The relationship between suction and swelling properties in a heavily compacted unsaturated clay

Engineering Geology 50 (1998) 31–48

The relationship between suction and swelling properties in a heavily compacted unsaturated clay P. Delage *, M.D. Howat 1, Y.J. Cui Ecole Nationale des Ponts et Chausse´es, Soil Mechanics Research Centre (CERMES), Paris, France Received 8 January 1997; accepted 26 September 1997

Abstract In order to study the hydro-mechanical behaviour of highly compacted unsaturated clays used in engineered clay barriers for nuclear waste disposal, some experimental techniques for controlling suction were extended into the range of high suctions. The technique of control by imposing a given relative humidity was considered for very high suctions (several hundreds of MPa), and calibration data from the literature were examined. Some corrected values were proposed, based on a calibration made with sodium chloride. The extension of the osmotic technique to higher suctions was made, and a value of 10 MPa was attained, enlarging considerably the range of this method. The study of the water retention and swelling properties of the FoCa7 clay under controlled suction and zero applied stress is presented. A good continuity between the two methods of suction control was observed. Fairly reversible responses to suction cycles were observed, in terms of water content and volume change. During these changes, it was observed that the air volume remained constant. The reversibility is related to the predominant role of a saturated microstructural level, strongly influenced by the physico-chemical bonds existing between water and the active clay minerals. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Clay; Engineered barrier; Osmotic technique; Nuclear waste; Suction; Swell

1. Introduction The mechanical behaviour of engineered clay barriers made with highly compacted expansive clays used for nuclear waste disposal at great depth, has generally been studied by conducting standard soil mechanics tests. In current practice * Corresponding author at present address: ENPC-CERMES, 6–8 Avenue Blaise Pascal, 77455 Marne la vallee Cedex 2, France. Tel.: 33 1 64 15 35 42; Fax: 33 1 64 15 35 62; e-mail: [email protected] 1 This work is dedicated to M.D. Howat, who suddenly died in August 1994.

much attention has been paid to the volume changes observed during combined sequences of hydration and loading. Oedometer tests were usually performed, since they allow simple volume measurements and load control. Furthermore, the one-dimensional strain condition imposed in the oedometer is suitable to solve problems related to shallow foundations on expansive soils. Hydration sequences have most often been performed by soaking, starting from the initial moisture content and ending at a zero suction state. Mechanical properties have generally been studied either at a constant water content equal to the initial one, or after soaking, at a zero suction. These conditions

0013-7952/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 7 ) 0 0 08 3 - 5

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

only represent the two extreme conditions of the soil. The intermediate stages between the initial water content and the zero suction state, which does not necessarily correspond to full saturation, should also be investigated. This can be achieved by running controlled suction tests. Little information is available on the effects of suction changes on the behaviour of expansive soils, and more particularly of engineered barriers. The understanding of hydration of the clay barrier, from both hydraulical and mechanical points of view, is necessary for a proper evaluation of the containment properties of the barrier. Recent advances in the field of unsaturated soil mechanics have provided a general framework for a better understanding of the various aspects of their mechanical behaviour. Modern concepts of saturated soil mechanics have been successfully extended to include the effects of suction on the behaviour of unexpansive unsaturated soils. This was made possible by the development of the elasto-plastic framework derived from critical state soil mechanics proposed by the Barcelona group, and its extension to the volumetric behaviour of unsaturated expansive soils (Alonso et al., 1990; Gens and Alonso, 1992). Experimental data from suction controlled tests on heavily compacted expansive clays are now necessary to examine these concepts. In order to get a full description of the mechanical behaviour, oedometer tests results need to be complemented by triaxial testing. As far as the volume change behaviour of expansive soils is concerned, comparisons between these two tests have already been made, and the advantages and draw-backs of each technique discussed. More efforts have to be made to study the effects of deviatoric stress, and to obtain a more complete description of the behaviour in the (q; p) plane, for various void ratios and suctions [q=s −s , 1 3 and p=(s +2s )/3 being the deviatoric stress and 1 3 the mean stress, respectively]. In this paper, an experimental study on the influence of suction on the swelling properties of an expansive, highly compacted clay is presented. Due to very high suction values measured in this material, particular attention was paid to the development and improvement of experimental techniques for controlling high suctions. The

capabilities of the controlled relative humidity technique and of the osmotic technique were investigated. Finally, both methods are used to study the water retention and swelling properties of a french dense compacted clay (FoCa7) under zero applied stress.

