Experimental evaluation of mechanical behavior of ...research.iaun.ac.ir/pd/bayat/pdfs/PaperM_9855.pdf · Testing of soil in unsaturated conditions is not common in soil mechanics

Engineering Geology 141–142 (2012) 45–56

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Experimental evaluation of mechanical behavior of unsaturated silty sand underconstant water content condition

Mohammad Maleki ⁎, Meysam BayatDepartment of Civil Engineering, Bu-Ali Sina University, Hamedam, Iran

⁎ Corresponding author. Tel.: +98 811 8257410; fax:E-mail addresses: [email protected] (M. Maleki), m.

(M. Bayat).

0013-7952/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.enggeo.2012.04.014

a b s t r a c t
a r t i c l e i n f o
Article history:Received 13 July 2011Received in revised form 24 April 2012Accepted 28 April 2012Available online 9 May 2012

Keywords:Unsaturated silty sandShear resistanceMatric suctionVolume changeConstant water content triaxial test

There are very few experimental data on the mechanical behavior of unsaturated soils, particularly in con-stant water content condition, because of the technical difficulties and time-consuming nature of measuringsuction and deformation. This paper presents the results of a series of constant water constant triaxial testson the specimens of an unsaturated silty sand. Constant water content tests correspond to a field conditionwhere the rate of loading is much quicker than the rate at which the pore water is able to drain out of theunsaturated soil. The axis translation technique and a double-walled triaxial cell have been used to measurethe soil matric suction and variation of pore air volume respectively. Test specimens were prepared at twodifferent compaction conditions prior to testing to achieve different initial density. It is found that the me-chanical behavior of the soil mainly depends on the initial density, the mean net stress and the initial matricsuction. Also the volume and pore water pressure changes are significantly different in specimens with dif-ferent initial condition. The results of tests indicated that the shearing strength of silty sand increases non-linearly with matric suction.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In geotechnical practice most soils are in unsaturated condition,for instance the compacted soils used in several engineering con-structions (such as earth dams, highways, embankments, and airportrunways). Many of these soil structures often do not attain fully satu-rated conditions during their design life. Testing of soil in unsaturatedconditions is not common in soil mechanics laboratories. The me-chanical behavior (i.e. shear strength and volume change) of unsatu-rated soils is relevant in the design of several geotechnical and geo-environmental structures. Conventional soil mechanics theory treatssoil as either fully saturated or dry. However, a large number of engi-neering problems involve the presence of unsaturated soil zoneswhere the voids between the soil particles are filled with a mixtureof air and water. These zones are usually ignored in practice and thesoil is assumed to be either fully saturated or completely dry. Thisis despite test results indicate significant differences between the me-chanical behavior of unsaturated soils and the mechanical behavior offully saturated or completely dry soils (such as Adams and Wulfsohn,1998; Miao, 2002; Wang et al., 2002; Chiu and Ng, 2003; Ng and Chiu,2003; Rahardjo et al., 2004; Kayadelen et al., 2007; Jotisankasa et al.,2009). So far many studies have been conducted on critical stateand mechanical behavior of saturated soils (Schofield and Wroth,

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1968; Wood, 1991; Maatouk et al., 1995; Newson, 1998) but not un-saturated soils.

In the 1950's, research at Imperial College London, was directedtoward a fundamental understanding of unsaturated soil behaviorwithin the classical framework of saturated soil (Bishop et al., 1960).The research involved careful laboratory studies, primarily on theshear strength of unsaturated soil. These studies have provided thecatalyst for further studies in various countries of the world. This inter-est has accelerated during the last 17 years, and has been accompaniedby the development of extensive experimental laboratory testing pro-grams leading to the formulation of various constitutive models forunsaturated soils.

It is well known that the mechanical behavior of saturated soils canbe interpreted and explained by a single stress state variable, called theeffective stress (Terzaghi, 1936). Conversely, unsaturated soils are char-acterized by the presence of air phase, water phase and air–water inter-face in voids. Due to the additional pore phase, there are difficultiesin applying the same approach to unsaturated soils (Jennings andBurland, 1962) and it is thus difficult to define convenient stress statevariables. During the past three decades there has been an increasinguse of two independent stress variables to describe the behavior of un-saturated soils (Coleman, 1962; Bishop and Blight, 1963; Burland, 1965;Aitchison, 1967; Matyas and Radhakrishna, 1968; Barden et al., 1969;Brackley, 1971; Fredlund and Morgenstern, 1977).

