2. 2 Test material, method and sample preparation
Baise expansive soil was used as test soil. Liuyang
Jiuxi quicklime and Yueyang Power Plant fly ash were
used as modifiers. The main properties of the materials
are shown in tables 1 to 3. In this paper, the modifier is
uniformly mixed with the expansive soil, and blending
dose refers to the mass ratio of former to the air-dried ex
pansive soil. Based on previous studies [II J, considering
the decrease in the expansive characteristic of the soil
and the quantity and period of the tests, the dosage of
preparing specimens using the aforementioned compac- modifier can be determined as follows: mixing contents of
tion method, which not only guarantees the reliability of quicklime are 0% , 3 % , 5 %, and 7 %, whereas those
the test results in terms of evaluating the findings on of fly ash are 0% , 5 % , 10% , and 15 % .
Tab. 1 Physical properties of Baise expansive soil
Specific gravity
2. 75
Liquid limit ( % )
58.2
Plastic limit ( % )
24.2
Plasticity Free expansive index ( % ) ratio ( % )
34 63
Tab. 2 Chemical properties of lime (unit: %)
Name Content of CaO and MgO Content of CaO Grade
Quicklime 76 73.5 Calcium level 3
55.1
Tab. 3 Chemical properties fly -ash (unit: % )
Content of active ingredient
4.65 23.0 7
CaO
5.89
MgO
4.93
Loss on ignition
1. 02 4.84
Free expansion ratio was measured according to the
Test Methods of Soils for Highway Engineering (JTG
E40-2007) [IZJ. The compaction test index refers to the
Test Methods of Soils for Highway Engineering (JTG E
40-2007) . SWCC tests were performed by using a
pressure plate produced by the Soil Moisture Company,
United States. The gravity water content of the samples
was tested at suctions of 5, 10, 25 , 50, 100, 200, 400,
600, 800, and 1 000 kPa.
2.3 Test results
2.3. 1 Compaction and free expansion ratio tests
Table 4 shows the results of compaction test, where
as compaction curves are shown in figure 1.
Linear Volume Particle size distribution ( mm ) ( % )
contraction ( % ) contraction ( % ) <0.002 0.002 - 0.0 75 >0.0 75
6.0 19.00 46.2 4 7.6 6.2
Tab. 4 Results of the compaction test
Content of lime ( % ) 0 3 5 7
Optimum water content ( % ) 14. 7 14.1 11. 7 11. 2
Maximum dry density ( glcm ) 1. 90 1. 88 1. 86 1. 85
Content of fly ash ( % ) 0 5 10 15
Optimum water content ( % ) 14. 7 12.2 11. 2 8.4
Maximum dry density ( glcm ) 1. 90 1. 89 1. 81 1. 80
-+- Prue soil ... 10% fly ash i::: �. 5� flyaSh
� 1.78 �\ -+- Prue soil ... 3% lime � 1.92 ___ 5% lime ___ 7% lime
�1.88 � .� 1.84 � 1.80 g 1.76
8 10 12 14 16 18 C 1.74 ��-��-��
20 Ci 5 8 11 14 1 7 20 Water content (%)
(a) Lime-improved soil
Water content (%) (b) Fly ash-improved soil
Fig. 1 Compaction curves
An increase in modifier content evidently facilitates
a decrease in the maximum dry density of improved soil.
This finding can be attributed to a number of reasons, in
cluding the exchange of Caz + with Na + , the presence of
K + in soil, the clay of soil agglomerates, and the genera
tion of hydration silicate calcium and calcium aluminate
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38 Journal of Highway and Transportation Research and Development
gel. The dispersed soil particles overlap and fill in the
pores for form a space-frame structure, which the com
paction test aims to destroy. Meanwhile, maximum dry
densities and optimum water contents are reduced with
increasing modifier content. This reduction may be at
tributed to the short chemical reaction time ( only 24 h ) ,
which is insufficient for improved soil. This finding a
grees with past test results [II]. The free swelling test re
sults of the soil samples are shown in table 2.
The free expansion ratio of improved soil evidently
decreases with increasing modifier content. Therefore,
soil expansion can be partly or even completely elimina
ted by increasing modifier content. In the national stand
ard Construction in Expansive Soil Area Specifications
(GBJl12-87) [13]
as well as in the literature [13-14],
free expansion ( < 40%) is used as a determinant for ex
pansive soils. Table 5 demonstrates that expansive soil
can be improved as a non-expansive soil after mixing with
a lime content of 5% or fly ash content of 15% .