2. Soil characteristics and sample preparation The clay studied is an expansive clay of the Parisian basin, named FoCa7, considered as an option for making clay type engineered barriers, and for backfill of the deep geological disposal of nuclear waste in France. From a mineralogical point of view, this clay is composed mainly of interlayered kaolinite-smectite minerals (ca. 80% of the total, with a 50–50 distribution), and of kaolinite (7%), quartz, iron oxides, and some other minerals in low quantities. The values of its plastic limit, liquid limit and plasticity index are, respectively: w =50%, w =112%, I =62 (Atabek p L p et al., 1991). The samples of dense compacted clay were prepared in the laboratory of CEA (Commissariat a` l’Energie Atomique-French Agency for Research on Atomic Energy). A large quantity of natural clay was ground and mixed in order to provide enough homogeneous powder. The powder was hydrated under a constant relative humidity of 60% in a temperature and humidity controlled chamber, and put in a latex bag. The bag was immersed in an oil bath, vacuum was applied within the bag and the oil pressure was increased up to a value of 60 MPa, to provide isotropically compacted samples. The compacted soil was extracted from the bag, and cylindrical samples were machined. The average values of dry unit weight, water content and degree of saturation of the compacted FoCa7 clay are, respectively: r =1.85 Mg m−3, w=13% and S =80%. d r The storage of the samples in standard plastic bags is not really satisfactory. Due to the very high internal suction, the samples absorb water from the ambient air through the plastic membrane, and the water content slowly increases. For a good preservation of the water content of the soil, all the plastic bagged samples were put

P. Delage et al. / Engineering Geology 50 (1998) 31–48

together into one well isolated container. Metal containers are best adapted for this.

3. Experimental methods The axis translation method (Richards, 1941) is most often used for controlling suction in the range of several hundreds kPa. An alternative technique is the osmotic technique ( Kassif and Ben Shalom, 1971; Delage et al., 1987), which will be described in more details later. The osmotic technique is able to produce values of suction up to 1500 kPa in a simpler and safer manner, since no air pressure is involved in the system. A suitable method for the application of very high suctions consists of controlling the relative humidity of the air surrounding the sample. Samples are placed in a desiccator containing an aqueous solution of a given chemical compound. According to the physico-chemical properties of the compound, a given relative humidity is imposed within the dessicator. Water exchanges occur by vapour transfer between the solution and the sample, and a given suction is applied to the sample when vapour equilibrium is reached. One can use either the same product at various concentrations (sulphuric acid for instance), or various saturated saline solutions. Practically, their is no upper limit for this technique, since very high suctions can be applied using dry atmospheres. Concentrated sulphuric acid at a density of 1.79 Mg m−3 can impose a 7% relative humidity, corresponding to a suction of 1000 MPa ( EstebanMoratilla, 1990). However, this technique suffers some limitations at suctions <10 MPa. In the following, the controlled relative humidity technique and the osmotic technique will be considered. In the first stage, the calibration of the controlled relative humidity technique will be made; in the second stage, the osmotic technique will be extended to higher values of suction, ensuring continuity between the two methods.

4. Suction control by imposed relative humidities Suction control by imposing relative humidity was initially developed by soil scientists. The first

33

application to geotechnical testing was by Esteban and Saez (1988) [see also Oteo-Mazo et al. (1995)]. They put an oedometer in a cell containing a solution of sulphuric acid at a given concentration, and were able to investigate the combined effect of stress and high suctions on the volume changes of several expansive soils. They observed that vapour exchanges were quite slow, leading to testing periods of several months. The relationship between relative humidity and suction is given by the Kelvin law, as follows: RT P u −u = ln a w Mg P 0

(1)

where u and u are the air and water pressures, a w respectively; R is the constant of perfect gases (R=8.3143 J mol K−1); T is the absolute temperature (T=293 K at 20°C ); M is the molar mass of water (M=18.016 g mol−1); g is the gravity acceleration ( g=9.81 m s−2); P/P is the relative 0 humidity, equal to the partial water vapour pressure P divided by the saturated water vapour pressure P . Let RT/Mg=137.837 MPa. 0 For simplicity and safety, saturated saline solutions were preferred to sulphuric acid. A bibliographic study showed that, for a given salt, different values of relative humidities have sometimes been reported, as shown in Table 1, where values from six papers and handbooks temperature between 10 and 35°C are sometimes given, as well as the rate of change with temperature near 20°C, in % °C−1. The following references were consulted: $ National Research Council [ref. A, from Spencer (1926)]; $ Chemical Rubber Company [ref. B, from Weast (1968)]; $ Schneider (1960) (ref. C ); $ International Standard Organization ISO R 483 (ref. D); $ Tessier (1975) (ref. E ); $ AFNOR NF 15 014 (ref. F, French normalization organization). Different values are given for the following salts: K SO (1 point difference), KNO (1 point), 2 4 3 NH Cl ( lower difference of 0.3), Mg(NO ) (1 4 32 point), Na Cr O (3 points) MgCl (1 point), 2 2 7 2 CaCl (1.7 point). 2

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

Table 1 Relative humidities of various saturated saline solutions, taken from various references Reference

AB A BE AC F C ABE AB A DF ABC A AB AB A AB ABC AB A ACDF AB A A ABCDE F A B AB AB AB ACDF A C AB A AB B ACEF AB F A ABC AB C DF AB ABC DF AB AB

