Volume change behaviour of clays: the influence of mineral composition, pore fluid composition and stress state

Mechanics of Materials 36 (2004) 435–451 www.elsevier.com/locate/mechmat

Volume change behaviour of clays: the influence of mineral composition, pore fluid composition and stress state C. Di Maio *, L. Santoli, P. Schiavone Dipartimento di Strutture, Geotecnica, Geologia applicata all’Ingegneria, Universit a della Basilicata, c.da Macchia Romana, Potenza 85100, Italy Received 7 August 2002; received in revised form 27 November 2002

Abstract This paper analyses the influence of mineral composition, pore fluid composition and stress state on volume change behaviour of four different clayey soils: the Ponza bentonite, a commercial kaolin, the Bisaccia and the Marino clays. In order to investigate the role of the smectite content, also artificial bentonite–kaolin mixtures were tested. Oedometer tests were carried out on the materials ‘‘reconstituted’’––according to BurlandÕs suggestions [Geotechnique XL (3) (1990) 329–378]––with distilled water, concentrated NaCl solutions and a non-polar fluid (cyclohexane). Some tests were carried out on dry materials. The results show that volume change behaviour of artificial mixtures and natural soils reconstituted with distilled water is strongly influenced by mineral composition and, in particular, by the smectite fraction. The influence on compression and swelling indices decreases with increasing axial stress, whereas, with respect to the coefficients of consolidation and swelling, it remains high in the whole considered stress range. An increase in pore solution concentration causes a reduction in compressibility. The effect increases with the smectite content, thus reducing the differences among the different soils. In particular, compressibility of the bentonite reconstituted with concentrated salt solutions becomes more similar to that of the commercial kaolin––which, in turn, is poorly influenced by pore solution concentration––than to that of the water-saturated bentonite. The dependence on the stress level of the effects of pore solution composition on both volume change indices and coefficients, is qualitatively similar to that of the effects of mineral composition. The materials prepared with cyclohexane behave similarly to the dry ones and they both are less compressible than the materials prepared with aqueous solutions. The coefficient of consolidation increases dramatically and swelling is negligible in the whole considered stress range. The changes seem to depend on an increase in resistance to interparticle sliding. In the case of the Ponza and Bisaccia clays, this hypothesis is supported also by the results of direct shear tests. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Clays; Pore fluid composition; Mineral composition; Compressibility; Swelling; Shear strength

1. Introduction

*

Corresponding author. E-mail address: [email protected] (C. Di Maio).

Volume changes in clays are the effect of complex interactions between the solid skeleton and the pore fluid. At a given temperature, the type of

0167-6636/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0167-6636(03)00070-X

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Nomenclature As e cv cs Cc Cs c.f. D eL

specific surface void ratio coefficient of one-dimensional consolidation coefficient of one-dimensional swelling compression index swelling index clay fraction dielectric constant void ratio at liquid limit

interaction depends on mineral and pore liquid composition, on void ratio and on stress level. Experimental results show that smectite behaviour is greatly controlled by pore liquid composition. Macroscopic evidences of these interactions are the changes in volume and shear strength caused by ion diffusion, under constant external stresses (Barbour and Fredlund, 1989; Chatterji and Morgestern, 1989; Mitchell, 1991; Sridharan, 1991; Di Maio and Fenelli, 1994; Di Maio, 1996a,b, 1998). Furthermore, changes in external loads produce effects which depend on pore fluid composition. In particular, compressibility and swelling––which depend also on the type of exchangeable cations––decrease with increasing pore liquid ionic force, or with decreasing dielectric constant (among others: Jimenez Salas and Serratosa, 1953; Bolt, 1956; Kenney, 1967; Kinsky et al., 1971; Olson and Mesri, 1970; Mesri and Olson, 1971; Sridharan, 1991; Mitchell, 1993; Di Maio, 1996b). Most processes relative to salt solution saturated material are explained, at least qualitatively, in terms of diffuse double layer properties. The effects of organic non-polar solvents are attributed to the suppression of the double layer and are often analysed in terms of the pore liquid dielectric constant. Depending on the particular value of its specific surface, kaolinite may or may not be influenced by pore liquid composition. Sridharan and Ventakappa Rao (1973) found that kaolinite may undergo consolidation as an effect of an increase in the dielectric constant of the pore fluid. The

ea Gs cw w wL r0a r0n re sr

axial strain specific gravity unit weight of water percentage water content liquid limit effective axial stress effective normal stress electrical conductivity residual shear strength

authors hypothesized that kaolin volume changes are related to interparticle forces which control particle sliding. If the ionic strength of pore fluid decreases, or if the dielectric constant increases, the resistance to particle movements decreases, thus allowing a reduction in porosity. Chen et al. (2000) observed that the compression index of kaolin varies with the dielectric constant of the pore organic fluid similarly to the Hamaker constant, and it exhibits a minimum at about D ¼ 24. Moore and Mitchell (1974) found a similar trend for shear strength, although the minimum was attained at a different dielectric constant value. Illite behaves in a way which is similar to kaolinite. Mitchell (1960) showed that the type of influence of pore liquid composition greatly depends on particle size. In the case of the ‘‘quick clays’’, the fabric after sedimentation in marine environment is of ‘‘cardhouse’’ type, with edge-to-face associations. Due to a decrease in ion concentration of the pore liquid, the shearing resistance at the particle contacts decreases. Under these conditions, remoulding may produce a collapse that turns the soil into a viscous fluid (Bjerrum, 1954, 1955; Bjerrum and Rosenqvist, 1956; Moum and Rosenqvist, 1961; Rosenqvist, 1966). Most results reported in the technical literature refer to one or more specific aspects of clay volume change behaviour. The results are not always easily comparable because of the different procedures in sample preparation. The purpose of this paper is to systematically examine volumetric response at different mineral and pore fluid compo-

