On the swelling potential of compacted high plasticity clays

Engineering Geology 104 (2009) 200–210

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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e n g g e o

On the swelling potential of compacted high plasticity clays Valéry Ferber a,⁎, Jean-Claude Auriol a, Yu-Jun Cui b,1, Jean-Pierre Magnan c,2 a

Laboratoire Central des Ponts et Chaussées, Division for Soil Mechanics and Site Survey, Route de Bouaye, BP4129, 44341 Bouguenais Cédex, France Centre d'Enseignement et de recherche en Mécanique des Sols, Ecole Nationale des Ponts et Chaussées, 6 et 8, avenue Blaise Pascal, Cité Descartes-Champs-sur-Marne, F 77455 Marne-la-Vallée Cedex 2, France c Laboratoire Central des Ponts et Chaussées, 58, Bd Lefebvre, 75732 PARIS Cédex 15, France b

a r t i c l e

i n f o

Article history: Received 29 November 2007 Received in revised form 13 October 2008 Accepted 27 October 2008 Available online 5 November 2008 Keywords: Clays Embankments Expansive soils Laboratory tests Microstructure Partial saturation

a b s t r a c t In earthworks engineering, predicting the influence of the water content and the dry density on compacted soils deformation is a fundamental issue, which is particularly important for the use of clays in embankments. This paper presents the experimental results and a microstructural interpretation of swelling tests performed on four clays, compacted at different water contents and dry densities. The influences of dry density and water content on the swelling potential are described, showing the coupled effect of these two parameters. In order to quantify this coupling, the initial air void ratio is defined and it is observed that this air void ratio has an apparent linear relationship with the void ratio after swelling. This observation enables analysing all the tests in a synthetic manner, by plotting the origin ordinate and the slope of the linear relationships versus the initial hydrous state. Moreover, one of the clays microstructure was analysed using mercury intrusion porosimetry (MIP). The results show that swelling leads to a micropores increase and a macropores decrease. In addition, micropores volume increase does not depend on the initial dry densities, whereas the macropores volume increase does. For further discussion, a simplified microstructural model is proposed allowing the interpretation of the soil swelling using MIP results. The comparison between the model prediction and the microstructure observation supports the interpretation of the swelling tests and shows that the origin ordinate and the slope of the relationship between the initial air void ratio and the final void ratio can be linked to micropores and macropores volume variations. However, the comparison also shows that the simplified microstructural model presents some limitations when applied to very dry or wet soils. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In earthworks engineering, using all kinds of natural materials in embankments is an important economical and environmental issue. From this point of view, one of the main problems is the use of clayey materials, which are known as being very sensitive to water content variations. It has been shown that such materials can be the source of short-term and long-term disorders in road or railways structures (Auriol et al., 2000), thus generating non negligible maintenance costs. The swelling behaviour of these materials in embankments slopes is one of the phenomena related to these disorders. One of the characteristics of road and railways embankments is the significant heterogeneity in terms of dry density, water content and soils nature. Thus it is important to assess the influence of these parameters on the deformation behaviour of embankments. Holtz and Gibbs (1956) presented a study which highlighted the influential parameters on swelling potential of soils. The subsequent works on ⁎ Corresponding author. Tel.: +33 2 40 84 57 85; fax: +33 2 40 84 59 97. E-mail address: [email protected] (V. Ferber). 1 Tel.: +33 1 64 15 35 50. 2 Tel.: +33 1 40 43 52 60. 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.10.008

Table 1 Geotechnical and physico-chemical characteristics of the clays AMign — C1 AVLGV — C2 AVA34 — C3 ATours — C4 C400 µm (%) C80 µm (%) C2 µm (%) Liquid limit, wL (%) Plastic limit, wP (%) Plasticity index, Ip French soil classification Methylene blue absorption value (g/100g) Clay fraction specific surface (m2/g) (Eq. (1)) Density of solid particles, ρs (Mg/m3) Std proctor optimum water Content content (%) Std proctor optimum dry density (Mg/m3) Smectites (%) Illite (%) Kaolinite (%) Other clay minerals

94.5 82.4 70 55.9 21.7 34.2 A3 5.1

98 94.8 76.1 96.1 38.8 57.3 A4 8.1

100 99 66 98.1 37.1 61.1 A4 10.7

88 84 63 85.5 43.3 42.2 A4 12.9

152

223

339

421

2.7

2.71

2.72

2.73

24.5

38

28

38.4

1.58

1.28

1.44

1.29

– 35 65 –

– 45 – 55

60 40 – –

65 30 – 5

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results are used to elaborate the relationship between soils microstructure and swelling deformation. 2. Materials and procedure 2.1. Geotechnical characteristics of the clays For this study, four plastic sedimentary clays from the secondary to tertiary eras were taken in earthwork sites in different areas of France. These materials are characterized by a liquid limit higher than 50 and plasticity index higher than 30 (Table 1). Their high plasticity properties can be explained by their high clay fractions (N60%) and the presence of active clay minerals such as smectites and palygorskite. The clay fraction specific surface calculated based on the methylene blue value (Eq. (1), after Cuisset, 1980), is consistent with the clays mineralogy since the clays C3 and C4, which contain smectites, have the largest clay fraction specific surface. Fig. 1. Results of the standard Proctor tests performed on the four tested clays.

