Osmotic suction of highly plastic clays

RESEARCH PAPER

Osmotic suction of highly plastic clays

Yulian Firmana Arifin Æ Tom Schanz

Received: 8 October 2008 / Accepted: 29 June 2009 / Published online: 28 July 2009

� Springer-Verlag 2009

Abstract This paper presents a study on osmotic suction

of compacted highly plastic clays. Two different types of

bentonite (i.e. Calcigel and a bentonite from India called

herein as Indian bentonite) were used. Squeezing technique

was utilized to obtain soil pore-water of the specimen.

Using relationship between electric conductivity and

osmotic pressure of salt solution, osmotic suction of soil

pore-water was obtained. Additional tests (i.e. total and

matric suction using filter paper method and swelling

pressure using constant volume swelling pressure test)

were performed. The results show that osmotic pressure of

soil pore-water obtained decreases by increasing squeezing

pressure. Based on experimental result, osmotic suction of

the specimen is osmotic pressure of the first drop of

extracted soil pore-water. An empirical method was sug-

gested to determine the squeezing pressure in squeezing

technique. In addition, roles of osmotic suction in thermo-

hydro-mechanical behavior of highly plastic clays were

presented and discussed in the paper.

Keywords Highly plastic clays � Osmotic suction �Squeezing technique � Swelling pressure � Temperature

1 Introduction

In unsaturated soil mechanics, osmotic suction is one of

important components influencing the behavior of soils.

Miller and Nelson [26], for instance, found that at given

water content on shrinkage curve, the volumes of specimen

prepared with sodium chloride giving higher osmotic suc-

tion are smaller than those of specimen prepared with

distilled water. This indicates additional stress acting on

the sodium chloride specimen. In swelling potential

investigation, Rao and Shivananda [35] found that the

difference in osmotic suction did not influence the mag-

nitude of swelling strain of salt-amended clay. However,

specimen with high-osmotic suction showed delay to reach

maximum swelling strain due to migration of cation from

the soil specimen to the reservoir. This delay was also

observed in the osmotic consolidation and swelling of

sodium montmorillonite [12].

One of the soil components to be influenced by tem-

perature is dissolved salt solution (or pore-water chemis-

try). Pusch et al. [33] stated that the increase in temperature

of a bentonite decreases the hydration force due to a

reduction in the number of hydrates in the smectite surface

within bentonite and increases the osmotic pressure in the

molecular system. This leads to reduction or increase in

the swelling pressure of compacted bentonite depending on

the dominant factor in the type of bentonite used. The

swelling pressure will increase when the increase in the

osmotic pressure is dominant and it will decrease when

the reduction in the hydration force is dominant.

The wide use of compacted unsaturated bentonite as

sealing material, e.g. as clay liner in landfill and as ben-

tonite barrier in high-level waste repository, leads to con-

sider osmotic suction as an important issue. This is due to

the fact that the material will be in contact with salt

Y. F. Arifin

Laboratory of Soil Mechanics, Bauhaus-University Weimar,

Coudraystrasse 11C, 99421 Weimar, Germany

e-mail: [email protected]

T. Schanz (&)

Laboratory of Foundation Engineering,

Soil and Rock Mechanics, Ruhr-Universitat Bochum,

Gebaude IA 4/126, 44780 Bochum, Germany

e-mail: [email protected]; [email protected]

123

Acta Geotechnica (2009) 4:177–191

DOI 10.1007/s11440-009-0097-0

solution from leachate in the landfill or water containing

salinity of surrounding rocks in the high-level waste

repository. Therefore, it is important to investigate the

magnitude of osmotic suction and the cation in the pore-

water of bentonite used. Some researchers have investi-

gated the osmotic suction of low plastic clay [11, 18, 26,

31, 40]. However, so far the method to investigate osmotic

suction of highly plastic clay such as bentonite has not been

clearly described.