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

sitions, as well as at different stress levels, for a given reconstitution procedure. The experimentation has been performed on two practically ‘‘pure’’ clays, the Ponza bentonite, mostly composed of montmorillonite, and a commercial kaolin composed of kaolinite, on their mixtures at several percentages, and on two natural soils with a composite clay fraction. The influence of pore liquid composition has been evaluated first on the materials reconstituted with distilled water and NaCl solutions at various concentrations. In order to evaluate the behaviour of the solid skeleton alone, experimentation has been subsequently carried out also on dry materials. But the ‘‘dry’’ state is difficult to obtain for smectite without changing its mineral structure, and difficult to maintain at room temperature and humidity. So, one cannot be sure about pore pressure and effective stress. To the aim of keeping under control pore pressures and to obtain electrical interparticle interactions reasonably similar to those in dry conditions, some tests were carried out on the materials prepared with an organic nonpolar liquid, cyclohexane. In order to evaluate whether this fluid influences the shearing resistance at the particlesÕ contact, some shear tests were carried out on the Ponza and Bisaccia clays.

437

with only 30% clay fraction, as shown by Fig. 1, which reports the particle size distribution curves of all the soils under consideration and also the curve relative to a sand which was used as a term of comparison. Table 1 reports mineral composition determined by X-ray powder diffraction, using Cu-Ka radiation, and the liquid limit evaluated by means of the fall cone test on the powdered soils mixed with distilled water. The limit water content was evaluated also by mixing the soils with NaCl solutions at various concentrations. The results show (Fig. 2) that wL decreases with increasing salt solution molarity, with the exception of the commercial kaolin whose liquid limit is practically constant. However, noticeable and unavoidable deformations of the kaolin specimen surface during cone penetration make this type of test unsuited for evaluating water adsorption capability. Moreover, previous experimental results showed that pore liquid composition may have opposite influence on the liquid limit of kaolinite and illite with respect to bentonite (Bjerrum and Rosenqvist, 1956; Sridharan, 1991; Anson and Hawkins, 1998). In order to get a more accurate evaluation of water retention properties, some settling tests were carried out following Sridharan and Prakash (1998). Several suspensions were prepared by mixing 25 g dry material with different quantities

2. Materials

Ponza bentonite

Bisaccia clay

kaolin

sand

Marino clay

100

% finer by dry weight

Three of the considered soils are already known in the literature. The Bisaccia clay has been studied by many authors, among whom Fenelli and Picarelli (1990), Picarelli et al. (2000) and Di Maio and Onorati (2000a,b). It is a very active smectitic clay whose behaviour is strongly influenced by physicochemical processes. The Marino clay is a less active clay; nevertheless, previous studies indicated that its behaviour is noticeably influenced by pore liquid composition (Di Maio, 1996a). The Ponza bentonite, as it is composed mainly by Na-montmorillonite, is the most active among the considered materials. The effects of ion diffusion on its compressibility and residual shear strength are particularly strong (Di Maio, 1996b). The considered kaolin, although it contains about 80% kaolinite, is composed mainly by silt size particles,

80 60 40 20 0 0,0001

0,001

0,01

0,1

1

10

grain size(mm)

Fig. 1. Grain size distribution curves of the considered materials.

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C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

Table 1 Specific weight Gs , liquid limit wL , mineral percentage, specific surface As determined by an approximate method based on methylene blue adsorption, electrical conductivity re of the materials prepared with distilled water at the liquid limit Gs

wL (%)

Kaolinite (%)

Illite (%)

Smectite (%)

Ponza bentonite Bisaccia clay

2.77 2.78

390 110

20 10

– 20

Na-smectite 70–80 (Ca-smectite) 30

Commercial kaolin Marino clay

2.63

50

75–80

8–10

<5

2.75

50

30

10

Mixed layer illite–smectite 10

– Chlorite 10% Quartz 15% Calcite 10% Feldspars 5% Quartz and feldspars 10% Chlorite 10% Quartz 30–40%

As (m2 /g)

re (S/m)

500 190

0.40 0.42

17

0.03

130

0.39

400

400 final water content wf(%)

Ponza bentonite Bisaccia clay

300

Marino clay

wL %

kaolin Asrum clay (Bjerrum and Rosenqvist,1956)

200

300

200

Bisaccia clay in distilled water, wL = 110% Bisaccia clay in sat.NaClsol.,wL= 51% kaolin in distilledwater,wL= 50% kaolin in sat.NaClsol.,wL = 49%

wi = wf

100

100

0

0

0

0

1

2

3

4

5

6

NaCl solution molarity

200

400

600

800

1000

initial water content wi(%)

Fig. 2. Liquid limit wL against pore solution molarity. The limit, determined by fall cone test, is defined as the weight of water divided by the weight of solid (without salt).

Fig. 3. Equilibrium water content of the sediment volume wf against initial water content of the suspension wi . The water content is defined as the ratio of the weight of water in the pore solution to the weight of solid, without salt.

of distilled water or saturated NaCl solution, and they were left to settle. Fig. 3 reports the water content of the sediment volume at equilibrium wf against the initial water content of the suspension wi for the commercial kaolin and, for comparison, for the Bisaccia clay. In the case of this latter, for any wi , water retention capability wf in the concentrated salt solution is much lower than in distilled water, consistently with the fall cone test results. It can be observed that, on the contrary, water retention capability of kaolin in the salt solution is higher than in distilled water. The settling limit wSL is defined by Sridharan and Prakash as the maximum water content of the soil–water suspension for which there is not a decrease in

porosity caused by settling. It is given by the intersection of the curve interpolating the experimental data with the wf ¼ wi line. It can be observed that such a limit is wf ¼ 85% for kaolin in distilled water and wf ¼ 109% in the salt solution. So, during the settling process, pore liquid composition influences the kaolin behaviour, and, for the considered liquids, the type of influence is opposite to that on smectic clays, as expected on the basis of the literature. However, stresses are very low in this type of experimentation. As we shall see later, under stress levels slightly higher than those acting in settling tests, the considered kaolin is negligibly influenced by pore liquid composition.