Ss;2μm = this subject confirmed this influence (Seed et al., 1962a,b; Grim, 1962; Cox, 1978; Komine and Ogata, 1996; Derriche and Kebaili, 1998; Al Shaeya, 2001). Furthermore, the chief role of soils microstructure in the phenomenon has been evidenced (Gens et al., 1995). This explains the introduction of the microstructure effects in some constitutive models (Gens et al., 1995; Alonso et al., 1999). In summary, the studies in this field have showed that soil swelling deformation depends mainly on the following parameters: -

the nature of soils, namely the clay fraction, the mineralogy, etc.; the dry density; the water content; the amount of water intake.

The aim of the present work is to study soils swelling properties under different initial conditions, by performing swelling tests in oedometer on four high plasticity clays compacted at various water contents and dry densities. These tests were completed by microstructure investigations using MIP on one of the four clays, in order to describe the swelling phenomenon at a microscopic scale. All these

20:93MBV C2μm

ð1Þ

where Ss,2µm is the clay fraction specific surface, MBV is the methylene blue value (in g/100 g) and C2µm is the clay content. As a result of the high plasticity indexes, the optimum moisture contents (OMC) are higher than 20%, almost reaching 40% for clays C2 and C4 (Fig. 1). These high values of OMC are associated with low values of maximum dry density (MDD), particularly for clays C2 and C4 (MDD lower than 1.3 Mg/m3). Nevertheless, it can be seen that OMC is not directly correlated with plasticity index since the OMC of C2 (OMC = 38%) is clearly higher than the OMC of C3 (OMC = 28%), despite a similar value of plasticity index (respectively 57.3 and 61.1). 2.2. Experimental procedure 2.2.1. Swelling tests All the clays were air-dried for about one week. They were then crushed and sieved at 2 mm. Finally, they were moistened to different water contents and kept in waterproof bags for more than 2 days prior to use.

Fig. 2. Characteristics of the miniaturized Proctor rammer.

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Table 2 Dry densities of the C3 soil specimens submitted to the MIP tests (w = 21%)

Before swelling Before swelling Before swelling After swelling After swelling After swelling

Dry density after compaction (direct measurement)

Dry density after swelling (direct measurement))

1.29 1.37 1.46 1.25 1.34 1.43

– – – 1.01 1.08 1.11

The specimens were compacted using a miniaturized Proctor rammer (Fig. 2) in a conventional oedometric mould. The compaction was applied by series of seven blows, in order to cover the whole surface of the specimen (Fig. 2-C). The compaction energy was controlled by modifying the number of series of seven blows. The most compacted specimens were submitted to eight series of seven blows and the less compacted to two. Consequently, the controlled parameter is not the dry density but the compaction energy. At the OMC and beyond, it was difficult to obtain a large range of dry densities. On the contrary, at low water contents, dry densities generally range between 80 and 100% of the MDD (Appendices 1–4). The swelling tests were performed in conventional oedometric devices, under a 3 kPa vertical stress. After a short time (not more than one hour) under the 3 kPa stress, the initial height of specimens was measured and then, de-ionised water was poured in the oedometric

cell. Specimens deformation was followed by the means of conventional displacement sensors. For each soil, the swelling tests were performed at four different initial water contents (Appendices 1–4), ranging generally from 50% to 110% of the OMC (40% of the OMC minimum for the clay C2). These different hydric states are supposed to lead to different microstructure: flocculated for soils dry of the OMC, dispersed for wet soils. Each series was characterized by a given water content and different dry densities resulting from the different compaction energies (Appendices 1–4). 2.2.2. Mercury Intrusion Porosimetry Mercury Intrusion Porosimetry (MIP) tests were performed only on specimens of clay C3, compacted at a water content of 21%, because of an imposed limitation of the number of tests. Prior to MIP observation, the specimens were lyophilized using the freeze-drying method which has been regarded as the most effective for soils microstructure preservation (Delage and Pellerin, 1984). The analyses were performed in two stages, corresponding to the different pressure ranges: - a low pressure phase (below atmospheric pressure), which corresponds to pore diameters ranging from approximately 9 to 200 µm; - a high pressure phase (above atmospheric pressure), which corresponds to pore diameters ranging from 7 nm to 9 µm.

Fig. 3. Influence of initial dry density and water content on the compacted clays swelling deformation.

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In order to describe the microstructure changes due to swelling, the MIP tests were performed on: - three specimens compacted at three different dry densities (1.29, 1.37 and 1.46 Mg/m3) (specimens named “before swelling”); - three specimens compacted at the same water content and different compaction energies as mentioned above (leading to dry densities of 1.25, 1.34 and 1.43 Mg/m3) and then submitted to swelling tests (specimens named “after swelling”). The dry densities of specimens were measured after compaction and, for the concerned specimens, after swelling (Table 2).

more significant than at high water contents. In the latter case, the dry density has only a slight influence. These observations confirm the complex coupled influence of water content and dry density on the swelling potential. For further description of the swelling behaviour, the void ratio of specimens after swelling was plotted versus the void ratio before swelling (Fig. 4). In this diagram, the ordinate (void ratio after swelling) represents the final state of specimens, independently of the initial state, contrarily to the previous diagrams (Fig. 3) where the swelling potential depends on both the initial and the final state (Eq. (2)). SP =