Salt

Pb(NO ) 32 CaSO 4 CuSO 4 K SO 2 4 K SO 2 4 KH PO 2 4 Na SO 2 3 Na HPO 2 4 KNO 3 KNO 3 NH H PO 4 3 4 Na SO 2 4 K HPO 2 4 NaBrO 3 Na C H O 2 4 4 6 Na CO 2 3 ZnSO 4 K CrO 2 4 NaKC H O 4 4 6 KC1 KHSO 4 KBr Na SO 2 4 (NH )2SO 4 4 (NH )2SO 4 4 NH C1 4 NH C1 4 Na S O 2 2 3 NaC H O 2 3 2 HCO 2 2 4 NaC1 NaNO 3 NaNO 3 NaC1O 3 KCHO 2 4 4 6 NH C1+KNO 4 3 CoC 12 NaNO 2 Mg(C H O2) 2 3 2 NH NO 4 3 NH NO 4 3 NaBr Mg(NO ) 32 Mg(NO ) 32 Mg(NO ) 32 NaHSO 4 Na Cr O 2 2 7 Na Cr O 2 2 7 Ca(NO ) 32 KCNS

Molecules HO 2

Temperature (°C ) 5

0 5 5 0 0 7 12 0 0 0 10 0 0 2 10 7 0 4 0 0 0 0 0 0 0 0 5 3 2 0 0 0 0 0.5 0 6 0 4 0 0 2 6 6 6 1 2 2 4 0

10

15

18.5

% °C−1

0.1 98

98

0.2 95

94

92

0.3

94.7 −0.2 88

87

82

81

76

76

73

−0.1 −0.3 −0.2

−0.3 −0.2

69

56 54 57

−0.3 −0.2

56 −0.3

58

56 56

20 98 98 98 97 96 96.5 95 95 94 93 93.1 93 92 92 91 91a 90 88 87 86 86 84 82 81.0 81 79.2 79.5 78 76 76 76 75 75 75 74 72.6 67 66 65 65 63 58 55a 54 55 52 52 55 55a 47

% °C−1

24.5

25

30

35

97

96

96 96

92 93.0

91 92.9

89

85

85

84

0.03 −0.02

81.1 80 79.3 79.3

81.1 80 79.5 77.5

−0.1 −0.3 −0.2

75

75

−0.28 −0.5 −0.3

71.2

68.6

65

63

62

62

59

55

53

52

50

54

52

51

0 −0.1 −0.1 −0.2 −0.02

0 −0.8 −0.3

87

0 −0.2

0.2 −0.01

−0.3 −0.2 −0.67 −0.3 −0.3

80

75

52 52

0.1 −0.3 −0.83 51

35

P. Delage et al. / Engineering Geology 50 (1998) 31–48 Table 1 (continued ) Relative humidities of various saturated saline solutions, taken from various references Reference

AB ACDF AB AB ADF C AB(19) AB(15) E C ADF AB CDF A C DF

Salt

KNO 2 K CO 2 3 Zn(NO ) 32 CrO 3 MgC 12 MgC 12 CaC 12 CaC 12 CaC 12 CH COOK 3 KCH CO 3 2 LiC1 LiC1 ZnC1 ZnBr 2 KOH

Molecules HO 2 0 2 6 0 6 6 6 6 6 0 0 1 X 1.5 2 0

Temperature (°C ) 5

10

15

18.5

47

44

44

34

34

33

% °C−1

20

% °C−1

−0.2

45 44 42 35 33 34 32.3 34a 36.5 20 20 15 12 10 10 9

−0.2

0 −0.1 −0.57

38 35

21 14

21 13 −0.2

13

10

0 −0.1

24.5

25

30

35

43

43

43

43

33

33

33

18 22

17 22

21

12

12

12

8

7

6

−0.7 31 −0.3 −0.1 −0.2 −0.1

References: A, Spencer (1926); B, Weast (1968); C, Schneider (1960); D, ISO R 483; E, Tessier (1975); F, NF X 15 014. aInterpolated value.

The relationship between relative humidity and suction uncertainties is calculated by differentiating Eq. (1), leading to: RT d(P/P ) 0 d(u −u )= (2) a w Mg (P/P ) 0 The absolute uncertainty on the suction expressed in MPa is equal to the relative uncertainty on the P/P value, multiplied by the term RT/Mg, say 0 137.837 MPa. So a 1% relative uncertainty on the relative humidity gives a 1.38 MPa absolute uncertainty for the suction. This is only acceptable for higher suctions, and shows the limitations of the method for suctions ≤10 MPa. The sensitivity of the relative humidity to temperature changes is not only dependent on the absolute temperature T in Eq. (1). It is also a physical property of the chemical component, as shown in Fig. 1 (Schneider, 1960). This aspect is important in choosing the salt; it confirms the necessity of good temperature control during vapour suction control. For this reason, all desiccators were immersed in a temperature controlled bath, at 20°C ±0.1°C.

In order to select the salt solution, and to ensure compatibility between calibration data coming from different sources, some selected salts were calibrated in the range of low suctions (4–24 MPa) with a solution of very pure sodium chloride (NaCl ) at various concentrations. The widely accepted calibration curve of Lang (1967) was used. Saturated solutions of the salts referenced in Table 2 were put in vapour contact with solutions of NaCl at various concentrations until equilibrium was achieved. In order to reduce equilibration time, the NaCl concentration was initially adjusted, for each salt, at a relative humidity close to that given in Table 1. Calibration results are presented in Table 2. Important differences between the reference values given in Table 1 and NaCl calibrations were observed for CuSO (2.3 points) and (NH ) SO 4 42 4 (2.5 points). Two different values were obtained for Na SO · 7H O. Other observations showed the 2 3 2 chemical instability of this salt, which was discarded. Differences from reference values may be due to different quality and purity of the salts. In order to get a consistent set of values, the results of Table 2 were used in this study.