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

3. Methods and results Compressibility was evaluated by means of fixed-ring oedometer tests on four sets of materials reconstituted with four different fluids. The first set consisted of the natural soils and artificial bentonite–kaolin mixtures reconstituted by mixing the air-dried powders with distilled water. The materials were not previously ‘‘washed’’ with distilled water, with the exception of the commercial kaolin. The natural salt content of the Bisaccia and Marino claysÕ pore solution, determined by squeezing undisturbed samples with water contents of about 30%, was of the order of 3 g/l. Salt content of the Ponza bentonite––determined on the solution squeezed from a sample prepared at the liquid limit with distilled water––was about 0.5 g/l. In order to evaluate the intrinsic compressibility, all the specimens were prepared at an initial water content higher than the liquid limit evaluated with distilled water (Burland, 1990). The second set was constituted by the air-dried powdered materials mixed with NaCl solutions at various concentrations. Also in this case, the initial solution content was equal or higher than the liquid limit evaluated with the same solution. Another set of tests was carried out on the materials saturated with cyclohexane, an organic solvent whose dielectric constant is D ¼ 2:0 at 20 °C. The aim was to observe volume change behaviour when the interstitial water––but not the strongly chemically combined water––is substituted by a non-polar liquid. So, the powders were oven-dried at 105 °C for three days. The reconstitution procedure was such as to account for both the tendency of the liquid to evaporate very rapidly at room temperature, and the great reduction in the paste plasticity. The materials were rapidly mixed with the maximum liquid content at which apparently there was no drainage of the liquid from the solid nor self-weight consolidation. They were placed in the consolidation cells, immersed in cyclohexane, and the cells were sealed. A preliminary experimentation showed that different reconstitution procedures, such as air-drying, oven-drying at 105 °C or hydration starting from natural water content, do not cause differences in the behaviour of the water-saturated specimens.

439

So, the results of these tests can reasonably be compared to those mentioned above. The last set of tests was carried out on ‘‘dry’’ specimens. The materials, dried at 105 °C for 3 days, were prepared in thin layers, taking care that a given weight of soil would occupy always a given volume, with an initial void ratio close to that of the materials prepared with cyclohexane. The test on the sand was carried out only to have a term of comparison with poorly graded and coarse grained materials which are known to be much less compressible. The specimens––2 cm thick––were loaded and subsequently unloaded by steps, doubling and halving respectively the external load. Only in the case of the Bisaccia clay reconstituted with distilled water, a great tendency of the material to extrude required smaller axial stress increments. Each load was sustained long enough for the completion of primary compression or swelling and the development of secondary volume strains. In all the cases, the upper and lower porous stones were in contact with the same fluid as the pore fluid. 3.1. Water-saturated materials Extensive work has been reported in the technical literature about the role of mineral composition in the compressibility and swelling behaviour of clay soils (among others: Lambe and Whitman, 1979; Mesri and Olson, 1971; Yong and Warkentin, 1975; Mitchell, 1993). The influence of mineral composition is particularly strong when the pore liquid is distilled water, as shown by Fig. 4 which compares the compression and swelling curves of the bentonite, kaolin and sand prepared with and immersed in distilled water. As expected, the behaviour of the two ‘‘pure’’ clays is very different. In particular, kaolin behaves more similarly to the sand than to the other clay, as far as compression and swelling lines slopes are concerned. Fig. 5 reports the compression curves of the bentonite–kaolin mixtures and of the Bisaccia and Marino clays. It can be observed that both initial void ratio and compressibility gradually increase with increasing bentonite percentage. With stress level increasing, the compression curves converge towards a narrow range of void ratio. The Bisaccia

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C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451 5

7

Ponza bentonite

6

commercial kaolin loose sand

bentonite - kaolin mixtures Bisaccia clay Marino clay kaolin

4 100%

5

void ratio e

void ratio e

8

4 3

3

80%

50%

2

2

20%

1

10%

1 0 10

100

1000

10000

σ 'a (kPa)

Fig. 4. Oedometer compression and swelling curves of watersaturated Ponza bentonite, commercial kaolin and sand.

8

bentonite - kaolin mixtures Bisaccia clay Marino clay kaolin

7

void ratio e

6

100% bentonite

5

80%

4 50%

3 2

10%

20%

1 0 10

100

1000

10000

σ 'a (kPa)

Fig. 5. One-dimensional compression curves of the considered materials reconstituted with distilled water. The percentages reported on the curves refer to bentonite dry weight.

clay compression curve, consistently with its 30% montmorillonite content, lies between the 20% and 50% bentonite mixturesÕ curves. The Marino clay compression curve, consistently with its low smectite content lies below the compression line of the 10% mixture, but it is also below the curve relative to 100% kaolin, notwithstanding the similarity of grain size distribution. This is probably explained by different particle shape as well as by the fact that smectite, even in small percentage, makes the resistance to sliding at interparticle contacts decrease.

0 10

100

σ 'a (kPa)

1000

10000

Fig. 6. One-dimensional swelling curves of the considered materials reconstituted with distilled water.