3. Experimental results 3.1. Swelling tests For each clay, the swelling deformation was plotted versus the initial dry density for different initial water contents (Fig. 3). These data are consistent with the results reported by Seed et al. (1962b), Komine and Ogata (1996) and Cox (1978), i.e.: - at a given initial water content, the higher the dry density, the larger the swelling potential; - at a given dry density, the lower the initial water content, the larger the swelling potential. It can be also observed that the influence of the dry density on the swelling potential depends highly on the initial water content: at low water contents, the increase in swelling potential due to dry density is

203

Δe 1 + e0

ð2Þ

where SP is the swelling potential, ei is the initial void ratio and Δe the void ratio change due to swelling. In this second diagram (Fig. 4), a linear relationship can be observed between the void ratio after swelling and the void ratio before swelling for a given initial water content. In other words, if one specimen is denser before swelling, it will remain denser after swelling. This also means that the void ratio before swelling partly controls the void ratio after swelling. But it can also be seen that, at a given initial void ratio, the lower the initial water content, the larger the void ratio after swelling. This indicates that the final void ratio is controlled by both the initial void ratio and the initial water content. In order to take into account this coupled influence of initial void ratio and water content, the void ratio after swelling was plotted versus the initial air void ratio (Fig. 5). As indicated by Eq. (3), air void

Fig. 4. Influence of initial void ratio on void ratio after swelling.

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ratio is defined by both the void ratio and the water content. In this new diagram a linear relationship is observed for a given water content. This linear relationship can be considered as the same than that in Fig. 4, since air void ratio and void ratio are directly linked (see Eq. (3)). But at a given initial air void ratio, the influence of initial water content is not as significant as in the previous graph. For clays C1 and C2, there is still an influence of initial water content: the higher the initial water content, the larger the final void ratio. But for the clays C3 and C4, the series are almost merging. eair =

Vair w = e−ρs : ρw Vs

ð3Þ

ratio of initial water content to the OMC, and that the trends are similar between clays C2, C3 and C4. For these four clays, the slope reaches a value of approximately unity for a ratio of unity, and a slope of approximately 0.6 for a ratio of 0.6. All the values are lower than unity except for clay C1. The origin ordinate seems to be insensitive to the initial water content, as opposed to the slope. In this regard, a clear difference appears between clay C1 and the three others. The plot of the average origin ordinate versus the liquid limit for each clay (Fig. 7) suggests that the origin ordinate could be mainly controlled by the soils clay plasticity properties. 3.2. Mercury Intrusion Porosimetry

Since a linear relationship is observed between initial air void ratio and final void ratio at a given water content, linear regression parameters were determined for the four clays. The regression coefficient is bigger than 98% for the two intermediate water contents, but lower for the very dry and very wet series. The particular difficulty of preparing identical dry specimens (especially for clay C1, at a water content of 10.1%) and the limited range of void ratios for wet specimens obtained by dynamic compaction (see Section 2.2.1) could explain the low values of regression coefficient. The origin ordinate and the slope of the regression lines were plotted versus the ratio between the initial water content and the OMC (Fig. 6), which expresses the hydration state of specimens. It can be seen that the slope of the linear relationship increases with the

The intrusion curves of the MIP tests (Fig. 8) give the mercury cumulative intrusion volume by dry mass of soil in the pores, from the largest pores to the smallest ones. Consequently, the cumulative intrusion increases when the pore diameter decreases and the incremental intrusion between two pore diameters gives the volume of the pores of this size range. This incremental intrusion (dV), divided by the variation of the logarithm of the diameter (dLogD) is plotted versus the logarithm of the diameter, which is called the pore size distribution (PSD). The PSD curves are given here for the specimens before swelling (Fig. 9) and after swelling (Fig. 10). The void ratios calculated on the basis of the cumulative intrusions of mercury (Eq. (4)) were compared to the void ratios of specimens,

Fig. 5. Influence of initial air void ratio on void ratio after swelling (eair,i/ef diagram).

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Fig. 8. Mercury intrusion porosimetry tests performed on the clay C3, before and after swelling on specimens compacted at three different dry densities.

MIP tests which cannot fully represent the big specimen used for swelling tests. eMIP = e=

Vintrusion = ρs Vintrusion Vs

ρs −1 ρd

ð4Þ ð5Þ

From the cumulative curves (Fig. 8), it can be seen that:

Fig. 6. Influence of initial water content on slope (A) and origin ordinate (B) of linear relationships in the eair,i/ef diagram.

- the total intrusions of the specimens after swelling is higher than that of specimens before swelling, which illustrates the pore volume increase due to the swelling process; - for the specimens before and after swelling, the higher the dry density, the higher the cumulative intrusion, showing that the cumulative intrusion is linked to the void ratio; - the intrusion curves seem to be close from each other in the range of pore diameter from 5 to 200 µm. On the contrary, a clear difference is observed between the specimens before and after swelling in the small diameter range. These observations can be detailed using the PSD curves:

calculated on the basis of the measured dry density (Eq. (5), Fig. 11). This comparison shows that MIP tests clearly underestimate the void volumes. This can be explained by the very small soil volume used for

Fig. 7. Influence of the liquid limit on the average value of the origin ordinate.

- before swelling, the distribution of micropores (from 7 nm to 10 µm diameter) is similar for the three dry densities, whereas the distribution of macropores (from 10 to 200 µm) is clearly influenced by the dry density. The increase of initial dry density

Fig. 9. Pore size distribution of the clay C3, compacted at three different dry densities (w = 21%).

