moisture migration tests on unsaturated expansive clays in hefei, china

Applied Clay Science xxx (2013) xxx–xxx

CLAY-02607; No of Pages 6

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Applied Clay Science

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Research paper

Moisture migration tests on unsaturated expansive clays in Hefei, China

Ming Wu Wang ⁎, Jian Li, Song Ge, Shu Ting LiSchool of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, China

⁎ Corresponding author. Tel.: +86 551 2901434; fax:E-mail address: [email protected] (M.W. W

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Please cite this article as:Wang,M.W., et al.,Mhttp://dx.doi.org/10.1016/j.clay.2013.02.024

a b s t r a c t
a r t i c l e i n f o
Article history:Received 30 December 2012Received in revised form 31 January 2013Accepted 22 February 2013Available online xxxx

Keywords:Expansive clayUnsaturated soilMoisture migrationVaporous water

Expansive clay in semiarid area behaves features of typical unsaturated soil, which is sensitive to climate andmoisture, and appears multi-fissures, deterioration of mechanical performances with the change of moisture.Consequently, to ensure the rational design of foundation treatment, and safe operation, research onmoisturemi-gration of expansive clays is of great significance. Herein, test models of mixed water migration and vaporouswater migration were presented to find out the mechanism. Series of tests, in which the moisture content inleft soil column was the same as 24%, and in right soil column was 0%, 6%, 12% and 18% respectively, wereconductedunder constant temperature. It is concluded thatmoisture content changedwith the increase ofmigra-tion time in both migration models. And the migration behaves nonlinearly when the initial water content in-creases up to 6% for mixed water migration model and 12% for vaporous water migration model. But themigration characteristics of mixed water and vaporous water were different. In the initial 30 days the changeof moisture water was larger in mixedmigration model, however within 60 to 90 days, it was larger in vaporouswater migration model. The research results have been applied to guide relevant engineering practice.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Expansive clay is one special clay where there are noticeable changesin volumewhen the soil absorbswater or dries, and thismakes the foun-dation treatment of expansive clays a global issue, known as cancer ofengineering (Du et al., 1999; Wang and Chen, 2011; Wang et al., 2003).Moreover, expansive clay in the semiarid area is more of in unsaturatedstate. The engineering behavior of unsaturated expansive clays is depen-dent on the soil–water–air phase relationships and residual state (Fityusand Buzzi, 2009). Van Genuchten (1980), Fredlund and Xing (1994),Fredlund et al. (1994), Alonso et al. (1999), Gardner (1958), Wheeleret al. (2003) conducted researches on unsaturated clay and sand, andgot some benefit results. Van Genuchten described a relatively simpleequation for the soil-water content–pressure head curve, which enablesresearchers to get a closed -formexpressionof the relative hydraulic con-ductivity, and Fredlund and Xing proposed a general empirical equationto describe soil-water characteristic curve. Some other models were putforward to study the permeability and shear strength of unsaturated soilbased on soil-water characteristic curve (Azmatch et al., 2012). Up topresent, various laboratory testing techniques for measuring the soil-water characteristic curves and the water hydraulic conductivity of un-saturated soils were developed (Langroudi and Yasrobi, 2009; Masrouriet al., 2008; Ng et al., 2002; Puppala et al., 2006). The above researchespointed out that themoisturemigrationwill result in worsening of engi-neering properties because the corresponding proportion of water andair, and suction and strength will change with moisture migration. For

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oisturemigration tests onun

example, the above change may lead to differential settlement andcracks on buildings and causes a huge economic cost. Therefore, themechanism of moisture migration of unsaturated expansive clays hasbeen a topic for engineers.