Critical state frameworks for unsaturated soil mechanics havebeen proposed and compared with those of saturated soil mechanics(Alonso et al., 1990; Maatouk et al., 1995; Wheeler and Sivakumar,1995). The smooth transition from the two stress state variables for

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Fig. 1. Details of: (A) used triaxial cell for testing unsaturated soils; (B) base plate ofunsaturated triaxial test apparatus.

Table 1Physical properties of compacted soil specimen.

Group-1 Group-2

wet (KN/m3)γ 19.11 17.24dry (KN/m3)γ 17.94 16.26(Sr) initial (%) 37.73 26.26e 0.46 0.61GS 2.67 2.67(ω) initial (%) 6.5 6

46 M. Maleki, M. Bayat / Engineering Geology 141–142 (2012) 45–56

an unsaturated soil, net stress (σ−ua) and matric suction (ua−uw),(where, σ is the total stress, ua is the pore air pressure, and uw isthe pore water pressure) to the single stress variable effective stress(σ−uw) for a saturated soil (Fredlund and Morgenstern, 1977),forms the basis for the extension from saturated to unsaturated soilbehavior. The concepts of yielding, hardening, and critical state arethe key elements comprising the critical state framework for saturatedsoils. These concepts can be extended using two independent stressstate variables, (σ−ua) and (ua−uw), to build a critical state frame-work for unsaturated soils. These variables have been suggested asthe critical state variables for unsaturated soils by several researchers(Maatouk et al., 1995; Wheeler and Sivakumar, 1995; Rampino et al.,1998). Given the diversity of results from previous research, morestudy is required on the behavior of soils with varied values ofwater content (ormatric suction), initial density and net stress to inves-tigate their influence on shear strength and volume change behavior.

This paper presents test data from triaxial tests on an unsaturatedsilty sand with measurements of matric suction. It summarizes thefindings from an experimental program concerning the mechanicalbehavior of two groups of silty sand with different initial density test-ed under low mean net stress. Constant water content triaxial testswere carried out on unsaturated specimens. In addition, consolidatedundrained tests on saturated samples were performed to study soilbehavior in saturated condition. The mechanical behavior of unsatu-rated specimens is studied and compared with the behavior patternfor saturated specimens and their variation is considered with respectto matric suction.

2. Testing apparatus

The stress–strain behavior of unsaturated soils is often interpretedfrom results of triaxial or direct shear tests. These tests are generallyperformed by controlling matric suction (Alonso et al., 1990; Fredlundand Rahardjo, 1993; Aversa and Nicotera, 2002) in the soil specimenby using the axis translation technique. In this technique, the pore-airpressure is artificially raised above atmospheric pressure to increasethe pore water pressure by the same amount to a positive value. Inthis way, the cavitation of water in the measuring system is prevented(Fredlund and Rahardjo, 1993).

In the current study, the triaxial compression apparatus developedat Bu-Ali Sina University is used to study the mechanical behavior ofunsaturated soils. The details of the triaxial including its top cap andbase plate are presented in Fig. 1. High-air entry ceramic disks, with500 kPa of air entry values are sealed into the base pedestal and topcap using epoxy resin along their periphery. The two-way waterflow (i.e. through ceramic disks in the cap and pedestal) causes a con-siderable decrease in test time and also create a more uniform distri-bution of moisture in the specimen. Two ceramic disks connected tothe measuring system water compartment. The grooves inside thewater compartment run as water channels for flushing air bubbles ac-cumulated due to diffusion. The diffused air volume was measured byusing the diffused air volume indicator (DAVI) proposed by Fredlundand Rahardjo (1993), and the measurement of pore-water volumechange was corrected accordingly.

3. Testing program and procedures

3.1. Experimental details

In this work two groups of triaxial tests under saturated and un-saturated conditions were carried out for investigating the influenceof initial density, matric suction and net confining pressure on soil be-havior. The differences between the two groups are the specimen'sinitial density and compaction water contents. The initial physicalproperties of specimens are given in Table 1. The values in this tableare average values obtained from all specimens in this study. The

grain size distribution and compaction test result (ASTM D, 1557,2002, Method A) obtained by using the Standard Proctor Compactionmethod are shown in Fig. 2. The soil consists of 51% sand, 20% silt and29% clay.