This paper presents the osmotic suction investigation for

highly plastic clays such as bentonite. Since bentonite

behaviors are highly influenced by the type of its

exchangeable cation, two different types of bentonite (i.e.

sodium bentonite and calcium-magnesium bentonite) were

used. In this paper, swelling pressure, one of osmotic

suction effects on the hydro-mechanical behavior, of the

two types of bentonite is presented and discussed. Fur-

thermore, the correlations between osmotic suction of soils

and physico-chemical characteristics of soils and change in

osmotic suction related to temperature effects are also

presented and discussed in this paper.

2 Literature review related to osmotic suction of soils

Soil suction consists of two main components, i.e. matric

component and osmotic component. The matric component

of soil suction comes from capillary and hydration force

mechanisms, and the osmotic component comes from

dissolved solutes in soil pore-water. The sum of these two

components is referred as total suction.

In nature, evaporation can cause decrease in the amount

of pore-water in soil. This results in concentration increase

of dissolved ion or the osmotic suction of the soil. At low

water content in which there is no dissolved salt in the soil

pore-water, the osmotic suction is zero and negligible [40].

Mata et al. [25] proposed a cut-off law that the osmotic

suction is being constant at water content lower than micro-

structural water content. Micro-structural water is defined

as water contained in the intra-aggregate pores. This is not

easy to determine in practice and may need a complex

micro-structure investigation.

Miller and Nelson [26] and Sreedep and Singh [40]

determined the osmotic suction of clayey soils from sub-

traction of the total and matric suction that are measured

independently. Miller and Nelson [26] used filter-paper

technique and pressure-plate apparatus, whereas Sreedep

and Singh [40] used chilled-mirror hygrometer and pres-

sure membrane extractor for measuring total and matric

suction, respectively. Pressure plate and pressure mem-

brane devices operate by imposing a suction value (i.e.

applied air pressure minus water pressure at atmospheric

condition) on a given specimen which can be a soil or filter

paper [9]. This condition is associated with the capillary

component of total suction. That the difference between

total suction and matric suction is equal to osmotic suction

might be true for low plastic clay, because the hydration

force component of this type of clay is insignificant in

developing the matric suction component. For high plastic

clay, subtraction of the total suction and capillary compo-

nent results in osmotic suction plus hydration force com-

ponent. Therefore, an independent measurement of osmotic

suction should be performed where the result gives an

osmotic component due to salt concentration in the free

soil-pore water.

Osmotic suction of soil can be determined by measuring

the electrical conductivity of a soil pore-water extract.

Squeezing technique has been widely used to squeeze out

the soil pore-water [14, 18, 21, 28, 31, 35, 37]. For different

purposes, the squeezing technique was also used for mea-

suring the soil-water chemistry and soluble salt content in

soils (e.g. [17, 23, 24, 45]; ASTM D 4542-95 [6, 38]). The

squeezing technique shows reasonable result compared to

other method (e.g. saturation extract method) for measur-

ing osmotic suction [18] and soil pore-water chemistry

[17]. The problem in using squeezing technique is that the

magnitude of the squeezing pressure applied influencing

the result obtained [15, 17, 38]. Interestingly, almost all

researchers mentioned above used a single squeezing

pressure for different water content of soils. In case of clay

with small montmorillonite content, when squeezing

pressure is increased until a certain value (i.e. approxi-

mately 50 MPa), the salt concentration in extracted soil

pore-water is constant [23]. However, the salt concentra-

tion of the pore-water squeezed out from the soil with high

montmorillonite content decreased by increasing squeezing

pressure [45]. For different clay mineral, Di Mariano et al.

[11] observed important changes in the osmotic suction

when squeezing a low-activity tectonised clay that contains

kaolinite and illite minerals at stresses between 5 and

23 MPa. Therefore, it is important to investigate the

magnitude of squeezing pressure applied without affecting

the result obtained.