Fig. 6 reveals similar tendencies for swelling curves, the slope of which increases with the smectite percentage. Fig. 7 shows that the differences among the compression and the swelling indices, Cc and Cs respectively, of the different materials decrease with increasing axial stress. Such a decrease is predicted qualitatively by the double layer model, as shown by Fig. 7 which reports the curve e– log r0a determined to interpret the Ponza bentonite behaviour. According to earlier results and theoretical considerations, the curve should interpret swelling and re-compression more than first compression. The differences among the theoretical and experimental swelling curves are due to the fact that the clay is not so ‘‘pure’’ and ‘‘ideal’’ as required by the model. An even stronger influence of the smectite content can be observed with respect to time evolution of volume change. Fig. 8 reports swelling and consolidation against time for all mixtures and, respectively, for a decrease in axial stress from 40 to 20 kPa and for an increase from 20 to 40 kPa. It shows that 10% bentonite is sufficient to change dramatically time trend of volume change, acting mainly on permeability, as it can be readily verified. The coefficients of consolidation cv and swelling cs ––determined on the experimental curves of displacements by the log-time method based on the Terzaghi model––are reported against axial stress in Fig. 9. For bentonite contents equal to 50% and 80%, the curves are very

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

Ponza bentonite 80% bentonite 50% bentonite 20% bentonite 10%bentonite kaolin Bisaccia clay Marino clay theoretical curve

2

As = 500 m /g

5.0

-2

10 M NaCl

4.0

100% bentonite

swelling ∆σ'a = 20 kPa - 40 kPa

80% bentonite

1.5

50% bentonite 20% bentonite

1

Cc

3.0

2

∆ h (mm)

6.0

441

10% bentonite 100% kaolin

0.5 0 -0.5

2.0

-1 consolidation ∆σ'a = 40 kPa - 20 kPa

-1.5

1.0

-2 1

0.0

(a)

10

100

1000

σ 'a (kPa)

5.0

4.0 2

As = 500 m /g -2

10 M NaCl

Cs

3.0

10

100

Ponza bentonite 80% bentonite 50% bentonite 20% bentonite 10% bentonite kaolin Bisaccia clay Marino clay theoretical curve

1000

10000

100000

t (min)

10000

Fig. 8. Consolidation and swelling curves of the bentonite– kaolin mixtures for an increment of axial stress from 20 to 40 kPa and for a decrement from 40 to 20 kPa respectively.

1.E-01 cv

kaolin

cs

1.E-02

2.0

b

cv . cs (cm /s)

1.E-03

0.0 10

(b)

10%

2

1.0

100

1000

10000

σ 'a (kPa)

Fig. 7. Compression (a) and swelling (b) indices against axial stress for the materials reconstituted with distilled water. The dotted line derives from Bolt model (1956) evaluated with the parameters used by Mitchell (1993) for montmorillonite, and for As ¼ 500 m2 /g and a solution 10
2 M NaCl.

close to that of the bentonite; furthermore, cv decreases with axial stress increasing. For bentonite percentages equal to or lower than 20%, cv and cs increase with decreasing bentonite content, as expected on the basis of previous experimental results (among others: Yin, 1999), and increase with axial stress. It is worth noting that the values of cv relative to the bentonite are slightly lower than those found by Robinson and Allam (1998) (curve a in Fig. 9) on a less plastic Na-montmorillonite ðwL ¼ 321%Þ. Furthermore they are in the range between the curves found by Marcial et al. (2001) for a Ca-smectite clay ðwL ¼ 112%Þ and a

10%

1.E-04

20%

1.E-05

50%

20%

80%

1.E-06

100%

50% 80% 100%

Marcial et al.(2001)

a

1.E-07 10

100

σ 'a (kPa)

1000

10000

Fig. 9. One-dimensional coefficients of consolidation cv and of swelling cs against axial stress for the bentonite-kaolin mixtures reconstituted with distilled water. cv curves a and b were determined by Robinson and Allam (1998) for a montmorillonite and for a kaolinite respectively.

Na-smectite clay ðwL ¼ 471%Þ. In the case of kaolin, the values of cv are very close to those found by Robinson and Allam (curve b) on a kaolinite with a similar liquid limit ðwL ¼ 53%Þ. Fig. 10 reports analogous curves for the Bisaccia and Marino clays, showing that the values of both, cv and cs are close to those obtained for the bentonite–kaolin mixtures containing similar percentages of montmorillonite. The values of cv

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C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451 1.E-02

2

c v . cs (cm /s)

1.E-03

cv Marino clay

cs Marino clay

cv 10% bentonite

cs 10% bentonite

cv Bisaccia clay

cs Bisaccia clay

cv 50% bentonite

cs 50% bentonite

1.E-04

1.E-05

1.E-06

1.E-07 10

100

1000

10000

σ 'a (kPa)

obtained for the Bisaccia clay are lower than those obtained by Abdullah et al. (1997) on a highly plastic clay with a similar liquid limit ðwL ¼ 108%Þ. The difference may depend on the different mineral composition as well as on the preparation procedure. In fact the clay studied by Abdullah et al., constituted mainly by illite and mixed layer illite-smectite, had been compacted. 3.2. Salt solution-saturated materials The influence of pore solution concentration on clay compressibility has been studied by many authors (among others: Bolt, 1956; Taylor, 1959; Moum and Rosenqvist, 1961; Olson and Mesri, 1970; Di Maio, 1996a). Most results refer to dilute solutions. This paper aims to investigate also the case of concentrated salt solutions, whose influence on clay behaviour is not interpretable by classical double layer models. Fig. 11 reports the oedometer curves for specimens of the Ponza bentonite reconstituted at about the liquid limit with NaCl solutions at various concentrations and immersed in the same solutions. It can be observed that the influence of pore solution concentration on compressibility is comparable to that of mineral composition and of stress level. In particular, the curves of the specimens reconstituted with the concentrated solutions are more similar to that of kaolin than to that of

Fig. 11. Oedometer curves for the Ponza bentonite reconstituted with and immersed in NaCl solutions at various concentrations.

the water-saturated bentonite. The different compression curves tend to converge towards a narrow range of void ratio in the considered stress range, as expected on the basis of the diffuse double layer model. A similar effect, although lower, can be observed in the case of the Bisaccia clay (Fig. 12). For both soils, the initial fluid content was about the liquid limit evaluated with the appropriate solution, so, it was different for the different solutions. The liquid limit state was found to be a

3

distilled water 0.5 M NaCl

2.5

1 M NaCl sat. NaCl solution

2 void ratio e

Fig. 10. One-dimensional coefficients of consolidation cv and of swelling cs against axial stress for the Bisaccia and the Marino clays reconstituted with distilled water.