Tab. 5 Test result of free expansion ratio (unit: %)
Content of lime
Free expansion ratio
Content of fly ash
Free expansion rat.io
2.3. 2 SWCC test
o 63
o 63
3
51
5
55
5
34
10
47
7
25
15
38
The results of the SWCC test for improved soil with
different contents of lime and fly ash are shown in table
6. For analysis and comparison, figures 2 and 3 show the
SWCCs of the two improved soil samples.
Tab. 6 Test result of the SWCCs of improved soil
Different matric suction ( kPa ) gravity water content ( % ) Improved soil
Improved soil with lime
Improved soil with fly ash
Pure soil
3% lime
5% lime
7% lime
Pure soil
5% fly ash
10% fly ash
15% fly ash
0 5
24.5 24.4
23.7 23.6
23.0 22.9
22.5 22.4
24.5 24.4
24.2 24.1
23.5 23.4
23.0 22.8
25� ______ �� � 24� _____ ����, �23 t==========:=;�� � 22 § 21 u � 20
� 19 " C 18
.� 17 o 16
15 0.1
-+- Pure soil --- 3% lime soil ... 5% lime soil • 7% lime soil
10 100 Malric suction (kPa)
10
24.3
23.6
22.8
22.3
24.3
23.9
23.2
22.7
1 000
Fig.2 SWCC bunch of lime-improved soil
3 Fitting and analysis of SWCC liner system
3.1 Fitting of SWCC linear system
20
23.9
23.5
22.5
22. I 23.9
23.7
23
22.6
To achieve the improvement effect through the un
saturated soil theory, the variance gamma (VG) model
( S-shaped curve model ) is widely used for the fitting of
SWCC. The fitting formula is as follows:
50
22.8
22.7
22.0
21. 8
22.8
22.5
22.4
22.3
100 200 400 600
20.8 18.8 17.5 17
2l. 6 19.8 18.3 17.6
21. 3 20.3 18.7 17.7
21.5 20.7 19.0 17.9
20.8 18.8 17.5 17.1
20.9 19.3 17.8 17.3
20.2 20.3 18.8 18.3
20.5 20.5 19.3 18.5
� 25 � 24���������� C 23f � 22 8 21 � 20
� 19 C 18 ';; 17 � 16
-+- Pure soil ___ 5% fly ash soil ....... 10% fly ash soil -+- 15% fly ash soil
800
16.7
16.9
17.2
17.5
16.7
17.0
18.0
18.2
15 L-������������� 0.1 10 100 1000
Matric suction (kPa)
Hg. 3 SWCC bunch of fly ash-improved soil
() = (), + [1 + (0'1/1) n ] m '
1 000
16.2
16.5
16.7
16.9
16.2
16.5
17.6
18.0
where ex, n, m are fitting parameters; () is gravity water
content; ()s is the saturated water content; (), is the resid
ual water content; I/J is the matric suction; ex is the recip
rocal of air entry value I/J b; m, n are experIence con
stants, which can reflect the degree of changes of water
content with varying suction and describe the shape of soil
water characteristic curve, as known as the shape factor.
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YANG Heping,et al : Analysis of the Improvement Effect of Expansive Soil through the Soil- water Characteristic Curve 39
Accordingly, When, m = 1 - 1/ n, the best fitting
effect is achieved based on observed values, Thus, the
method is adopted to fit the curves. Figures 4 and 5 show
the results of fitting, whereas the calculated fitting pa
rameters are shown in table 7.