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

Fig. 1. Sensitivity to temperature of various salts (Schneider, 1960).

5. Extension of the osmotic method to the high suction range As mentioned above, the control of suction by imposed relative humidities is slower than techniques involving liquid transfers, due to the very low kinetics of gas transfers. The technique also

suffers some limitations regarding accuracy in the range of lower suctions (10 MPa). For this reason, extension of the osmotic method into the high suction range was investigated. The osmotic technique for the control of suction is not common practice in geotechnical testing of unsaturated soils. With this technique, drainage of

P. Delage et al. / Engineering Geology 50 (1998) 31–48

37

Table 2 Calibration of different saturated salt solutions using a calibrated NaCl solution, according to Lang (1967) Saturated saline solution

K SO 2 4 CuSO · 5H O 4 2 KH PO 2 4 Na HPO · 12H O 2 4 2 KNO 3 ZnSO · 7H O 4 2 Na SO · 7H O 2 3 2 Na SO · 7H O 2 3 2 (NH ) SO 42 4

NaCl concentration measured at equilibrium (g salt per g water)

Water potential, according to Lang (1967) (MPa)

Relative humidity (%)

Relative humidity, according to references (%)

0.050 0.073 0.076 0.108 0.108 0.151 0.157 0.165 0.299

4.2 6.1 6.3 9.0 9.0 12.6 13.1 13.7 24.9

97.0 95.7 95.5 93.7 93.7 91.3 90.9 90.5 83.5

97 (AC )–96 (F ) 98 (BE) 96.5 (C ) 95 (AB) 94 (A)–93(DF ) 90 (ABC ) 93 (A) 93 (A) 81 (A–F )

the sample is caused by the process of osmosis, whereby the sample is placed on a cellulotic, semipermeable membrane which is permeable to water, and a solution of polyethylene glycol (PEG) is circulated beneath the membrane. The large sized PEG molecules cannot penetrate the membrane, resulting in an osmotic suction applied to the sample through the membrane. Various sizes of PEG molecules exist, they are defined by their molecular weight. The most commonly used PEG solutions have molecular weights of 6000 and 20 000 g. In order to avoid any penetration of PEG molecules through the membrane, a PEG solution should be used with a semi-permeable membrane corresponding to its size. Semi-permeable membranes are defined by their molecular weight cut-off (MWCO); a MWCO 14 000 membrane is used with PEG 20 000, and a MWCO 4000 membrane with PEG 6000. Initially developed by biologists (Lagerwerff et al., 1961), the technique was later adopted by soil scientists ( Zur, 1966). The first adaptation to geotechnical testing was by Kassif and Ben Shalom (1971) in an oedometer, to study expansive soils. Subsequent work has been done on a hollow cylinder triaxial apparatus by Komornik et al. (1980), and on a standard triaxial apparatus by Delage et al. (1987). Some improvements on the oedometer have been made by Delage et al. (1992), involving the use of a closed circuit and a pump to ensure the circulation of the solution, and to allow a more precise control of the volume of the

water exchanged. More recently, Dineen and Burland (1995) proposed a system controlling the mass of the water exchanged. The adaptation of the osmotic technique for determination of the water retention properties of soils is described in Fig. 2 (Cui and Delage, 1996). Cylindrical samples (triaxial type, with a 38 mm diameter) are carefully inserted in a tube-shaped membrane that had previously been wetted. The membrane and sample are submerged in a magnetically stirred PEG solution, whose concentration

Fig. 2. Use of the osmotic technique for the determination of the water retention properties (Cui and Delage, 1996).

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

corresponds to the desired suction value. The weight change of the sample with time is monitored, and it has been observed that a period of 1 week is necessary to achieve suction equilibration. In the osmotic system, the suction value depends on the concentration of the solution; the higher the concentration, the higher the suction. The relationship between osmotic pressure and PEG concentration is well known for two molecular weights (PEG 6000 and 20 000), since consistent results have been obtained by different authors, as shown by Williams and Shaykewich (1969). The range of possible suctions is confined to 1500 kPa, for concentrations increasing from 0 to 28 g of PEG per 100 g of solution, for both PEG 6000 and 20 000. A suitable method for measuring the PEG concentration of the solution is the measurement of its refractive index (Suraj de Silva, 1987). Fig. 3 was plotted using the experimental results of Heydecker, Lagerwerff and Zur, collected by Williams and Shaykewich in two diagrams giving the suction as a function of the concentration (in g of PEG/100 g of solution), for PEGs 6000 and 20 000. In Fig. 3, all points of PEG 6000 and 20 000 were plotted together, in a diagram giving the square root of the suction as a function of the concentration in g of PEG per g of water. All points fit one line, and the following parabolic relation is obtained: s=11c2