1.5

1 0.5

0 10

100

1000

10000

σ 'a (kPa) Fig. 12. Oedometer curves for the Bisaccia clay reconstituted with and immersed in NaCl solutions at various concentrations.

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451 1.6

2 w = wL w = 1.5wL

1.2 void ratio e

1.5

distilled water 1 M NaCl

1.4

w = 1.2wL

void ratio e

443

1

1 0.8 0.6

0.5

0.4

0 100

σ 'a (kPa)

1000

0.2

10000

Fig. 13. Oedometer curves relative to the Bisaccia clay reconstituted with 1 M NaCl solution at three different initial solution contents.

reference state for the Ponza bentonite and the Bisaccia clay even when they were prepared with concentrated solution. In fact the compression curves converge at low r0a values, as shown by Fig. 13 which refers to the Bisaccia clay prepared at some different solution contents higher than wL . Fig. 14, which reports the results obtained for the Marino clay, shows that the compression curves obtained with the two different fluids do not converge. A similar behaviour was observed in the case of the commercial kaolin (Fig. 15). Furthermore, in the case of this latter clay, the oedometer curve position depends on the initial water content more than on the pore liquid composition, as shown by the results relative to a specimen reconstituted with distilled water and two specimens reconstituted with 1 M NaCl solution, one of which prepared at wL and the other at 1.5 wL . As far as ion concentration increases, the differences among the materials reduce dramatically, as shown by Fig. 16 which reports the compression index against pore solution molarity for an increment of axial stress from 150 to 300 kPa. The influence of pore solution concentration on cv is noticeable in all the considered stress range (from Figs. 17–19). The difference between the coefficient for the material reconstituted with distilled water and that reconstituted with the saturated NaCl solution is of about two–three orders

10

100

σ 'a (kPa)

1000

10000

Fig. 14. Oedometer curves for two specimens of the Marino clay, one reconstituted with and immersed in 1 M NaCl solution, the other reconstituted with and immersed in distilled water.

1.8 distilled water sat. NaCl sol.wi = 1.5wL sat. NaCl sol.wi = wL

1.6 1.4

void ratio e

10

1.2 1 0.8 0.6 0.4 0.2 10

100

σ 'a (kPa)

1000

10000

Fig. 15. Oedometer curves for two kaolin specimens reconstituted with 1 M NaCl solution at different initial solution contents and for a specimen in distilled water.

of magnitude (depending on the axial stress level) in the case of the Ponza bentonite, one order in the case of the Bisaccia clay, lower for the Marino clay. No differences were found for kaolin. The coefficient of swelling cs , reported in the same figures, has a more complex variation: it decreases with increasing pore solution concentration at very low stress levels and for high smectite contents, whereas it increases in all the other cases.

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C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451 5 Ponza bentonite Bisaccia clay

4

Marino clay

1.E-03

kaolin

3

cv distilled water

cs distilled water

cv 0.5 M NaCl

cs 0.5 M NaCl

cv 1 M NaCl

cs 1 M NaCl

cv sat. NaCl sol.

cs sat. NaCl sol.

Cc

∆σ 'a = 300 kPa - 150 kPa 2

2

1

cv . cs (cm /s)

1.E-04

0 0

1

2

3

4

5

6

1.E-05

NaCl solution molarity 1.E-06

Fig. 16. Compression index for an increment of axial stress from 150 to 300 kPa against NaCl solution molarity. 1.E-07 10

1.E-02

cv distilled water

cs distilled water

cv 0.6 M NaCl

cs 0.6 M NaCl

cv 1 M NaCl cv sat NaCl sol.

cs 1 M NaCl cs sat. NaCl sol.

100

σ 'a (kPa)

1000

10000

Fig. 18. One-dimensional coefficients of consolidation cv and swelling cs against axial stress for the Bisaccia clay reconstituted with distilled water and with NaCl solutions.

1.E-03

2

cv . cs (cm /s)

1.E-03 1.E-04

1.E-05

2

cv . cs (cm /s)

1.E-04

1.E-06

1.E-07

1.E-05 cv distilled water cs distilled water

1.E-06

cv 1 M NaCl

1.E-08 10

100

σ 'a (kPa)

1000

10000

Fig. 17. One-dimensional coefficients of consolidation cv and swelling cs against axial stress for bentonite reconstituted with distilled water and NaCl solutions.

3.3. Cyclohexane saturated materials and dry materials One of the parameters influencing clay behaviour is the pore medium dielectric constant. The influence of such a parameter on volume change behaviour of clays has been investigated by many authors. Jimenez Salas and Serratosa (1953) presented oedometer compression and consolidation curves for a natural bentonite saturated with three

cs 1 M NaCl 1.E-07 10

100

1000

10000

σ 'a (kPa) Fig. 19. One-dimensional coefficients of consolidation cv and swelling cs against axial stress for the Marino clay reconstituted with distilled water and with 1 M NaCl solution.

organic solvents with different values of dielectric constant. The authors showed that initial void ratio and compressibility decrease with decreasing dielectric constant, whereas time rate of consolidation dramatically increases. Consistently with these results, Mesri and Olson (1971) showed that the Wyoming bentonite saturated with non-polar

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

fluids is characterized by much lower void ratios and much higher coefficient of permeability than with water. Sridharan and Ventakappa Rao (1973) found analogous results. Furthermore, they showed that the type of influence on kaolinite can be opposite to that on montmorillonite, and that, irrespective of the soil type, shear strength increases as the dielectric constant decreases. More complex variations of Cc and shear strength of kaolinite with dielectric constant were found by Chen et al. (2000) and Moore and Mitchell (1974) respectively. The next tests were carried out in order to observe whether materials saturated with non-polar