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V. Ferber et al. / Engineering Geology 104 (2009) 200–210

leads to a decrease of the global volume of macropores and to their average diameter; - after swelling, the distribution of micropores (from 7 nm to 10 µm diameter) is also similar for the three dry densities, even though a slight discrepancy is observed for the most compacted specimen. The macropores (from 10 to 200 µm) volume and mean diameter are still the largest for the loosest specimens. 4. Discussion According to many authors (Olsen, 1962; Diamond, 1969; Sridharan et al., 1971; Tessier, 1984; Delage et al., 1996; Alonso et al., 1999; Wan et al., 1995; Delage et al., 2006; Sivakumar et al., 2006), compacted clayey soils are characterized by common features in terms of microstructure, which can be summarized as follows: - clay particles, which constitute clayey soils, naturally gather in aggregates and the voids inside and outside aggregates are respectively described as micropores and macropores as observed in the PSD (Fig. 9); - the compaction process mainly leads to a decrease of the macropores volume (Fig. 9), even though micropores and particles orientation can also be modified by compaction, particularly in soils compacted beyond the optimum water content (Cetin et al., 2007). This is also related to the evolution of microstructure with water content, from a flocculated to a dispersed microstructure. At low water contents, it can be seen that micropores volume is constant, and the decrease of global void ratio with compaction would be almost exclusively the consequence of the macropores volume decrease (Fig. 9). Moreover, since compaction of dry soils is a process of air expulsion and macropores volume reduction, the macropores in soils compacted at a water content lower than the OMC is probably mainly filled by air. Water would be located mainly, or perhaps even exclusively, inside aggregates, i.e. in micropores, where suction are the highest because of the small size of pores (capillary component of the suction) and of the physico-chemically active clay particles (adsorption component of the suction). Obviously, this statement is not relevant beyond the optimum water content, where aggregates are certainly saturated and the macropores are probably partly filled by free water. One important remaining point is whether aggregates are saturated. If the aggregates in dry specimens were saturated, the micropores should contain only water and the air volume should be only located in macropores. In other words, the macropores void ratio should be equal to the air void ratio.

Fig. 10. Pore size distribution of the clay C3, after the swelling tests, compacted at three different dry densities (w = 21%).

Fig. 11. Confrontation between void ratio measured before freeze-drying (“e specimen”) and void ratio estimated by the MIP cumulative intrusion (“eMIP”) for soil C3.

In order to clarify this point, two basic hypotheses are made for the discussion: 1) before swelling, the aggregates are saturated; 2) all the water is located in micropores and, consequently, the macropores void ratio, eM, can be quantified using air void ratio (Eq. (6)). As the total void ratio is equal to the sum of micropores and macropores void ratios (Eq. (7)), the micropores void ratio can be quantified using Eq. (8). eM;i = eair;i = ei −ρs : ei = eM;i + em;i

em;i = ρs :

wi ρw

wi ρw

ð6Þ ð7Þ

ð8Þ

On the basis of these hypotheses, the origin ordinate of the observed linear relationship between air void ratio before swelling and void ratio after swelling (Fig. 5) would correspond to the void ratio of a specimen with no macropores void ratio. Such a specimen would consist exclusively of perfectly arranged aggregates, i.e. aggregates with no micropores. Consequently, the origin ordinate would correspond to the void ratio of aggregates after swelling or, in other words, the micropores void ratio after swelling. Since the initial water content has only a limited influence on the origin ordinate (Fig. 6-B), and that the micropores pore size distribution is little influenced by the initial void ratio (Fig. 9), it can be suggest that the micropores void ratio after swelling could be an intrinsic parameter, which would express the intrinsic swelling potential of aggregates themselves. This idea is supported by the influence of the liquid limit on the average value of the origin ordinate (Fig. 7). Eq. (9) can be used to express the linear relationship observed between the air void ratio before swelling (or macropores void ratio before swelling, according to the assumptions of this discussion) and the void ratio after swelling, where α and β are, respectively, the slope and the origin ordinate of the linear relationship. Moreover, the void ratio after swelling, ef, can still be expressed as the sum of the micropores and macropores void ratios, after swelling (Eq. (10)). Consequently, the micropores void ratio after swelling would correspond to the origin ordinate (Eq. (11)), which leads to Eq. (12), where the macropores void ratio after swelling is proportional to the macropores void ratio before swelling, with a factor equal to the slope

V. Ferber et al. / Engineering Geology 104 (2009) 200–210

of the linear relationship. In other words, the slope α of the observed relationship would be used to quantify the remaining proportion of macropores volume after a swelling test. The analysis of the linear relationships for the four tested soils (Fig. 6-A) showed that the slope is generally lower than 1, which would mean that swelling generally leads to a macropores volume decrease. Moreover, the slope generally decreases with the increase in initial water content, which suggests that the remaining proportion of macropores after swelling is lower in dry specimens than in specimens compacted close to the OMC. ef = α:eM;i + β

ð9Þ

ef = eM;f + em;f

ð10Þ

em;f = β

ð11Þ

eM;f = α:eM;i

ð12Þ

In order to evaluate these ideas, the micropores and macropores void ratios (respectively em and eM) were determined (Tables 3 and 4) using cumulative intrusion volumes (Eqs. (13) and (14)) and plotted versus the initial void ratio (Fig. 12). A value of 10 µm was chosen as the threshold pore diameter between micropores and macropores (Fig. 9). em =