(3)

where s is the suction expressed in MPa, and c is the concentration (g PEG per g water). It is important to note that the calibration does not depend of the nature of the PEG. The extension of Eq. (3) to higher suctions is now examined. The study has been extended to PEG of lower molecular weights (4000 and 1500, using adequate membranes). In the first stage, the refractive indexes of these PEG solutions were calibrated at higher concentrations, up to the saturation of the solutions. Results are presented in Fig. 4. As previously, the refractive properties of the PEG appear to be quite independent of the molecular weight. Higher concentrations, above which precipitation occurs, are attained for lower molecular weights. As previously, solutions at various concentrations of PEG were placed in an atmosphere controlled at a given relative humidity by a saturated saline solution. In order to reduce the reaction time, concentrations of PEG were initially extrapolated from results of Fig. 3 in order to reasonably fit with the saline solution used for controlling the suction. When too high a suction was imposed, precipitation of PEG was observed, giving the maximum possible concentration. Calibration results are presented in Table 3 and plotted in Fig. 5 in the same way as in Fig. 3 (square root of the suction as a function of the concentration). It can be observed that the new points at higher suctions are consistent with the

Fig. 3. Suction/concentration calibration of PEG 6000 and 20 000 [after Williams and Shaykewich (1969)].

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

Fig. 4. Calibration of the refractive indexes of PEGs at high concentrations. Table 3 Calibration of various PEGs at high concentrations Salt

Hr (%)

Suction (MPa)

K SO 2 4 CuSO 4 KH PO 2 4 KNO 3 Na HPO 2 4 ZnSO 4

97 95.7 95.5 93.7 93.7 91.3

4.2 6.1 6.3 9 9 12.6

Concentration PEG 20 000 (g PEG per g water)

Concentration PEG 6000 (g PEG per g water)

Concentration PEG 4000 (g PEG per g water)

Concentration PEG 1500 (g PEG per g water) 0,599

0.714 Precipitation

0.758 0.990

0.963 0.952

Precipitation

Precipitation

Fig. 5. Suction/concentration calibration of PEGs for high suctions.

1.350

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

points at low suctions, although the linear relation [Eq. (3)] is no longer valid. A suction as high as 12.6 MPa was obtained with the lower molecular weight (1500). PEG 4000 and 6000 can develop a suction of 9 MPa, whereas PEG 20 000 can only attain 6.3 MPa. The highest concentration in Fig. 5 is around 1.4 g PEG per g water (PEG 1500), that is, more than three times higher than the maximum concentration observed in the data collected by Williams and Shaykewich (<0.4 g PEG per g water). These results considerably extend the range of suctions available with the osmotic technique, since almost one order of magnitude has been gained, providing continuity between the osmotic technique and control by relative humidity. As compared to this latter technique, there are two advantages associated with the osmotic technique in the 0–10 MPa range: better precision is obtained; and water exchanges occur in the liquid state, resulting in much faster rates.

6. Water retention properties of the FoCa7 clay The determination of the water retention curve of the FoCa7 clay was made, in the high range of suctions, by imposing controlled relative humidities to the samples. Samples were placed in desiccators containing saturated saline solutions. Some tests using the extended osmotic technique were also performed in the lower range of suctions. In some cases, heights and diameters of the samples were measured with a calliper square in order to determine volume. In the high range of suctions, four samples (1324iB, 1324iE, 1324iK and 1324iG) were tested. Their initial characteristics are given in Table 4.

The water content measurements were made by putting the sample in an oven at 105°C. The curve of Fig. 6 shows the change in sample weight as a function of drying time. It shows that a period of 24 h was not long enough to dry the sample entirely, and that 8 days were necessary. This phenomenon is related to the very high initial suction of the sample, and to the high quantity of energy necessary to withdraw all the pore water, due to very strong clay-mineral adsorption bonds. In the following, all water content determinations are made after an 8 days drying period in the oven. In order to study hysteretic effects during the wetting-drying cycles, each sample was submitted to a specific hydrous path. Paths and sample characteristics at each stage of the experiment are given in Table 5, together with the salts used, the corresponding relative humidities and the imposed suctions. Each step is numbered, and values of water contents and, when available, void ratios and degrees of saturation are given. Imposed suctions ranged between 6.1 and 262 MPa. The extreme values of water content, void ratio and degree of saturation were 6.8–28.1 (w), 0.392–0.846 (e) and 46.3–88.7% (S ), respecr tively, as compared to the average values at the initial state (w=13%, e=0.431, S =79.5%). The r maximum swelling observed is 96%, at s= 6.1 MPa; which occurs with a 15% increase of water content, and a final water content more than twice larger than the initial value of 13%. This confirms the considerable swelling properties of the FoCa7 clay. The details of each path is given below; the results are presented as suction/water content plots: $ sample 1324iB ( Fig. 7): the first suction imposition (point 1) at 139 MPa decreased the water

Table 4 Characteristics of the samples used for the determination of the water retention curve Ref.