Quantitative study ofmoisturemigration in soils had begun since theconcept of soil water potential was put forward in the 1950s. Thomas(1987) discussed nonlinear analysis of heat and moisture transfer inpartly saturated soil. Singh et al. (1989) investigated theheat conductionand moisture distribution through the different layers of unsaturatedsoil. Grifoll and Cohen (1996) studied the potential effect of rainfalland evapotranspiration on contaminant migration on various watertransport mechanisms, and suggested that transport mechanisms suchas the diffusion, dispersion and convection are significant. Shoop andBigl (1997) used a coupledmodel of heatflow andmoisture flow to sim-ulate large-scale freeze–thaw experiments and to predict soil moistureconditions, and concluded that thawed moisture content depended onthe location of water table. Poulose et al. (2000) investigated moisturemigration in a silty soil by means of geotechnical centrifuge tests.Dobchuk et al. (2004) discussed the prediction of water vapor move-ment through waste rock. Zhang et al. (2004) analyzedmoisture migra-tion in seasonally frozen ground region based on experiments. Wang etal. (2004) discussed the influences of surface temperature and rainfallintensity on moisture migration of short duration in unsaturated soil.Liu et al. (2005) developed mathematical model to describe simulta-neous moisture transfer in the porous soil, and relevant experimentwas conducted. Comparing with the calculated and measured results,it indicated that thedry surface layer has an important effect onmoisturemigration in soil. By a systematic experiment on heat andmass transfer-ring in one dimension vegetable yellow soil column, Chen et al. (2006)

saturated expansive clays in Hefei, China, Applied Clay Science (2013),

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Fig. 1. SWCC of expansive clays used in the tests.

2 M.W. Wang et al. / Applied Clay Science xxx (2013) xxx–xxx

discussed the dominant mechanism of capillary flow that governs themoisture transfer. The above researches have made some significantconclusions. However, the moisture migration of unsaturated expansiveclays is affected by many factors, and the previous reports are mainlyfocused on mixed water migration, few pay attention to migration ofexpansive clays, and especially to vaporous water migration. In a word,themoisturemigration in unsaturated expansive clays is not fully under-stood up to now. Thus,many issues andmuch researchwork still need tobe done to develop foundation treatment plan for expansive clays.Therefore, to choose the rational foundation treatment plans for expan-sive clays and ensure safety, research on themechanism ofmoisturemi-gration in unsaturated expansive clays is important in practice.

The objective of this paper is to investigate the moisture migrationinduced by different water contents between the soil columns of unsatu-rated expansive clay. Themodels ofmixedwatermigration and vaporouswater migration, which were made up of different initial water contentswith unsaturated expansive soil in Hefei, were discussed to carry out-migration tests. The resultswill provide a basis formaking a rational foun-dation treatment plan for major engineering.

2. Test model of moisture migration

2.1. Basic properties and soil-water characteristics

The expansive clays used in tests were taken from runway founda-tion of international airport site in Hefei, China. The expansive clay isof medium or low shrinkage and swelling grade. The composition ofclay minerals composed mainly of montmorillonite and illite fromXRD tests. Their physical properties and swelling characteristics basedon laboratory experiments were listed in Table 1 (Wang et al., 2011,2012, 2013). The measured SWCC (soil-water characteristic curve) ofreconstituted sample duringwetting process as shown in Fig. 1 behaveshysteresis with reference to that during drying process from SWCC testsby GDS triaxial stress path testing system. Based onMualemmodel andVan Genuchten model, the computed relative hydraulic conductivityand soil-water diffusivity were shown in Fig. 2.

2.2. Test model

The moisture migration induces change in water content of ex-pansive clays, which leads to variations of suction and shear strength.Obviously, the moisture migration plays a role on engineering behaviorsof unsaturated expansive clays. However, there are many complicatedand uncertain contributive factors to moisture migration. To removethe interferences from temperature difference and gravity in tests, laterallying soil columns of uniform, sealed and under constant temperatureconditionwereused to investigate themechanismofmoisturemigration.The tests of different initial water contents and migration times werecarried out to discuss the distribution principle of water content aftermoisture migration, and its effect on engineering properties. The PVC(Polyvinyl Chloride) pipe of 2 mm thickness was used to make themodel because it is of good sealing, low cost and easy tomake testmodel.

In the test, the key problem is how to distinguish mixed water andvaporous water duringmigration. To overcome this problem, matchingmodel and test procedurewere presented for two types ofmigration. Thedetails of models are described as shown in Figs. 3 and 4. To investigateinfluences of different initial water content and migration time onthe water content in unsaturated expansive clays, test models listed

Table 1Physical properties and swelling characteristics of expansive clays.