Previous investigations have indicated that sample preparationmethods affect the behavior of soils (Ladd, 1974; Mulilis et al., 1977)and thus the choice of a suitable sample preparation technique is im-portant in determining the behavior of soils. Current field samplingtechniques are not readily able to produce high-quality undisturbedunsaturated soil specimens for laboratory testing at an affordable cost.Accordingly, numerous sample reconstitution methods have been de-veloped and, among thesemethods, wet compaction has the advantageof providing relatively easy control of specimen density, even for loosespecimens (Frost and Park, 2003).

In this work, wet compaction has been used for preparation of sam-ples. Compacted specimens were prepared using a compaction moldspecifically designed for static compaction. The purpose of using staticcompaction as opposed to dynamic compaction is to obtain a more ho-mogenous specimen in terms of density and shear strength.

During sample preparation, dry sand, silt and kaolinite have beenmixed at weight ratios of 51, 20 and 29% respectively. A bulk soil sam-ple was prepared by spraying fine droplets of the required quantity of

Fig. 2. The silty sand tested: (A) grain size distribution curve; (B) compactiontest result.

47M. Maleki, M. Bayat / Engineering Geology 141–142 (2012) 45–56

water into the air dried soil. Water spraying and soil agitation weredone simultaneously to ensure a uniform distribution of water. Thesoil was transferred to plastic bags and placed without further agita-tion in a controlled environment room for about 5 days to achievemoisture equilibration before preparing samples for testing. Triaxialsoil specimens were formed by statically compacting soil at lowwater content of 6 or 6.5% in seven uniform layers, using a speciallyfabricated unit. The low water content is selected for achieving de-sired initial suctions without any drying process. Suitable thicknessof soil of each layer is necessary for achieving uniform specimen.Layer thickness depends upon type of soil, dimensions of specimenand compaction method. In this study, a layer thickness about 1 cmis optimum for achieving material homogeneity. The loose soil is con-fined in a mold and compacted using the gradual movement of a pis-ton. In the used compaction method a static force is gradually appliedto each layer until the required thickness (volume) is achieved. This

Table 2Initial stress values for the constant water content tests.

Test σ3 ua uw σ3−ua ua−uw

(G1 or G2)-S 25-25 275 250 225 25 25(G1 or G2)-25-50 300 250 225 50 25(G1 or G2)-25-100 350 250 225 100 25(G1 or G2)-50-25 275 250 200 25 50(G1 or G2)-S 50-50 300 250 200 50 50(G1 or G2)-S 50-100 350 250 200 100 50(G1 or G2)-S 100-25 275 250 150 25 100(G1 or G2)-S 100-50 300 250 150 50 100(G1 or G2)-S 100-100 350 250 150 100 100(G1 or G2)-S 162-25 275 250 88 25 162(G1 or G2)-S 162-50 300 250 88 50 162(G1 or G2)-S 162-100 350 250 88 100 162

procedure produced uniform 38 mm diameter by 70 mm long cylin-drical specimens all having the same material fabric. A length to diam-eter ratio of about 2 was selected in order to minimize the frictionaleffects due to the end platens of the apparatus and to reduce the likeli-hood of buckling during testing.

3.2. Saturated triaxial compression tests

Consolidated undrained triaxial compression tests were performedunder saturated conditions. The saturated stress–strain behavior of soilspecimens was measured by means of a conventional triaxial compres-sion test apparatus. Prior to the tests, the soil specimens were saturateduntil a pore pressure coefficient (Bw) exceeding 0.95 (ASTM D 854-02,2002) was measured. For this purpose, after placing and sealing thespecimen inside the triaxial chamber and taking measurements of di-ameter and height, the specimens were first subjected to a flow of CO2

from the bottom to the top for at least 3 h and then saturated by de-aired water. Then a cell pressure and a saturation back pressure wereapplied and increased gradually. A difference of 10 kPa between cellpressure and back pressure was maintained so that accidental swellingof the specimen resulting from a high back pressure or consolidationof the specimen due to high cell pressure is prevented. At the end ofthe saturation process, the soil specimens were consolidated under aconfining pressure (σ3). Then, they were sheared at a constant strainrate. In consolidated undrained triaxial tests with pore water pressure

Fig. 3.Wetting curves during equalization stage: (A) the specimens of group-1; (B) thespecimens of group-2.