� 22 " o u t 20 � C 18 .� o 16
--+-- Pure soil --- 3% lime soil � 5% lime soil --- 7% lime soil
10 100 Matric suction (kPa)
1 000
Fig.4 Fitted SWCC bunch of lime-improved soil
� 24 c: " 22 c: 0 u --+-- Pure soil � 20 � ___ 5% fly ash soil " 18 ........ 10% fly ash soil C . ;;: --- 15% fly ash soil e 16 0
10 100 1 000 Matric suction (kPa)
Fig. 5 Fitted SWCC bunch of fly ash-improved soil
Tab.7 Parameters of SWCC of improved soil
fitted by the VG model
Fitting IJ, IJ, Correlation ifih a n m parameters (% ) (% ) coefficient ( kPa )
Pure soil 0.016 1.775 0.437 24.5 15.3 0.999 5 64.4
3%lime soil 0.010 l. 524 0.344 23.7 13.6 0.999 7 99.5
5%lime soil 0.009 1. 117 O. 105 23.0 12.6 0.999 7 116.8
7% lime soil 0.004 l. 462 0.316 22.5 11.5 0.998 4 238. I 5% Oy ash soil 0.015 1.583 0.368 24.2 14.8 0.999 73 65.0
10% Oy ash soil 0.014 l. 356 0.263 23.5 14.0 0.999 15 70.0
15% Oy ash soil 0.009 1.470 0.320 23.0 13.2 0.999 21 116. I The column on the correlation coefficient evidently
shows that the correlation coefficient of experimental data
and the model is above O. 998. The fitting curves are
consistent with the actual SWCCs, indicating that the
effect is sufficient based on the fitting of the SWCC by
the VG model.
3. 2 Analysis of SWCC linear system
Table 7 shows that the three parameters are consist
ent. With increasing modifier content, ex becomes m
creasingly smaller ( i. e. , air entry value becomes m
creasingly larger ) , and saturated water content and re-
sidual water content both decrease.
Two kinds of improved SWCC linear systems are
plotted separately in figures 4 and 5. A comparison of
figures 4 and 5 reveals that the SWCCs of the improved
expansive soils' are similar despite the existence of dis
tinct turning points. Two feature points ( i. e. , air entry
value and residual water content ) are considered to di
vide the curve into three sections; each section corre
sponds to different water conditions.
In analyzing the two SWCC linear systems, the
curves are found to become flatter as the modifier increa
ses, indicating that improved soil dehydration speed de
celerates and water holding capacity is strengthened.
These observations indicate that the fine, powder-like
modifiers occupy the pore and combine with the clay at
the contact point, thus altering the particles and the pore
size distribution. Thus, gas is barred from entering the
pores, and air entry value consequently increases.
A convergence of approximately 80 kPa exists at the
suction. This finding suggests that water holding capaci
ties of soils are altered at the convergence point, which
disagrees with traditional theoretical conclusions a higher
dry density results in higher water holding capacity. The
antinomy may be attributed to the limited number of tests
conducted in the past as well to the fact that only single
soil sample testing had been conducted by most studies to
obtain and fit SWCC for the establishment of an appropri
ate theoretical equation. Given the differences in the
slopes of curves ( slope of air entry value and residual
water content ) , the curves are not parallel and create an
intersection. The slopes of curves should be an evalua
tion index in the analysis of the improvement effect.
4 Assessment of chemical modification
4. 1 Selection of evaluation index
The influence factors of SWCC have been found to
be the mineral composition in soils, pore structure, tem
perature, confining pressure, initial water content, dry
density, compaction work, etc. [10]. The effects are
mainly reflected by the air entry values, residual water
content, and dehydration rate.
The foregoing analysis reveals that with increasing
modifier content, the SWCC of expansive soil becomes
flatter, which indicates that the dehydration rate of ex-
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40 Journal of Highway and Transportation Research and Development
panslve soil decelerates after the modification, whereas
water holding capacity is strengthened. Along with in
creasing air entry value, gas is hindered from accessing
the soil samples. As a result, residual water content con
tinuously decreases. Therefore, the slope of the curve
can be used as an evaluation index for improvement
effects.
Soil sample saturation IS the first step in non-satu
rated SWCC tests. Different modifier contents of compac
ted expansive soil must correspond to different gravity
saturated water contents. Thus, the gravity saturated wa
ter content can also be used as an evaluation index for
the modified effect of SWCC.
Based on the above findings, this paper uses the
four indexes : gravity saturated water content, aIr entry
value, residual water content, and slope of curve, to e
valuate the improvement effects of expansive soil.
4. 2 Bases for the establishment of index systems
For the quantitative analysis of the rationality of im
provement effects based on the evaluation of the four in
dexes and the collection the fitting result of the SWCC
test, table 8 summarizes the gravity saturated water con
tent, air entry value, residual water content, as well as
the slope of the curve of lime- and fly ash-improved soil.
The slope value considers not only the size but also the
direction.
Table 4 shows that changes in the index values are
regular. The saturation gravity water content and residual
water content of improved soil decreased with the increas
ing modifier content. By contrast, the air entry value in
creases, and the slope is flatter by comparison. These
observations show that the choice of the four indexes IS
reasonable and viable for an improvement evaluation.