Vmicropores V7nm −V10μm = Vs Vs

ð13Þ

eM =

VMacropores V10μm = Vs Vs

ð14Þ

207

Table 4 C3 soil micropores and macropores void ratios after swelling according to MIP tests (“MIP”) and according to microstructural equations (“model”) ρd,i

ei

ef

ef “MIP”

em,f “MIP”

em,f “model” Eq. (11)

eM,f “MIP”

eM,f “model” Eq. (12)⁎

1.25 1.34 1.43

1.176 1.030 0.902

1.015 1.077 1.111

1.437 1.390 1.300

1.211 1.193 1.106

1.212 1.212 1.212

0.226 0.197 0.194

0.393 0.299 0.214

⁎α = 0.8, according to the results on clay C3 (Fig. 6), with wi/wOPN = 21/28 = 0.75.

The following remarks can be made concerning the basic hypotheses and the resulting equations (Eq. (8) to (14)): - these hypotheses enable a quantitative microstructural analysis of the swelling tests results. The comparison of the results of this analysis with MIP results (Fig. 12) confirms that, before swelling, the compacted clay can be considered as an arrangement of aggregates and that the macropores void ratio controls the global void ratio. After swelling, the conclusions of the theoretical microstructural approach are only partly confirmed by the MIP tests: the micropores void ratio increases and does not depend much on the initial void ratio, whereas the macropores void ratio decreases but does not seem to be linearly dependent on initial void ratio as suggested by the model; - the elaborated model is based on the ideas that, before swelling, aggregates are saturated and the whole macropores volume is

where V7nm and V10µm are, respectively, the cumulative intrusion volumes for pore diameters of 7 nm and 10 µm, given by the MIP tests. These data are compared to micropores and macropores void ratios calculated using the equations presented above (Tables 3 and 4) (Fig. 12). This comparison shows that: - at a given initial void ratio, the MIP tests seem to confirm the predicted decrease of the macropores void ratio due to swelling (Fig.12-B). However, this decrease is not as significant as predicted by the model. More generally, the model over-estimates the macropores void ratios observed in the MIP tests, which could be explained by the under-estimation of the global void ratio by MIP intrusion volume (Fig. 11); - though the macropores void ratio before swelling seems to increase linearly with the void ratio (Fig. 12-B), this linear relationship does not remain clear after swelling, as opposed to the model prediction. Consequently, the idea that the macropores volume decrease is a proportion of the initial macropores volume (Eq. (12)) is not clearly evidenced by these data; - the micropores void ratios before swelling given by the MIP are constant, as predicted by the model; but they are not rigorously constant after swelling. It is slightly lower for the most compacted specimens. However, the micropores void ratios after swelling are of the same order, which would support the idea that micropores void ratio after swelling does not depend much on initial void ratio.

Table 3 C3 soil micropores and macropores void ratios before swelling according to MIP tests (“MIP”) and according to microstructural equations (“model”) ρd,i

ei

ei “MIP” em,i “MIP” em,i “model” Eq. (8) eM,i “MIP” eM,i “model” Eq. (6)

1.29 1.105 0.731 1.37 0.978 0.657 1.46 0.862 0.621

0.442 0.445 0.449

0.571 0.571 0.571

0.282 0.212 0.180

0.534 0.407 0.291

Fig. 12. Micropores (A) and macropores (B) void ratios calculations for soil C3, before and after swelling, according to MIP tests (“MIP”) and according to microstructural equations (“model”).

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V. Ferber et al. / Engineering Geology 104 (2009) 200–210

filled by air. These two assumptions are probably not applicable in all cases. In particular, for wet specimens, i.e. for water contents close or beyond the OMC, it is possible that the macropores volume contains free water and, consequently, the air void ratio is lower than the macropores void ratio (Eq. (15)), with δ N 0). Eqs. (15) and (9) lead to Eq. (16), which would explain the slope values bigger than that obtained for the wettest series of specimens (Fig. 6). On the other hand, in very dry specimens, it is possible that aggregates are not saturated and, consequently, that air void ratio is larger than the macropores void ratio (Eq. (15), with δ b 0). In this case, the ratio between macropores void ratio after swelling and macropores void ratio before swelling, given by the slope α, is probably over-estimated, which means that the remaining proportion of macropores volume would be lower than the slope value (Eq. (17), with δ b 0). This could partly explain the small values of macropores void ratio after swelling given by MIP tests (Fig. 12); - finally, the idea that the micropores void ratio after swelling is independent of the initial conditions (Eq. (13)) has not been rigorously demonstrated here, since a little influence of void ratio on the final micropores void ratio was observed in MIP tests (Fig. 12), particularly for the most compacted specimens. However, the mean value of origin ordinates calculated from the swelling tests (Fig. 6-B) seems to be controlled by the liquid limit (Fig. 7), which suggests that the origin ordinate could be an indicator of the aggregates swelling potential. eair;i = ð1 + δÞeM;i