Fig. 4. Result of undrained tests on saturated specimens in group-1: (A) stress–strainrelationship; (B) pore water pressure–strain relationship.

Fig. 5. Results of CW tests of group-1 at the initial matric suction of 25 kPa, plottedagainst axial strain: (A) deviator stress, q; (B) volume change; (C) suction.


measurements, the rate of strain ranges from 0.05 to 1%/min.We used astrain rate of 0.07%/min for consolidated undrained (CU) tests onsaturated specimens.

3.3. Unsaturated triaxial compression tests

Soil specimens were enclosed in two rubber membranes with alayer of silicon grease between them. In this way, air diffusion into cellwater through the rubber membrane was eliminated (Alonso et al.,1990). After placing and sealing the specimen inside the triaxial cham-ber, the specimen is consolidated prior to shearing by applying matricsuction and net confining pressure. A wetting process was then startedby decreasing the value of matric suction to different levels of 25, 50,100 and 162 kPa.

As shown in Table 2, specimens are referred to according to theconditions in which they were tested (initial density, matric suctionprior to shearing and net confining pressure). For example, G1-S50-100 represents a specimen of group-1 that was tested under constantwater content at a matric suction prior to shearing of 50 kPa and netconfining pressure of 100 kPa.

4. Results and discussions

4.1. Equalization stage

In order to confirm whether or not equilibrium has been reachedduring suction equalization, the moisture content of the specimen ismonitored with respect to time. Fig. 3 shows the volumetric watercontentwith respect to time and indicates that specimens have reachedequilibrium after a certain time. In Fig. 3, the specimen with the initialmatric suction of 25 kPa takes longer to equalize than the others, atsuctions of 50, 100, and 162 kPa. Also, for the specimens of group-2,the equilibrium time is generally shorter than for the specimens ofgroup-1. By comparing the two figures (i.e. Fig. 3(a) and (b)), we findthat the specimens of group-1 absorb more water than the specimens

of group-2 at the same matric suctions. These differences indicate ofinfluence of initial density on soil capillarity.

4.2. Shearing behavior

After consolidation, the specimen is subjected to deviator stress.Specimens were sheared until 30% axial strain under constant gravi-metric water content (CW). In these tests the pore-water valve wasshut off while pore-air was allowed to drain freely from the specimen.We used a strain rate of 0.01%/min for these constant water contenttests. This value of strain rate cases a suitable coherence betweenpore water pressure of specimen and pressure transducer in everymoment. During CW tests, matric suction varies as changes in pore-water pressure occur during shearing. Figs. 5–8 and 10–13 show thevariation of deviator stress, volume change and suction against theaxial strain during the shearing stage for the unsaturated specimensof group-1 and group-2, respectively. Also Figs. 4 and 9, show the




deviator stress and pore-water pressure versus axial strain for thesaturated soil specimens of group-1 and group-2, respectively.

For a given density, the deviatoric stress increases with an in-crease in net confining pressure at a given level of suction (the (a)sections of Figs. 4–13). Also, for the same net confining pressure,the strength of the unsaturated specimens is significantly greaterthan the strength of a saturated specimen. The general trend for alltests (group-1 and group-2) is that the specimens with higher initialsuctions were stiffer and they experienced higher ultimate deviatoricstresses. Although matric suction does not have important influenceon the general shape of the stress–strain curve, it does influence themaximum and final values of shear stress. Many other similar resultsabout the relationship between suction and shear strength have beenpresented in the literature (Miller and Nelson, 1993; Melinda et al.,2004; Rahardjo et al., 2004; Indrawan et al., 2006; Oh et al., 2008).