445

fluids, characterized by values of dielectric constant close to that of air, behave similarly to dry materials. Fig. 20a reports the oedometer compression and swelling curves for two specimens of the Ponza bentonite reconstituted at different initial contents of cyclohexane, and the curve relative to a dry specimen. For comparison, the figure reports also the curves of the material prepared with distilled water and with saturated NaCl solution. It can be observed that, at difference to the material saturated with aqueous solutions, the position of the e– log r0a curves relative to the system bentonite–cyclohexane depends on the initial void ratio. Furthermore, the curves intersect the normal 1.8

3.5 3

distilled water

distilled water

sat. NaCl solution

sat. NaCl solution cyclohexane

cyclohexane (1P)

dryBisaccia clay (1B)

cyclohexane (2P)

2.5

dry

2

void ratio e

void ratio e

dry Bisaccia clay (2B)

1.3

1.5

0.8

1 0.5

Bisaccia clay

Ponza bentonite 0.3

0 10

100

(a)

1000

σ 'a (kPa)

10000

10

100

1.4

1000

2.5 distilled water

distilled water sat. NaCl sol.wi = 1.5wL cyclohexane dry kaolin

1M NaCl

1.2

2.1

cyclohexane

dry (1M) 1

dry (2M) void ratio e

void ratio e

10000

σ 'a (kPa)

(b)

0.8

1.7

1.3

0.6 0.9

0.4

Marino clay

kaolin

0.2

0.5

10

(c)

100

1000 σ'a (kPa)

10000

10

(d)

100

1000

10000

σ'a (kPa)

Fig. 20. Comparison among oedometer curves relative to the materials reconstituted with distilled water, saturated NaCl solution, cyclohexane and to dry materials.

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451 0.8 kaolin

0.7

bentonite (1P)

0.6

Marino clay Bisaccia clay

0.5

Cc

compression lines of the materials reconstituted with aqueous solutions, and converge at high r0a . In particular, specimen 1P, prepared at an initial void ratio very close to that of the dry specimen, exhibited higher compressibility up to about 1000 kPa with respect to the dry specimen. Specimen 2P, with lower initial void ratio, exhibited lower compressibility. Both specimens 1P and 2P swelled negligibly on unloading, similarly to the dry specimen. In the case of the Bisaccia clay (Fig. 20b), the cyclohexane saturated specimen and the dry specimen 2B, prepared at the same initial void ratio, exhibited practically the same compressibility. Furthermore, as in the case of the Ponza bentonite, swelling was negligible. The Marino clay prepared with cyclohexane at the same void ratio as the dry specimen 2M, at low r0a values reached the same conditions as the dry specimen 1M (Fig. 20c). Differently from the previous two clays, the compression curves of the dry specimens do not show any trend to converge. Contrarily to the other materials, the compressibility of kaolin reconstituted with cyclohexane is slightly higher that that obtained by using aqueous solution, consistently with its higher initial void ratio. It is interesting to observe that the compression curve of the cyclohexane saturated specimen merges with that of the dry specimen at very low values of axial stress (Fig. 20d). Similarly to the other materials, swelling is negligible. Fig. 21 shows that the dependence on axial stress of the compression index of the materials reconstituted with cyclohexane is qualitatively similar to that of the loose sand and very different from that obtained for the materials prepared with aqueous solutions. For all the materials prepared with the organic fluid, consolidation occurs in few seconds, as shown by Fig. 22 which compares consolidation curves obtained for the Bisaccia clay reconstituted with the different fluids under an increment of axial stress from 40 to 80 kPa. Only under the highest stress levels for the bentonite and Bisaccia clay it was possible to determine cv by the log-time method, and in this case a value of 0.015 cm2 /s

loose sand

0.4 0.3 0.2 0.1 0 10

100

1000

10000

σ 'a (kPa) Fig. 21. Compression index of the cyclohexane-saturated materials against axial stress.

t (min)

0.1

1.0

10.0

100.0

1000.0

10000.0

0.0 0.2 0.4

- ∆ h (mm)

446

0.6 0.8 1.0 1.2 1.4

(dry (2B) cyclohexane distilledwater 1 M NaCl solution sat. NaCl solution

∆ 'σ = 80 kPa - 40 kPa

1.6 1.8

Fig. 22. Consolidation of the Bisaccia clay reconstituted with five different fluids, for an increment of axial stress from 40 to 80 kPa.

was obtained. This value is much higher than those obtained for the materials reconstituted with aqueous solutions. The decrease of smectitic soils compressibility seems to depend on an increase in shear resistance at the particle contacts. In fact, such a decrease is noticeable also for void ratios higher than those of the materials prepared with the saturated solution. This latter aspect has been investigated further for the Ponza and Bisaccia clays, through some shear tests.

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

3.4. Some data on residual shear strength Fig. 23 reports the residual shear strength of the Ponza bentonite and of the Bisaccia clay reconstituted with and immersed in distilled water, saturated NaCl solution and cyclohexane. Furthermore, it reports some data relative to dry materials. All the specimens were prepared with the same procedures followed for oedometer testing. The specimens (area ¼ 36 cm2 ) were consolidated under given values of normal stress and were sheared back and forth––in a direct shear apparatus––over a distance of 6 mm until the minimum strength was obtained, with a rate of displacement of 0.005 mm/min. A time interval of about 24 h was allowed to elapse between the end of one cycle and the beginning of the next. The figure shows that, for a given pore fluid, the two materials behave very similarly. The residual friction angle u0r obtained in distilled water is very low (about 4° for the Bisaccia clay and about 6° for the Ponza bentonite); it increases to about 15° for both materials in the saturated salt solution. Some first results relative to cyclohexane as pore fluid, show that the residual shear strength increases dramatically with respect to the values obtained with aqueous solutions. In ‘‘dry’’ conditions, the shear strength is even higher. However, as it was men-

bentonite - distilled water

Bisaccia clay - distilled water

bent.- sat.NaCl sol.