Diameter (cm)

Height (cm)

Volume (cm3)

Wet weight (g)

Wet density

Dry weight (g)

Dry density

Water content (w, %)

1324iB 1324iE 1324iG 1324iK

7 7 7 7

1 1 1 1

38.465 38.465 38.465 38.465

81.433 81.356 80.677 80.623

2.117 2.115 2.097 2.096

72.094 71.936 71.337 71.340

1.874 1.870 1.855 1.855

12.95 13.09 13.09 13.01

P. Delage et al. / Engineering Geology 50 (1998) 31–48

41

Fig. 6. Kinetics of the drying of FoCa7 in the oven at T=105°C.

$

content from 12.5 to 11.63%. Hence, 139 MPa is an upper bound for the suction value in the initial state. Further imposed suctions at much lower values (points 2 and 3, s=57 and 24.9 MPa, respectively) significantly increased the water content to 15.75 and 19.73%, respectively. Finally, a drying path was followed at successively 57, 82, 113, 139 and 262 MPa (points 4, 5, 6, 7 and 8, respectively), and an ultimate wetting at 139 MPa ended the test. Observation of Fig. 7 shows that points lie quite close to one curve, showing a very slight hysteretic effect over the range of suction. The initial suction can be extrapolated from this curve since the initial water content is known. A value of 113 MPa is obtained, corresponding to a water content of 12.5%. sample 1324iE ( Fig. 8): this test was aimed at studying the response at a low suction, and the lowest possible suction was applied using CuSO , at a value of 6.1 MPa. A high water 4 content of 28.1% was obtained. A subsequent drying sequence was followed at suction values of successively 12.6, 24.9, 38, 57 and 82 MPa, followed by a wetting sequence at 57, 38 and 24.9 MPa. Fig. 8 also shows a slight hysteresis on the drying–wetting cycle between 24.9 and 82 MPa.

sample 1324iK ( Fig. 9): the same initial wetting at a suction value of 6.1 MPa was followed by a drying–wetting cycle between 24.9 and 38 MPa, showing a slight hysteretic effect. $ sample 1324iG (Fig. 10): here, the first wetting at 6.1 MPa gave a higher water content than previously, whereas a subsequent cycle between this value and 24.9 MPa showed a similar trend to that seen previously. The final value at 6.1 MPa gives a lower value than in the first step. Results from all samples are plotted together in Fig. 11. Excellent consistency between all results is observed, showing a good homogeneity of the samples, resulting from a uniform preparation procedure. A problem is encountered at the lower suction of 6.1 MPa with the point 2 of the 1324iG sample (Fig. 10). Since the three other points obtained at this suction from three different samples gave comparable values of water content, this point is not considered as being really representative. The slight hysteretic effect is confirmed at all suctions between 6.1 and 262 MPa, that is, over a very large range of suctions. The value of the initial suction may be extrapolated from the initial water content; a value of 113 MPa, corresponding to a water content w=13%, is considered representative of the initial state. $

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

Table 5 Hydrous paths followed, samples 1324iB, 1324iE, 1324iK and 1324iG No.

1324iB

1324iE

1324iK

1324iG

w i e 0 S r

12.95% 0.423 79.8%

13.09% 0.426 82.0%

13.01% 0.438 78.1%

13.09% 0.438 78.1%

Salt H r Suction

Drying S r w e

LiCl · H O 2 15% 262 MPa

8 S =46.3 r 6.8% 0.392 1 S =73.8 r 11.63% e=0.421 7 11.4% 6 13.2%

CaCl · 6H 0 2 2 32.3% 139 MPa

K CO · 2H O 2 3 2 44% 113 MPa Mg(NO ) · 6H O 32 2 55% 82 MPa NaNO 2 66% 57 MPa NaCl 76% 38 MPa (NH ) SO 42 4 83.5% 24.9 MPa ZnSO · 7H O 4 2 91.3% 12.6 MPa

KNO 3 93.7% 9 MPa CuSO · 5H O 4 2 95.7% 6.1 MPa

Wetting S r w e

Drying S r w e

Wetting S r w e

6 S =79.4 r 14.9% e=0.501 5

3

16.6%

16.2%

Drying S r w e

Wetting S r w e

Drying S r w e

Wetting S r w e

9 11%

10 12.5%

5 14.7%

11 13.9%

4 S =83.0 r 16.1% e=0.518 (

2 S =83.4 r 15.75% e=0.504 3

3 19.73%

4 S =88.2 r 18.7% e=0.566 3 S =87.3 r 19.7% e=0.603 2 S =89.0 r 22.3% e=0.669

(

7

8 17.7% 9

4 S =86.6 r 18.4% e=0.567 3

3

3

3

3

3

3

5 S =90.7 r 19.9 e=0.586 4 21.8

18.5%

19.6%

3

(

1 18.6% 5 19.3% 3

3

(

3

3 24.5%

2

2

27.7%

31.14%

1 S =88.7 r 28.1% 0.846

3

1 S =86.2 r 21.79% e=0.675 6 22.8% 3

7 28.4%

P. Delage et al. / Engineering Geology 50 (1998) 31–48

43

Fig. 7. Water retention properties of sample 1324iB.

Fig. 9. Water retention properties of sample 1324iK.