ð15Þ

α eM;i + β 1+δ

ð16Þ

α e 1 + δ M;i

ð17Þ

ef =

eM;f =

Thus, the main contribution of this study and the proposed approach is to show that compacted clays microstructure evolution during wetting can be quantitatively evaluated simply by adopting a simple microstructure model. This model, based on the idea that macropores volume can generally be estimated by air volume, also suggest explanations on the previously observed coupled influence of water content and dry density, which control the microstructure after compaction. 5. Conclusions Conventional swelling tests performed on four clays confirmed the coupled influence of the initial water content and dry density on the swelling potential of compacted clayey soils. The analysis of these data with different parameters, such as void ratio or air void ratio, showed a linear relationship between the air void ratio before swelling and the void ratio after swelling. MIP tests performed on specimens of one of the four clays suggested that the coupled influence of water content and dry density is related to the microstructure changes. In addition, the MIP observation showed that swelling leads to a micropores increase and a macropores decrease. These observations led to establish quantitative relationships between micropores/macropores void ratios and the conventional void ratio and water content. On the basis of these relationships, the swelling tests results were used to quantify theoretically micropores and macropores void ratios before and after swelling. The comparison of the calculated results with the MIP tests results partly supports the adopted analysis approach (Fig. 13), namely: - the void ratio controls the macropores volume after compaction; - the water content controls the micropores volume after compaction;

Fig. 13. Schematic interpretation of the slope and the origin ordinate of the linear relationship in the eair,i/ef diagram.

- swelling leads to a macropores void ratio decrease, which could be estimated by the slope of the linear relationship between initial air void ratio and void ratio after swelling. The slope would quantify the remaining proportion of macropores volume after swelling; - swelling is a consequence of a micropores void ratio increase, which can be roughly estimated by the origin ordinate of the linear relationship between initial air void ratio and void ratio after swelling. The conclusions of this analysis are consistent with the idea that reversible volume changes in clayey soils are linked to deformation of micropores, whereas irreversible volume changes are the results of macropores volume changes (Alonso et al., 1999). However, these conclusions must be moderated at least by the two following limitations: - the hypotheses stating that aggregates are saturated before swelling and that the macropores void ratio can be quantified by the air void ratio are probably not relevant in the case of very dry specimens and specimens compacted at a water content beyond the OMC. It seems that the equations lead to consistent results for water contents ranging from 80% to 100% of the OMC; - it is suggested in the discussion that the origin ordinate of the linear relationship could be an intrinsic parameter, independent of initial water content and dry density. Even though this idea is supported by the influence of the liquid limit on the origin ordinate, the MIP tests and the influence of the initial water content on the origin ordinate indicate that this micropores void ratio after swelling could also be influenced by the initial conditions. In particular, the micropores void ratio after swelling, as determined by MIP, was lower for the most compacted specimens than for the two others. However, the origin ordinate could possibly give a first order estimation of the micropores void ratio after swelling. These limitations show that complementary swelling tests and microstructure investigations should be performed in order to answer the remaining question on the microstructural approach suggested in the discussion part. In particular, MIP tests on specimens compacted at different water contents, before and after swelling, and investigations on the saturation of aggregates after compaction would be helpful.

V. Ferber et al. / Engineering Geology 104 (2009) 200–210

In terms of practical applications, these results show that irreversible volume changes occur preferentially in soils compacted dry of the OMC. This is not only the consequence of the low dry density of these materials (because they are difficult to compact) but it was seen here that the structure of these materials at the microscopic scale is also less stable than specimens compacted at close to the OMC. Thus as a conclusion, this study highlights once more the importance of compaction of clayey materials in embankments, in order to limit the possible irreversible deformations due to wetting. Notations e global void ratio. eair air void ratio. em micropores void ratio. EM macropores void ratio. α slope of the linear relationship between final void ratio and initial air void ratio. β origin ordinate of the linear relationship between final void ratio and initial air void ratio. OMC Optimum Moisture Content MDD Maximum Dry Density

Appendix 1. Characteristics and results of the wetting tests on Clay C1 (AMign) Series

wi (%)

wmoy/ wOPN

ρd,i/MDD (%)

ei

Sri (%)

Swelling deformation (%)

wf (%)

ef

1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4

10.4 11.1 9.9 9.2 10.2 16.7 16.6 16.5 16.6 16.8 16.6 20.9 21.4 21.1 21.4 21.2 22.2 25.2 25.5 25.8 25.6 25.7 26.1

41.4

89.5 85.9 87 90.4 83 97.2 91.9 89.9 85.8 81.2 78.4 107.6 105.1 101.8 101.8 95.9 89.2 100.5 98.8 99.3 98.9 98.3 98.1

0.890 0.910 0.964 0.988 1.058 0.758 0.860 0.900 0.991 1.105 1.179 0.589 0.625 0.678 0.679 0.781 0.916 0.701 0.729 0.720 0.727 0.738 0.742

30.8 30.1 28.4 27.7 25.9 59.2 52.2 49.8 45.3 40.6 38 76.2 71.7 66.1 66.1 57.4 49 64 61.5 62.3 61.7 60.8 60.4

6.8 7.9 4.4 6.9 5.7 7.4 6.8 6.2 5.6 3.7 1.7 2.8 2.4 2.7 4.5 3 2.6 1 0.8 0.8 0.8 0.6 0.6

35.0 36.7 35.6 37.2 36.4 29.9 32.8 32.7 33.5 38.3 40.0 23.2 23.9 24.9 24.9 27.2 31.2 25.8 26.2 26.4 26.4 26.4 26.9