Bisaccia clay - sat.NaCl solution

bentonite - cyclohexane

Bisaccia clay - cyclohexane

dry bentonite

dry Bisaccia clay

500

τ r (kPa)

400

300

200

100

0 0

200

400

600

800

σ 'n (kPa) Fig. 23. Residual shear strength of the Bisaccia clay and the Ponza bentonite dry and reconstituted with various fluids.

447

tioned above, the dry condition is difficult to define in very active clays, to obtain and to maintain at room temperature and humidity; hence, the influence of eventual suction cannot be excluded. Further experimentation is required on this aspect, so, data relative to dry materials have been shown here only to fix an upper bound beyond which, reasonably, materials prepared with cyclohexane cannot go. The increase in strength caused by the increase in pore solution concentration, probably depends also on the lower void ratio of materials prepared with the electrolyte. Under the considered stress level, void ratio of the material, either dry or in cyclohexane, is equal to or higher than that in the saturated salt solution, so the increase in shear strength reasonably depends on a particular particle aggregation or on shear resistance increase at the particlesÕ contact.

4. Discussion The largest differences due to mineral composition are observed on the materials prepared with distilled water. Because of its high water adsorption property, smectite is the most influencing fraction. As expected, its influence on compression and swelling indices decreases with increasing axial stress. On the contrary, the influence on the coefficients of consolidation and swelling increases. Robinson and Allam (1998), while analysing the behaviour of homoionised clays reconstituted with water, showed that cv increases with axial stress for kaolinite, whereas it decreases for montmorillonite. Such a different trend––which, in turn, is controlled by hydraulic conductivity––is attributed by the authors to the highest physico-chemical forces acting in the montmorillonite pore water. It is interesting to observe that the coefficients of consolidation and swelling assume almost the same values for the artificial mixtures with a bentonite content higher than 50%, corresponding to a montmorillonite content of about 35%, as for the Bisaccia clay whose smectite content is about 30%. So, this percentage is sufficient to control the transient process completely. The same percentage is sufficient for artificial mixtures

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

montmorillonite–quartz and montmorillonite–kaolinite to reach the same value of residual shear strength as that of montmorillonite (Kenney, 1967; Di Maio and Fenelli, 1994). As far as the difference between cv and cs is considered, it is worth mentioning that the values of cs considered in this paper refer to the swelling process as a whole, driven by both hydraulic and chemical forces. Differently from cv , which refers mainly to ‘‘primary’’ compression, cs determined on the experimental curves of displacements by the log-time method refers also to ‘‘secondary’’ swelling. This aspect was discussed by Mesri et al. (1978), who noticed that ‘‘excess negative pore pressures dissipate faster than predicted from the Terzaghi theory fitted to the observed swell data’’. Consistently, experimental results relative to the Ponza bentonite show that pore pressures dissipate over the same time during consolidation and swelling, whereas the swelling curve takes longer to reach the inflection point than the consolidation curve (Di Maio and Onorati, 2000c). Swelling induced by osmotic gradients, under constant external load, has been shown to be much slower than mechanical swelling, whereas osmotic consolidation time evolution is similar to that of mechanical consolidation (Di Maio, 1996b). So, it is plausible to infer that when volume changes are induced by both mechanical and chemical processes, mechanical and chemical consolidation occur practically simultaneously. On the contrary, the first part of the swelling curve is due to dissipation of negative pore pressures induced by unloading, and the second part to water flow induced by osmotic gradients, under constant TerzaghiÕs effective stress. Pore solution concentration influence on compressibility increases with the smectite content. As expected, Cc and Cs decrease dramatically for the Ponza bentonite and the Bisaccia clay, less for the Marino clay. A 1 M NaCl solution reduces the differences among the compression indices of the various materials of an order of magnitude. The difference between cv and cs increases as axial stress decreases. In terms of time scale, in the case of the Bisaccia clay prepared with 1 M NaCl solution, the inflection point is reached in about 40 min during consolidation for an increment of axial

stress from 20 to 40 kPa, whereas it is reached in about 1500 min for swelling caused by a decrement of r0a in the same range. In the case of the considered concentrated solutions, time evolution of swelling is probably influenced also by other factors––besides those mentioned for water-saturated materials––which are not simple to analyse and are currently under study. In fact, hetero-cations of the natural clays are substituted––by mass action––by Naþ . This process, which depends also on the stress level (Di Maio, 1998), causes further rebound. In the case of the kaolin used in this experimentation, the influence of pore solution concentration is practically negligible. In particular, the results show that its volume change behaviour is influenced by pore solution at very low vertical stress, as those arising in settling tests, but it is governed by grain size and, reasonably, by particle shape in the oedometer tests. It is known that the intrinsic compressibility Cc ¼ e100
e1000 (with e100 and e1000 the intrinsic void ratios at r0a ¼ 100 and 1000 kPa respectively), varies almost linearly with eL (void ratio at the liquid limit). Fig. 24, which reports Cc against eL for the considered soils reconstituted with water and with the NaCl solutions, compares these results to those reported by Burland (1990) and to the line Cc ¼ 0:256eL
0:04 found by the author as the best fit regression line to data reported in the literature. The point relative to the Ponza ben-

1.5 Ponza bentonite

Bisaccia - distilled water

Bisaccia clay Marino clay

1

kaolin data reported by Burland.1990

Cc*

448

0.5

C c * = 0.256e L - 0.04

0 0

1

2

3

4

5

void ratio atliquid limit eL

Fig. 24. Intrinsic compression index against void ratio at liquid limit eL . For each material the values of Cc obtained with different pore solutions are reported.