In unsaturated condition, compression under externally

applied pressure occurs due to expulsion of air. Since there

is a change in applied pressure, pores and the radius of air–

water interface changes. The soil attains a new equilibrium

state represented by changing in void ratio (e) and degree

of saturation (Sr). Nagaraj et al. [30] suggested a state

parameter, (e/Gs)HSr, acquired from the combination of

unsaturated clay state (i.e. e and Sr). This macro structural

parameter was formulated from micro structural consider-

ations and its relation with independent stress state vari-

ables (r - ua) and (ua - uw). Even different approaches

exist for all many different situations dealing with unsat-

urated soils; the parameter suggested by Nagaraj et al. [30]

178 Acta Geotechnica (2009) 4:177–191

123

is simple with the added advantage of its predictive capa-

bility with only initial water content, dry density, and

properties of soils as input parameter. Considering the

squeezing pressure as external pressure, this generalized

state parameter can be used to predict the squeezing pres-

sure required in osmotic suction investigation.

This paper presents the osmotic suction investigation for

highly plastic clays using squeezing technique. A method

to predict the squeezing pressure for highly plastic clay is

presented. The comparisons between osmotic suction

obtained from squeezing technique and subtraction of the

total and matric suction using filter paper method are also

presented and discussed in this paper.

3 Material used and specimen conditions

In this study, a sodium-type bentonite from India, named

Indian bentonite from here on, and a calcium–magnesium-

type bentonite (i.e. Calcigel) were used in order to study

the osmotic suction of different types of highly plastic clay.

The Indian bentonite was selected because the material is

sodium bentonite which has different behavior from Cal-

cigel. Table 1 summarizes the physico-chemical properties

of bentonite used in this study. According to Table 1,

Indian bentonite has Na? exchangeable cation of 64 meq/

100 g or almost 80% from the total basic exchangeable

cation. Calcigel has Ca–Mg exchangeable cation more than

90% from the total basic exchangeable cation.

Because osmotic suction and swelling behavior of soil

are influenced by its initial pore-fluid concentration [35],

the bentonites were mixed with distilled water. The

distilled water has electric conductivity of 1 9 10-2 mS/

cm which corresponds to an osmotic suction of 0.3 kPa

calculated using Eq. 1. Since the osmotic suction of dis-

tilled water is very small, it is thought that the effects of

initial pore-fluid on the osmotic suction of the compacted

bentonite can be reduced. After curing for 2 weeks, the

specimens were statically compacted to reach the required

dry density as shown in Fig. 1.

4 Experimental techniques and procedures

The squeezing process to extract the specimen’s pore-water

was performed using a squeezer that had equal shapes and

dimensions to the squeezer used by Krahn and Fredlund

[18]. Figure 2a shows the squeezer used in this study.

According to the figure, the squeezer is divided into three

main parts (i.e. ram, cylinder, and base). The squeezer was

equipped with accessories such as Teflon disk, rubber disk,

filter holder, and rubber washer. The Teflon disk was used

to separate the ram and rubber disk and to avoid rubber

disk destroy when high pressure was applied. The rubber

disk and rubber washer were used to avoid leakage in the

cylinder and between filter holder and base, respectively.

The filter holder was used for placing three layers of wire

mesh with 1 mm diameter and filter paper.

The osmotic pressure of the pore-water obtained from

the squeezing technique was inferred using an empirical

relationship between osmotic suction and electrical con-

ductivity (or a calibration curve) which was measured

using a conductivity meter. The calibration of the con-

ductivity meter was performed using molal NaCl solutions.

Table 1 Summary of material characteristics of bentonite used in this study

Properties Calcigel Indian Bentonite

Specific gravity 2.80 2.85

Liquid limit (%) 180 400

Plastic limit (%) 56 33

Plasticity index (%) 124 417

Shrinkage limit (%) 18 14

Clay content (%)a 40 82

Fine content (%)a 100 100

External specific surface area (m2/g)b 81 81

Total specific surface area (m2/g)c 525 400

Basic exchangeable cation Na?, Ca2?, Mg2?, K? (meq/100 g)d 2, 29, 17, 0 64, 10, 7, 1

Total basic exchangeable cation (meq/100 g) 48 82

a Determined using sedimentation method (ASTM D 422-63)b Brunette-Emmet-Teller (BET) methodc Determined using Ethylene Glycol Monoethyl Ether (EGME) methodd Determined using inductively couples plasma atomic emission (ICP)

Acta Geotechnica (2009) 4:177–191 179

123

The water potentials (or osmotic suctions) of the solutions

used were calculated according to the procedures and the

equation given by Lang [19].