Fig. 8. Water retention properties of sample 1324iE.

Fig. 10. Water retention properties of sample 1324iG.

At low suctions, several complementary points were determined with the extended osmotic method. Samples were initially wetted at a zero suction by vapour transfer in a desiccator containing free water, and dehydrated at various suctions ranging from 0.3 to 10 MPa. Each point corresponds to a specific sample. All results plotted together in Fig. 12 show a good compatibility between the relative humidity and the osmotic method. The good agreement of the osmotically suction controlled points, which were fully hydrated before being dried at a controlled suction, confirms that hysteretic effects are also small at low suctions. It shows that both vapour and liquid transfers lead to a similar final hydrated state. It

is also observed that the point hydrated at a zero suction (0.1 on the logarithmic axis) is not fully aligned with the other points, which results in a curve having the general shape of a water retention curve near zero suction. The combined use of the two suction control methods allows for the determination of the water retention curve over the range of suction from 0 to 262 MPa with sufficient precision.

7. Swelling under a controlled suction Sample volume changes during the wetting–drying cycles were obtained by measuring the diameter

44

P. Delage et al. / Engineering Geology 50 (1998) 31–48

Fig. 11. Water retention properties of all samples plotted together (vapour control of the suction).

Fig. 12. Global water retention curve, including osmotically suction controlled points.

and height of the sample, as mentioned already. This measurements allow the investigation of the isotropy of the swelling strains. In Fig. 13, radial strains are plotted as a function of axial strains, for the four samples presented in Table 4, at various suction values. It shows that the swelling of the isotropically compacted clay is reasonably isotropic, which seems normal.

Fig. 13. Evidence of isotropic swelling at various suctions.

Fig. 14. Void ratio versus log s plot for all samples.

All the points (vapour and osmotic control ) for which a volume measurement was made are plotted in Fig. 14, in a diagram giving the changes of void ratio as a function of the logarithm of the suction. As observed on the water retention curve ( Fig. 13) all samples give consistent results, and a unique and linear relation between void ratio and the logarithm of the suction is observed. Cycles in suction do not appear on the graph, and hysteretic

P. Delage et al. / Engineering Geology 50 (1998) 31–48

45

Fig. 15. Total volume changes as a function of water exchanges.

effects are slight in terms of deformation. This shows a reversible elastic deformability of the samples as a function of suction changes, under a zero stress. In order to better understand the swelling mechanisms, and to consider the changes of the degree of saturation during the hydrous cycles, water volume changes have been calculated and related to total volume changes, as shown in Fig. 15. Points submitted to a suction lower than the initial one correspond to swelling and are located above and on the right of the zero point. Points submitted to a suction higher than the initial one are on the left, in the negative zone. As shown in the figure, these points correspond to many different samples, and also include different points obtained from the same sample. All swelling points are aligned along the bisectrix. This demonstrates that total volume changes coincide exactly with the volume of absorbed water. In other words, swelling occurs

with a constant air volume. On the other hand, only a slight shrinkage is observed on points situated at the left of the zero, showing that water extraction has little effect on the total volume, for suctions higher than the initial value.

8. Discussion The reversible response observed on the FoCa7 clay in both water content and volume changes during cyclic suction changes, is not in full agreement with existing experimental data on unsaturated and expansive soils. As far as volume changes are concerned, results of Chu and Mou (1973) in Gens and Alonso (1992) showed that expansive soils submitted to suction changes had an approximately elasto-plastic response: whereas the first application of a low suction never previously supported by the sample induced large irrecoverable swelling strains, subsequent suction

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P. Delage et al. / Engineering Geology 50 (1998) 31–48

cycles in the same range induced approximately reversible cyclic strains. This is not the case for the FoCa7 clay which presents unusually reversible behaviour under suction cycles, even for strains as large as 50%. It is well known that hysteresis occurs in any porous medium submitted to wetting and drying cycles, due to the different mechanisms involved during draining and filling. These differences are related to the morphology and pore size distribution of the porous medium, and to pore interconnections. Simple porous models are able to account for these effects, which occur in inert porous materials (sandstones for instance) and are not a priori related to any solid–water physico–chemical interaction. The main characteristics of the porous medium of the FoCa7 clay are its high density, and the high activity of the clay minerals which compose the solid matrix. This activity is quantified by the high values of plasticity index (62%) and initial suction (113 MPa), and by the water retention properties of the clay. Hence, a large amount of water is strongly tied to the clay minerals by physico-chemical bonds. Some elements of interpretation are provided by Fig. 15, which shows that, for suctions lower than the initial value, the air volume remains constant, all volume changes coinciding exactly with the amount of exchanged water. In such conditions, one can suppose that microstructural changes only occur in the saturated zone of the sample (initially defined by the 83% degree of saturation), leaving the unsaturated zone full of air (the remaining 17%) unchanged. For an initial volume of 38.465 cm3, this constant volume of air is equal to 6.2 cm3. It corresponds, during swelling, to a decreasing degree of saturation, which may be easily related to the total volume and/or suction. More information on this constant air volume would be necessary for a better understanding of the swelling mechanism, on a microstructural point of view: this includes the investigation of its morphology [related to that of the (solid+water) phase, made of compacted grains of the powder hydrated at a 60% relative humidity], and information on possible morphological changes during hydrous cycles. In this regard, information on the