C. Di Maio et al. / Mechanics of Materials 36 (2004) 435–451

tonite reconstituted with distilled water has not been reported because it is out of the range of validity of the relation (which holds for 0:6 < eL < 4:5), and in fact it would lie well above the regression line. The figure shows that there is a good agreement among data obtained by mixing different materials with distilled water and those obtained by mixing the same material with different pore solutions. So, the state of the materials at the liquid limit can be considered as a reference state also in the case of pore liquids different from water. The Ponza bentonite and the Bisaccia clay prepared with concentrated salt solutions behave very differently from the same materials prepared with distilled water. However, they are still such that a unique normal compression line can be defined––for each system solid–pore liquid––even in the range of axial stress as low as 100–200 kPa. The behaviour changes when a non-polar fluid is used. In fact, the materials prepared with cyclohexane behave more similarly to the dry ones. The coefficient of consolidation increases dramatically and swelling is negligible in the whole considered stress range. Mesri and Olson (1971) explained the strong influence of pore organic fluids on the permeability of the Wyoming bentonite (prepared with carbon tetrachloride and benzene) in terms of changes in fabric and formation of tight domains. Actually, it was observed that the macroscopic aspect of the considered materials changed during preparation with cyclohexane. In fact they assumed fine sand typical feature. However, the materials were not analysed by scanning electron microscopy, so, the formation of tight domains for the cases under examination would be only a conjecture that needs further study. On the other hand, the hypothesis of a large increase in interparticle shearing resistance is confirmed by first results on residual shear strength and by earlier studies. In fact, Moore and Mitchell (1974) attributed to variation of net particle interaction force––resulting from electrostatic and electrodynamic forces––the changes in shear strength found on a kaolin. They analysed the effects of organic solvents in terms of dielectric constant and found that shear strength varied similarly to the Hamaker constant, exhibiting a minimum at a value

449

of D close to that of the solid skeleton. An analogous complex dependence on dielectric constant was found by Chen et al. (2000) on kaolin compressibility. The Authors carried out compression tests on kaolinite with water and nine organic fluids. They found that as the dielectric constant increased from approximately 2 in non-polar fluids to 80 in water, void ratio and compression index of the kaolinite decreased first, reaching a minimum at a dielectric constant of 24 in ethanol, and then increased. The swelling index also increased with the dielectric constant. Their results indicate also that under an overburden stress of 300 kPa, pore fluid properties had essentially no effect on the kaolin compressibility. In the case of our kaolin, the effects of the nonpolar fluid is negligible. On the other hand, the effects on smectitic soils are noticeable. For these latter soils, both cyclohexane and air cause noticeable decrease in compressibility with respect to the salt solutions, which in turn, cause similar decrease with respect to water. In the case of concentrated salt solutions, it seems reasonable to hypothesize that the strong reduction in double layer thickness is such that the pores can be considered as filled by ‘‘homogeneous’’ fluids. So, reasonably, the aqueous ionic solutions can be characterized by a macroscopic parameter such as dielectric constant. D decreases from water ðD ¼ 80Þ to saturated NaCl solution ðD ¼ 47Þ to cyclohexane ðD ¼ 2Þ and to air ðD ¼ 1Þ. Probably, to this decrease and to the related variation in the net interparticle electromagnetic forces can be attributed the further increase in residual shear strength and decrease in compressibility of dry materials and materials saturated with cyclohexane with respect to the solution saturated materials. This hypothesis is currently under study.

5. Conclusions Oedometric compression experiments are presented with different basic skeleton mineral, different smectite content, and finally different pore fluid. Compressibility and unloading moduli in one-dimensional strain states as well as the initial void ratio, are the study focus.

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Pore fluid in soils is generally a dilute composite solution, so, the use of concentrated salt solutions or organic fluids may seem excessive. With reference to the salt solutions, in over 97% of the seawater in the world, the salinity is between 33& and 37& (Stumm and Morgan, 1981). Furthermore, the results of basic research into the history of ocean basins (Ocean Drilling Program) show that pore solution concentration of Mediterranean submarine clays is often much higher than salt concentration in overlying sea water, and sometimes it is close to saturation. It is reasonable to hypothesize that also the marine-origin clayey soils we usually deal with once were formed in a concentrated salt solution. So, in order to understand their behaviour, it is important to take into account the original intrinsic properties which can be determined by using concentrated salt solutions. With reference to the organic liquid, it has been used for its low dielectric constant. Clay soils may come in contact with fluids characterized by dielectric constant lower than that of water either deliberately or accidentally. Besides its practical importance, the analysis of the interactions between solid skeleton and pore organic fluids is very important for the elucidation of soil behaviour itself. The results show: (a) Volume change behaviour of artificial mixtures and natural soils reconstituted with distilled water is strongly influenced by mineral composition and, in particular, by the smectite fraction. The influence depends on the stress level. With increasing axial stress, the influence on compression and swelling indices decreases, whereas it increases on the coefficients of consolidation and swelling. (b) An increase in pore solution concentration makes the materials compressibility decrease and the coefficients of consolidation and swelling increase, with the exception of cs at low stress levels and for high smectite content. The differences among the different materials reduce greatly. (c) The materials prepared with cyclohexane behave more similarly to the dry ones than to

those prepared with aqueous solutions. Compressibility is much lower, the coefficient of consolidation increases dramatically and swelling is negligible in the whole considered stress range. The changes seem to depend on an increase in resistance to interparticle sliding. This hypothesis is supported by the fact that, at a given axial stress, compressibility decreases and residual shear strength increases even if the void ratio is equal to or higher than that of the materials prepared with the salt solutions.

Acknowledgements The authors wish to thank Dr. Giovanni Mongelli who performed X-ray diffraction analysis, Dr. Rossella Coviello for her help in performing some of the tests and Dr. A. Brancucci who evaluated the specific surface As .

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