1.017 1.062 1.050 1.126 1.176 0.889 0.986 1.019 1.102 1.182 1.216 0.633 0.664 0.724 0.755 0.834 0.966 0.718 0.743 0.734 0.742 0.749 0.753

0.68

0.87

1.05

Appendix 2. Characteristics and results of the wetting tests on Clay C2 (AvLGV) Series

wi (%)

wmoy/ wOPN

ρd,i/MDD (%)

ei

Sri (%)

Swelling deformation (%)

wf (%)

ef

1 1 1 1 1 1 2 2 2 2 2 2

22.1 21.6 21.5 21.5 21.8 21.5 29.1 29.1 29.1 28.7 29.2 29.0

0.57

98.4 94.1 90.4 90.6 86.9 82.9 95 92 89.9 88.3 83.4 79.3

1.152 1.250 1.342 1.337 1.437 1.553 1.229 1.301 1.354 1.398 1.538 1.671

50.9 47.0 43.7 43.9 40.8 37.8 64.0 60.5 58.1 56.3 51.2 47.1

18.8 15.4 13.3 13.1 11.3 8.9 14.7 13.1 11.8 12.3 9.7 4.1

57.1 57.7 57.8 58.0 59.1 60.6 56.3 57.4 58.0 58.4 61.2 62.0

1.557 1.597 1.654 1.643 1.714 1.782 1.557 1.604 1.632 1.694 1.783 1.782

0.76

209

Appendix 2 (continued) Series

wi (%)

wmoy/ wOPN

ρd,i/MDD (%)

ei

Sri (%)

Swelling deformation (%)

wf (%)

ef

3 3 3 3 3 3 4 4 4 4 4 4

34.7 34.7 35.2 35.0 35.2 35.1 41.9 40.9 41.3 41.0 41.0 40.9

0.92

98.8 95 91.7 88.2 84.3 79.2 98.8 98.8 97.6 94.3 91.2 84.2

1.142 1.229 1.309 1.400 1.510 1.673 1.144 1.142 1.170 1.245 1.322 1.515

68.9 64.0 60.1 56.2 52.1 47.0 68.8 68.9 67.3 63.2 59.5 51.9

12.2 10.8 9.6 9.5 8.2 6.6 4.1 4.7 4.2 4.8 4.5 4.5

50.8 51.7 53.1 55.5 57.7 60.3 45.0 45.2 45.6 47.7 49.0 54.0

1.403 1.469 1.531 1.628 1.717 1.849 1.231 1.242 1.261 1.352 1.425 1.628

1.08

Appendix 3. Characteristics and results of the wetting tests on Clay C3 (AvA34) Series

wi (%)

wmoy/ wOPN

ρd,i/MDD (%)

ei

Sri (%)

Swelling deformation (%)

wf (%)

ef

1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4

20.5 20.2 20.1 20.3 20.1 19.9 24.5 24.7 24.3 24.3 24.8 24.7 28.7 28.6 28.8 28.3 28.4 29.2 32.5 32.1 32.3 32.1 31.7 32.2

0.72

100 98.5 96.8 94.7 91.9 83.4 101.2 99.1 97 94.4 91.8 81.7 102.7 99.8 98.1 96.8 92.8 82 99 99 98.2 97.8 96.2 85.6

0.882 0.911 0.945 0.988 1.047 1.257 0.859 0.900 0.940 0.994 1.050 1.305 0.832 0.885 0.919 0.943 1.027 1.294 0.900 0.902 0.917 0.925 0.957 1.198

62.0 60.1 57.9 55.4 52.3 45.7 77.5 74.0 70.9 67.0 63.4 51.0 93.4 87.8 84.6 82.4 75.7 60.1 96.7 96.6 94.9 94.2 91.0 72.7

30.4 28.9 26.5 25.8 25.6 21.1 24.6 24.4 23.4 23.1 21.8 18.4 19.0 21.0 19.3 19.2 18.1 15.2 12.9 14.1 13.3 14.2 13.1 11.1

51.6 53.1 52.7 53.9 54.9 60.2 48.5 49.9 49.8 51.4 52.6 58.9 45.6 47.0 46.7 47.3 49.4 55.7 42.2 42.8 42.7 43.5 43.8 48.5

1.454 1.463 1.460 1.501 1.572 1.734 1.317 1.363 1.393 1.454 1.497 1.729 1.181 1.281 1.288 1.316 1.394 1.642 1.145 1.171 1.173 1.197 1.214 1.443

0.88

1.02

1.15

Appendix 4. Characteristics and results of the wetting tests on Clay C4 (ATours) Series

wi (%)

wmoy/ wOPN

ρd,i/MDD (%)

ei

Sri (%)

Swelling deformation (%)

wf (%)

ef

1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4

25.0 25.3 25.3 25.1 25.1 24.7 31.7 31.9 31.9 32.0 31.6 32.1 35.5 35.4 35.6 35.6 35.4 35.1 43.0 42.2 44.1 44.2 43.6 43.9

0.65

93.6 89.5 87 81.4 81.8 76.1 100.8 96.4 90.2 89.1 83.7 77.1 102.8 101.3 98 96.6 91.1 87.9 94.8 95.4 92.1 93.4 92.7 92.2