Maximum dryunit density(g·cm−3)

Optimumwatercontent (%)

Freeswellingratio (%)

Swell–shrinkagetotal ratio (%)

Expansionfactor

Swellingpressure(kPa)

1.84 16.0 46.3 0.35 0.33 67.5

Please cite this article as:Wang,M.W., et al.,Moisturemigration tests onunhttp://dx.doi.org/10.1016/j.clay.2013.02.024

in Tables 2 and 3 were conducted, and concrete test procedures formixed water migration and vaporous water migration are depictedas follows.

2.3. Test procedure

Basing on migration features of mixed water and vaporous water,corresponding sample preparation method and test procedure weredeveloped respectively.

The sample preparation method and test procedure for mixedwater migration tests were interpreted as follows:

Step 1: Prepare a PVC pipe 200 mmin length, inner diameter of 36 mmand outer diameter of 40 mm with one end closed, and checkits air tightness.

Step 2: According to the test plan, fill expansive clays of relative highwater content into PVC pipe and compact at certain times till to

Fig. 2. Relative hydraulic conductivity and soil-water diffusivity of expansive clay.



Table 2Initial moisture content of soil column for mixed water migration test.

Case Left soil column (%) Right soil column (%) Migration time (d)

A 24 0 30, 60, 90B 24 6 30, 60, 90C 24 12 30, 60, 90D 24 18 30, 60, 90

Fig. 3. Images of test model.

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half-length of the pipe. Then lower water content clays werefilled into the pipe and compacted the same times.

Step 3: Seal another end and do leak detection. Then put them intocuring room after weighting the samples.

Step 4: Take out the sample from curing room to measure the watercontent of different sections in soil column after certain migra-tion time.

Meanwhile, the corresponding procedure for vaporous migrationtest was described as follows:

Step 1: Prepare two PVC pipes 100 mm in length with one end closed,whose inner diameter and outer diameter are 36 mm and40 mm respectively.

Step 2: According to test plan, prepare expansive soil clays of differentinitialwater contents for the left and the right pipe. Fill soils intothe left and the right PVC pipe and compact samples respective-ly. A ventilated grid was set on the surface of soil column of 0%or 6%water content, to avoid a collapse induced bymoisture ab-sorption during moisture migration.

Step 3: Connect and seal the left and the right PVC pipe by a sealingadapter. At the center of the adapter, an air zone of 10–20 mmin thickness needs to be reserved to ensure the migrationwater between the left soil column and the right only in gaseousstate.

Step 4: This stepwas similar tomixedwatermigration test. Put the sam-ples into curing room until named migration time, take out thecorresponding samples and do leak detection. Then, open thesamples and measure the water content at different sections inthe soil column.

3. Test results and comparisons

The results were illustrated in Figs. 5 to 9.It is observed from Fig. 5 that water content in the left soil column

decreased with the increase of migration, and the decreased ratiotended to slow down. However, the moisture content increased with

Fig. 4. The sketch of migration model test: (a) mixed water migration, and (b) vaporouswater migration model.


the growth of migration time in the right soil column of low initialwater content. Moreover, water content of soil in right column closerto the left soil column was higher. After 90 days of migration, in CasesA, B, C, and D, the average moisture content of left column decreasedfrom 24% to 15.47%, 20.24%, 12.81%, and 20.35% respectively. As to theright soil column, the water content of Case A increased from 0% to10.97%, and that of Case B increased from 6% to 10.86%. In Cases C andD, the growth is 28.36% and 8.83% respectively (Case C: from 12% to15.40%, Case D: from 18% to 19.59%).

Moisture content after migration of 90 days for different test modelwas shown in Fig. 6. It is clearly seen thatwater content of left soil columnamong tests decreased. But when the initial water content of the rightpart wasmore than 6%, the reducing value is not obvious. Thismay be in-duced by nonlinear characteristic of migration. The nonlinear was alsoobserved in the right side. Among the tests, the biggest water contentchange was found in the model of 0% initial water content.