Fig. 14 shows the peak shear strength of all unsaturated specimensversus initial matric suction. This figure suggests that there is a nonlinearincrease in peak shear strength of specimens as a result of increase in ini-tial suction. Also, equal increases of suction andnet confiningpressure donot produce equal increases of shear resistance. In particular, an increase

of net confining pressure produces a greater increase of resistance thanan equal increase of suction. A comparison between the two groupsshows that the shear strength of specimens is affected by initial density.The shear strength for the specimens of group-1 is greater than theshear strength of the specimens of group-2 at a given level of suctionfor all confining pressures. Also this figure shows that the specimensin group-1 show a larger increase of peak shear strength for a given in-crease of suction than the specimens in group-2.

Fig. 15 presents the shape of the two specimens of group-1 atthe end of test. As shown in the figure, the unsaturated specimensshowed a barrel failure mode at lowest suction (i.e. 25 kPa) similarlyto saturated specimens, while they showed a brittle failure mode ac-companied with a shear zone at highest suction (i.e. 162 kPa). Thisfigure shows that the stiffness and brittleness of the specimens ofgroup-1 increased as their initial matric suctions increased. In otherwords, the stiffness and brittleness of the specimens of group-1 in-creased as their initial matric suctions increased at the same net confin-ing pressure. Conversely, the shear stress of the unsaturated specimensof group-2 stabilizes at an axial strain between 20 and 30%, withoutdistinct failure planes being observed for all initial suctions.


Fig. 9. Result of undrained tests on saturated specimens in group-2: (A) stress–strainrelationship; (B) pore water pressure–strain relationship.


4.3. Volumetric behavior

The changes in volumetric strain of unsaturated specimens duringshearing are shown in the (b) section of Figs. 5–8 and 10–13 (dilationis plotted here as positive). These figures show clearly that a constant-volume condition was achieved at the end of test. All specimens ingroup-1 started to dilate after continued shearing regardless of theirsuction. For all specimens of group-1, an increase of net confining pres-sure reduces the dilation rate. In other words, during constant watercontent shearing, there was a tendency for reduced volume changesunder higher net confining pressures. This is largely attributed to thefact that higher net confining pressure caused a greater reduction ofthe pore spaces (i.e. compaction) during the preceding isotropic com-pression stage (such as Herkal et al., 1995; Adams et al., 1996). An ex-planation in terms of soil structure is that more closely packed particlesresult in reduced interconnections between pore spaces as well as re-duced porosity. The maximum value of dilation in specimens of group-1 occurred at specimens with initial water content equal to optimumwater content (i.e. 10.2%) at a given net confining pressure. Conversely,

in group-2, increasing water contents reduced the dilation rate and thespecimens with higher initial water content tended to compress more.In other words, the general trend for this group is that the higher thesuction, the greater was the dilation. This shows that initial density,matric suction and net confining pressure are all important factors inthe volume change of unsaturated soil.

4.4. Changes of suction or pore water pressure

Water is normally thought to have little tensile strength and maystart to cavitate when the magnitude of gauge pressure approaches−1 atm. As cavitation occurs, water phase becomes discontinuous,making the measurements of suction unreliable or impossible. Hilf(1956) introduced the axis-translation technique of elevating poreair pressure ua to increase porewater pressure to be positive, preventingcavitation in the water drainage system. Total stress, σ, is increasedtogether with air pressure of the same amount so the net stress,(σ−ua), remains unchanged. The variation of matric suction underconstant water content conditions during the shearing stage is investi-gated here. This variation is caused by the fact that pore-air pressureis maintained at a constant value while the pore-water pressurechanges continuously during shearing of the specimen. The (c) sectionof Figs. 5–8 and 10–13 show the variation of matric suction duringconstantwater tests for the two groupswith four different initialmatricsuctions (25, 50, 100 and 162 kPa) and three confining pressures (25,50 and 100 kPa). In most cases, the suction changes tend to stabilizeclose to the end of loading (i.e., 30% strain). Suction increases for allspecimens except for the highest matric suction in group-1 (i.e. G1-S162). For group-1, as shown in Fig. 8(c) the variation of matric suctionbecomes negative for thematric suction of 162 kPa, whereas for the ini-tialmatric suctions of 25, 50 and 100 kPa, the variation ofmatric suctionremains positive. It should be noted that a matric suction of 162 kPa




is close to the initial matric suction of the compacted specimens. Thisdecrease of suction did not occur in the tests of group-2 for which mat-ric suction increases to a relatively high value during shearing for alltests. The tests results indicate that the initial density of the specimenplays an important role in the variation ofmatric suction during loadingand this effect is particularly significant in the specimens with highestsuction value (i.e. 162 kPa). Also, in the first group of tests, as shownin Fig. 16(a) the relative variation of matric suction in the case of testsconducted at 25 kPa initial suction is larger than in those conducted atother values of suction for all confining pressures (i.e. 25, 50 and100 kPa). Conversely, as shown in Fig. 16(b), for specimens of group-2the largest percentage variation occurs for an initial suction 50 kPa atall values of confining pressure.