evolution with hydration of air continuity and air permeability would be quite interesting. The hypothesis of a microstructural saturated level reacting in a rather reversible way has been outlined by Gens and Alonso (1992) as part of a model for unsaturated expansive soils behaviour. This microscopic level is related to physico-chemical phenomena which mainly affects the water linked to the individual clay platelets and to larger units of closely packed particles. The model is completed at the macroscopic level, accounting for the larger scale structure of the soil, in relation to the existence and evolution of large pores of the inter-aggregate type. This macroscopic level is modelled in the same way as unexpansive, unsaturated soils (Alonso et al., 1990), using the concept of a LC ( loading-collapse) yield curve. According to the experimental results obtained in the present study, the FoCa7 clay seems to be an extreme case in this framework, for which all macrostructure effects related to the morphology of the porous medium have been erased by the very high compactive effort, and the resulting high density. This is consistent with results on the same clay by Atabek et al. (1991), who showed that a clear bimodal pore size distribution existing at a dry density of 1.6 kg cm−3 completely disappeared at densities >1.95 kg m−3, leading to a matrix dominated structure. Reversibility at a microstructural level is discussed by Gens and Alonso (1992), who mentioned that irreversibility in swelling tests is attributed, among other things, to a lack of parallelism. In this regard, the high compaction stress supported by the FoCa7 clay should favour parallelism, and consequently reversibility. Results by various authors mentioned by Gens and Alonso, including Branson and Newman (1983), showed slight hysteresis in the water content/suction curves of illite and montmorillonite, which is also consistent with the results obtained in this study. Another conclusion drawn from Fig. 15 is that the application of suction higher than the initial value does not lead to significant shrinkage of the sample. This is consistent with earlier results on a low plasticity silt by Vicol (1990), who showed that, when a sample has been consolidated or compacted above a particular density, suction increases are no longer able to induce any compres-

P. Delage et al. / Engineering Geology 50 (1998) 31–48

sive volumetric strain [see also Delage and Graham (1995)]. Results by Biarez et al. (1987) from tests on saturated consolidated remoulded unexpansive samples also showed that no more significant compressive strains were caused by suction increase when desaturation started, above the air entry value. In the case of FoCa7, the initial suction is a limit separating two zones of completely different behaviour. In the range of suctions at which the clay barrier is expected to work, that is, lower than the initial suction, the reversibility of both water retention and swelling properties is an interesting property which simplifies the analysis of the hydration process of the clay. This property has now to be studied under stresses different from zero, in order to provide a full description of the volumetric behaviour in the p/s plane (where p is the mean stress and s the suction). The main question is whether the constant air volume condition under stress will still prevail, or whether any air volume change will induce more irreversibility under suction changes.

9. Conclusion New technical developments were made in order to impose high, controlled suctions to unsaturated samples. Since the use of controlled relative humidities requires very long time periods in order to achieve desired suctions, and due to the lack of precision of this method <10 MPa, it was decided to examine the capabilities of the osmotic method at suctions >1500 kPa. The osmotic method appears to be suitable for suctions as high as 10 MPa, ensuring continuity with the method of suction control by imposed relative humidities. The water retention properties of the highly compacted unsaturated swelling FoCa7 clay were determined with both methods of suction control. A complete retention curve was obtained over the 0–262 MPa range of suction, showing a very good reproductibility between various samples, and very slight hysteretic effects at all suctions. Results plotted in a void ratio/log (suction) diagram showed fairly linear and reversible behaviour at all suctions smaller than the initial one (113 MPa).

47

A constant volume of air equal to the initial value was observed, showing that total volume changes were equal to the volume of exchanged water. The predominant role of a microstructural saturated level, strongly affected by the physico-chemical interactions between water and the active clayey minerals was outlined. It is suggested that the observed effects are due to the high compaction stress and density of the sample. All the macroscopic mechanisms in unsaturated soils related to the existence and evolution of larger pores and modelled by the standard LC model proposed by the Barcelona group were not observed due to the high density of the compacted clay. From a practical view point, the reversibility of both water content and total volume with suction changes under a zero applied load is an interesting and simple behavioural feature of engineered clay barriers made with highly compacted swelling clay. It will help in the understanding of the behaviour of the barriers during hydration. Tests with cyclic suction changes under stress are now planned, in order to investigate the combined effect of stress and suction change on the volumetric behaviour of the material, and to investigate possible reversible responses.

Acknowledgment This research has been financially supported by the French National Agency for Nuclear Waste Management (ANDRA). MM P. Lebon and F. Plas are gratefully acknowledged for their support, and for the interesting discussions which helped orienting the work. The conclusions and the viewpoints presented in the paper are those of the authors and do not necessarily coincide with those of the client. The authors also thank Drs Lajudie, Dardaine and Imbert of Commissariat a` l’Energie Atomique (CEA) for supplying the samples of FoCa7 clay.

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