Tab. 8 Summary of four indexes of fitted curves
Saturated Air entry Residual Slope of
Name gravity water value water curve
content ( % ) ( kPa ) content ( % ) Pure soil 24.5 64.4 15.3 6.64
3% lime soil 23.7 99.5 13.6 5.48
5% lime soil 23.0 116.8 12.6 4.70
7% lime soil 22.5 238. 1 11.5 4.55
5% fly ash soil 24.2 65.0 14.8 5.16
10% fly ash soil 23.5 70.0 14.0 4.72
15% fly ash soil 23.0 116.1 13.2 3.53
4. 3 Initial establishment of evaluation indicators
and criteria
Based on the above analysis, the modified effect of
expansive soil cannot be determines only from the shape
and four index values of the SWCC, which must be based
on the existing criterion to evaluate and establish rela
tionships among the four indexes. Such relationships
would determine the boundary value of the evaluation in
dexes.
In accordance with the assessment criterion of the
free expansion ratio, as given in table 5, the free expan
sion rates of 5% lime soil, 7% lime soil, and 15% fly
ash soil are all less than 40%, which proves that the
modification of expansive soil is successful. In this re
gard, as shown in table 8, the limiting value of the indi
cator is preliminary proposed. Careful analysis of the da
ta listed in the summary table reveals that saturated grav
ity water content is an essential index; its value must be
less than or equal to 23. 0%. Considering the differences
in the effect of modification after lime and fly ash are
mixed with the expansive soil, rates of change of the cor
responding three indexes: which are the air entry values,
residual water content, and slope of the curve, all differ.
The limiting values of the three indexes are given through
an integrated equilibrium analysis ( see table 9) .
Tab. 9 Critical values of four indexes for evaluating the
improvement effect
Saturated Air entry Residual Slope of
Name of index gravity water value water curve
content ( % ) ( kPa ) content ( % )
Expansi ve soil >23.0 < 116.0 > 13. 0 >4.0
None-expansive soil :;;:;23.0 �116.0 :;;:; 13. 0 :;;:;4.0
Tables 8 and 9 show that the saturated gravity water
content of the soils with 5% and 7% lime and 15% fly
ash satisfies the requirements. However, only two of the
remaining three indexes meet the requirements. The
slope of curve of the lime-improved soil is above the lim
iting value, whereas the residual water content of the fly
ash-improved soil does not meet the requirements. Based
on these findings, the saturated gravity water content is
proven to be an essential indicator. Moreover, a clear con
nection between changes in the four indexes of SWCC and
the swell-shrink characteristic of improved soil is evident.
The evaluation method employed in this study is
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YANG Heping,et al : Analysis of the Improvement Effect of Expansive Soil through the Soil- water Characteristic Curve 41
mainly based on whether the improved soil can meet the
requirements of free expansion ratio, the significance of
which reveals the necessary connection between the
change in the four indexes of SWCC and the swell-shrink
characteristic of improved soil. An increasing number of
experts and scholars are interested in unsaturated soil re
search, If SWCC tests are completed, the seepage,
strength, and volume change of unsaturated soil are pre
dicted, and the effect of improved soil is evaluated,
Thus, unsaturated soil mechanics can be truly applied to
the engineering practice,
5 Conclusions
( 1) This article attempts to establish a connection
between the theory of unsaturated soil and corresponding
engineering problems, We promote the application of the
water characteristic curve of unsaturated soil to engineer
ing projects,
(2) Through the study of the SWCC linear system
and expansion, the four-index system of evaluation is
proposed, However, given the wide distribution of ex
pansive soil in various climates and geological condi
tions, expansive soils of various regions often present
prominent regional distinctions, and the rationality of
each index requires further experimental verification,
(3) Only the drying curve has been studied in this
test. SWCC actually has a hysteretic nature, Thus, tests
that involve the drying and wetting of SWCC should be
performed to verify the conclusions in appropriate experi
mental conditions,
( 4) Tests of SWCC may become conventional or ob
ligatory, with comprehensive theoretical studies on unsat
urated soil and the development of tests for suction, Once
this evaluation method reaches maturity and is perfected,
it will enable SWCC to evaluate the improvement of the
effectiveness of unsaturated expansive soils.
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