In all specimens of groups-1and 2, the variation of matric suction isdependent on the confining pressure. In addition, it can be seen fromFig. 16(a) that under the same initial matric suction condition, the varia-tion in matric suction for specimens of group-1 under higher net confin-ing pressures is more pronounced than for specimens under lower

confinements. In otherwords, in all specimens of this group, there is a di-rect relationship between the value of the variation ofmatric suction andthe net confining pressure. However, high net confining pressures duringconstant water content shearing lead to a reduction in the volume of thespecimen. Different results of relationship between variation value inmatric suction and net confining pressure are shown in Fig. 16(a) and(b), so that this relationship is direct for of group-1 and indirect forgroup-2. These different results show the importance of initial densityof unsaturated soil in pore water pressure change during undrainedloading. In other words, the variation of matric suction under constantwater content is more sensitive to the initial density of specimensthan to the net confining pressure and the initial matric suction.

Fig. 17 shows the relationship between volume change and matricsuction change of the specimens at critical state. As shown in Fig. 17(a),the magnitude of matric suction change generally decreased with in-creasing dilation rate for the specimens of group-1. But the specimensof group-2 show different behavior in this respect. According toFig. 17(b), there is direct relation between the magnitude of suction




change and increasing dilation rate or reducing soil compaction of spec-imens at critical state. This means that the initial density of the soil isa decisive factor in the variation of matric suction during fast loading.

Finally, the changes of pore water pressure in the saturated testsare investigated. Figs. 4(b) and 9(b) show the pore water change ofsaturated specimens during shearing. As shown in these figures, a rapidincrease of pore water pressure occurred at the start of loading. Thetest results indicate that the confining pressure has an important rolein the variation of pore water pressure during loading, so that excesspore water pressure increases as confining pressure increases.

5. Conclusions

Triaxial experiments have been performed to investigate the varia-tion of pore pressure, volume change and axial strain in unsaturatedsoils samples with different values of initial matric suction.

The matric suction was controlled by the axis translation tech-nique and volumetric behavior of specimens was measured by using

a double-walled triaxial cell. The tests include consolidated undrainedtests on saturated specimens and constantwater content tests onunsat-urated specimens. Based on results, the following conclusions can bededuced.

1. Soil suction does play a role toward increasing the shear strengthof an unsaturated soil and the shear strength increases as a resultof increasing matric suction. The test results indicate a non-linearrelationship between shear strength and matric suction.

2. The shear strength increases as net confining pressure increasesfor both saturated and unsaturated conditions. The increase inshear strength with respect to matric suction is smaller than theincrease with respect to the net normal stress.

3. Matric suction and net confining pressure have significant influ-ence on the volumetric behavior of soil. The volume change of anunsaturated soil during shearing is more sensitive to the confiningpressure compared to the initial matric suction. An increase of con-fining pressure reduces the dilation while matric suction does nothave univocal effect on volume change of soil.

Fig. 14. Relationship between peak shear strength and initial matric suction: (A) in the specimens of group-1; (B) in the specimens of group-2.


4. Results of constant water content triaxial tests show that the rela-tionship between pore water pressure and total volume changeduring shearing is more complicated than that found in saturatedundrained triaxial tests. In other words, the change in pore waterpressure during shearing is not directly related to the overall vol-ume change of the specimen.

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Fig. 16. Relationship between initial matric suction and suction change in critical state: (A) group-1; (A=B) group-2.


Fig. 17. Relationship between volume change and suction change for a given initial matric suction: (A) group-1; (B) group-2.


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Experimental evaluation of mechanical behavior of ...research.iaun.ac.ir/pd/bayat/pdfs/PaperM_9855.pdf · Testing of soil in unsaturated conditions is not common in soil mechanics