1.260 2.364 1.433 1.600 1.586 1.780 1.099 1.195 1.346 1.374 1.528 1.738 1.058 1.090 1.159 1.192 1.322 1.408 1.231 1.219 1.297 1.266 1.284 1.296

54.3 50.2 47.8 42.8 43.2 38.5 79.1 72.7 64.6 63.3 56.9 50.0 91.4 88.7 83.4 81.1 73.1 61.8 96.5 97.4 91.6 93.8 92.5 91.6

15.3 15.8 13.9 12.7 11.2 7.2 11.5 13.0 11.0 10.6 9.8 6.1 8.4 9.2 9.1 9.2 9.2 7.8 2.3 1.8 2.4 1.3 1.4 1.7

56.4 57.8 59.6 64.2 61.8 64.2 47.6 50.1 53.0 54.3 57.4 60.8 44.3 44.9 46.5 47.8 51.0 52.3 45.5 44.9 46.9 45.6 45.3 46.0

1.605 1.738 1.770 1.929 1.874 1.980 1.099 1.195 1.346 1.374 1.528 1.738 1.231 1.282 1.356 1.393 1.536 1.596 1.282 1.259 1.352 1.295 1.317 1.334

0.83

0.92

1.13

210

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References Al Shaeya, N., 2001. The combined effect of clay and moisture content on the behaviour of remoulded unsaturated soils. Engineering Geology 62 (no. 4), 319–342. Alonso, E.E., Vaunat, J., Gens, A., 1999. Modelling the mechanical behaviour of expansive clays. Engineering Geology 54 (no. 2), 173–183. Auriol, J.C., Havard, H., Mieussens, C., Queyroi, D., 2000. Résultats d'enquêtes sur la pathologie des remblais en service. Routes/Roads no. 306, 57–74. Cetin, H., Fener, M., Soylemez, M., Gunaydin, O., 2007. Soil structure changes during compaction of a cohesive soil. Engineering Geology 92, 38–48. Cox, D.W., 1978. Volume change of compacted clay fill. Clay fills, Proceedings of the conference held at the institution of Civil Engineers, 14–15 nov 1978, London, pp. 79–86. Cuisset, O., 1980. Propriétés électrocinétiques des particules argileuses. Application de la méthode électrophorétique aux problèmes d'environnement et d'identification des sols. Rapport de recherche LPC n°96, Ed. LCPC. 107 pp. Delage, P., Pellerin, F.M., 1984. Influence de la lyophilisation sur la structure d'une argile sensible du Québec. Clay Minerals 19, 151–160. Delage, P., Audiguier, M., Cui, Y.J., Howat, M.D., 1996. Microstructure of a compacted silt. Canadian Geotechnical Journal 33 (no. 1), 150–158. Delage, P., Marcial, D., Cui, Y.J., Ruiz, X., 2006. Ageing effects in a compacted bentonite: a microstructural approach. Géotechnique 56, 291–304. Derriche, Z., Kebaili, M., 1998. Prévision du gonflement des argiles d'In-Aménas. Bulletin des Laboratoires des Ponts et Chaussées no. 218, 15–23. Diamond, S., 1969. Pore size distributions in clays. Clays and Clay Minerals 18, 7–23. Gens, A., Alonso, E.E., Suriol, J., 1995. Effect of structure on the volumetric behaviour of a compacted soil. Proceedings of the first international conference on unsaturated soils, UNSAT'95, Paris, France, 1995, 6–8 Sept, pp. 83–88.

Grim, R.E., 1962. Applied clay mineralogy. Mc Graw Hill series in the earth sciences, New York. 422 pp. Holtz, W.G., Gibbs, H.J., 1956. Engineering properties of expansive clays. Trans. ASCE 121, 641–663. Komine, H., Ogata, N., 1996. Experimental study on swelling characteristics of compacted bentonite. Canadian Geotechnical Journal 31 (no. 4), 478–490. Olsen, H.W., 1962. Hydraulic flow through saturated clays. Proceedings of the 9th national conference on clays and clay minerals, pp. 131–161. Seed, H.B., Woodward, R.J., Lundgren, R., 1962a. Prediction of swelling potential for compacted clays. Journal of the Soil Mechanics and Foundation Division 53–87 ASCE, SM3. Seed, H.B., Mitchell, J.K., Chan, C.K., 1962b. Studies of swell and swell pressure characteristics of compacted clays. Bull. of the Highway Research Board, Washington, 313, pp. 12–39. Sivakumar, V., Tan, W.C., Murray, E.J., McKinley, J.D., 2006. Wetting, drying and compression characteristics of compacted clay. Géotechnique 56 (no. 1), 57–62. Sridharan, A., Altschaeffl, A.G., Diamond, S., 1971. Pore size distribution studies. Journal of the Soil Mechanics and Foundation Division 8151, 771–787 ASCE, SM5. Tessier, D., 1984. Étude expérimentale de l'organisation des matériaux argileux. Hydratation, gonflement et structuration au cours de la dessiccation et de la réhumectation. INRA, Thèse de doctorat ès sciences de l'Université de Paris VII. 362 pp. Wan, A.W., Gray, M.N., Graham, J., 1995. On the relations of suction, moisture content and soils structure in compacted clays. Proceedings of the first international conference on unsaturated soils, UNSAT'95, Paris, France, 6–8 sept, pp. 215–222.