It is clear in Fig. 7 that test models for Case E were under the sametest condition except themigration time. It suggested that the soil columnof high water content behaved more significant shrinkage feature as mi-gration time varied. After a careful observation in Fig. 7(a) to (c), it can befound that fissure between the left soil column and PVC pipe, went on asmigration time, and the fissure appeared from disconnected and smallstate to continuous and wider style. However, soil column with dif-ferent interface colors in Fig. 7(d) and (e) in Case F test did notshow obvious shrinkage. Shrinkage characteristic did not behave inCase G (see Fig. 7(f)) comparing with the models from Cases E, F, and G,in which the left soil columnwas the samewith 24% initial water contentand the right columnswere 0%, 6%, 12% respectively. The interface color ofsoil columnwasdarker than that in Fig. 7(d) and (e). Thatmaybe inducedby little vaporous migration in Case G. The above phenomena did goodagreement with results from the measured moisture content.

It is concluded in Fig. 8 that for vaporous water migration test,water content in the left soil column decreased with the migrationtime, and the change during the duration from 60 to 90 days is larger,whichwas different frommixedwatermigration test, while the right soilcolumn shows the adverse tendency. Water content changed slightly atthe corresponding position for the mixed water migration test. After90 days of migration, in Cases E, F, G, and H, the average moisturecontent of the left soil column decreased from 24% to 21.85%, 22.32%,22.99%, and 23.88% respectively. As to the right soil column, the watercontent of Case E increased from initial 0% to 8.85%, and Case Fwitnessedan increase from 6% to 10.42%. In Cases G andH, the growth is 4.67% and7.33% respectively.

The distribution ofwater content in unsaturated expansive clay after90 days of vaporous water migration was described in Fig. 9. It may beconcluded that thewater content of the left soil column increased in ref-erence to initial water content, but the right side increased, andnonlinear migration characteristics were also found when the initialwater content was more than 12%. In general, the water content in the

Table 3Initial moisture content of soil column for vaporous water migration test.

Case Left soil column (%) Right soil column (%) Migration time (d)

E 24 0 30, 60, 90F 24 6 30, 60, 90G 24 12 30, 60, 90H 24 18 30, 60, 90



Fig. 6. Migration of mixed water in expansive soil after 90 days.

Fig. 7. Images of interface shrinkage after moisture migration. (a), (b), and (c) Images of 30, 6in Case F, and (f) image of 90 days of migration in Case G.

Fig. 5. Results from mixed water migration tests.



right soil column for vaporouswatermigrationmodel is less than that inmixed water migration model. When the right column is of larger suc-tion, it has a deep influence on the vaporous water migration.

4. Conclusions

The moisture migration in unsaturated expansive clays is a compli-cated problem. Herein, based on different types of moisture migrationmodel tests, moisture migration of lateral lying expansive soil columnsof different initial water contents was discussed under constant tem-perature, and some major conclusions may be drawn as follows:

1) From the test results, the presented test model, used to investigatethe migration mechanism of mixed water and vaporous water, iseffective and feasible.Meanwhile, the testmodel can control influencefactor and the test materials is of low cost.

2) The moisture content changed slowly as migration time goes on.Water content variation will appear nonlinear characteristic whenthe initial water content increased up to a certain value. Moreover,

0 and 90 days of migration in Case E, (d) and (e) images of 60 and 90 days of migration



Fig. 8. Results from vaporous water migration tests.

5M.W. Wang et al. / Applied Clay Science xxx (2013) xxx–xxx

the transition of moisture content due tomoisture migration agreedto different initial water contents for mixed water and vaporouswater.

3) The interface of the left expansive soil column behaved obviousshrinkage features in models of 0% initial water content in lowmoisture content soil column after 90 days of migration. Modelswith 6%, 12%, and 18% did not show fissure, but the color of theirinterface is different.

4) The moisture migration of different initial moisture content wasdiscussed. Nevertheless,moisturemigration in unsaturated expansiveclays is influencedbymany complicated factors. Extensive and furtherstudies are needed to further investigate the migration mechanismfrommacro and micro level by means of a comprehensive method.

Fig. 9. Migration of vaporous water in expansive soil after 90 days.


Acknowledgments

Financial supports provided by National Natural Sciences Foundation,China (no. 41172274) and the University Student Innovation Program ofHefei University of Technology (no. 2011CXCY238) are gratefully ac-knowledged. The authors would also like to express their sincere thanksto the reviewers for their thorough reviews and useful suggestions, andstudents Wu Han-jun, Hu Jin-yuan, Xia Yuan-zhe and Pan Liu-yang fortheir assistance throughout the tests.

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moisture migration tests on unsaturated expansive clays in hefei, china

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