Figure 3 shows the relationship between electrical

conductivity and osmotic suction or the calibration curve

determined in this study. The calibration curve obtained is

in a good agreement with the calibration curve given in

USDA [43] which was established for salt mixtures in

saline soil. Rao and Shivananda [35] found a single cali-

bration line for sodium chloride and calcium chloride. The

osmotic suction can be computed from the measured

electrical conductivity using the following equation:

p ¼ 38:54EC1:0489 ð1Þ

where p is osmotic suction (kPa) and EC is electrical

conductivity (mS/cm).

The osmotic suction measurement using the squeezing

technique was performed on several statically compacted

specimens. After compaction, the specimen with diameter

of 5 cm and height of 2 cm was trimmed to reach the same

diameter as the squeezer. Pressure was applied one

dimensionally using triaxial-load frame (Fig. 2b) until the

first drop of pore-water expelled. The pressure of 500,

1,000, 2,000, 4,000, 8,000, 15,000, 20,000, 25,000, 30,000,

35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000,

70,000, 75,000, and 80,000 kPa were applied with duration

of 5 min for each pressure. In an experiment where the

specimen was squeezed for duration of less than 5 min, it

was found that pore-water was not the only thing that came

out from the squeezer; the soil did too. Actually, low

pressure can be applied in order to squeeze out the soil

pore-water. However, lowering squeezing pressure takes

time longer to obtain the first drop of soil pore-water. This

will change the soil–water concentration due to, for

instance, evaporation or cation exchange in between ele-

mentary layers and in free pore-water in order to balance

the external pressure applied [38]. The soil pore-water was

collected using a syringe and was subsequently transferred

0.90

1.00

1.10

1.20

1.30

1.40

10 20 30 40 50 60 70

Mixture water content (%)

Dry

den

sity

(M

g/m

3 )

Calcigel

Indian Bentonite

Sr=100%Sr=80%Sr=60%

Fig. 1 Initial water content and dry density of the specimens used

Fig. 2 Device used in the squeezing technique (a) squeezer (b) squeezer in the triaxial load frame


123

to the conductivity meter to measure its electrical con-

ductivity. About 2 ml of soil pore-water was required for

the measurement and was carefully placed on the sensor

surface of the conductivity meter. In about less than 5 s

after the pore-water placement, the value of the electrical

conductivity in (S/cm or mS/cm was shown on the digital

display of the equipment. In order to investigate the

squeezing pressure effects, the pressure was subsequently

increased until the next drop of water was obtained.

Independent investigations of contact and non-contact

filter paper methods were also performed in order to obtain

total and matric suction of the specimen, respectively. The

filter paper method has been used by some researchers to

determine the total suction of soils (e.g. [1, 8, 16, 20, 22].

The uses of filter paper for measuring matric and total

suction are also found in ASTM standards (i.e. ASTM D

5298-94 [6]). In this study, filter paper of Whatman No. 42

was used since the calibration curves of the Whatman No.

42 obtained by different researchers, at different times,

with different batches of filter paper, show more consis-

tency than that of the S&S 589 [20]. The calibration curves

of the filter paper was established using a pressure plate

apparatus or using axis translation technique (ATT) for

suction lower than 1,500 kPa and using a desiccator with

salt solution or vapor equilibrium technique (VET) for

suction higher than 2,000 kPa.

A series of constant volume swelling pressure tests was

also performed in this study using an isochoric cell

developed at the Universitat Politecnica de Catalunya

(UPC), Barcelona, Spain. The cell was used to measure

swelling pressure of compacted bentonite and bentonite–

sand mixture from low to high density by Villar [44], Arifin

and Schanz [5], and Agus and Schanz [3]. The initial

conditions of the specimens were the same as those of

osmotic suction determination as shown in Fig. 1. The

specimen preparation, experimental techniques, and pro-

cedures follow the one-step swelling pressure test proce-

dures described by Arifin and Schanz [5].

5 Results and discussion

5.1 Squeezing pressure effects on the osmotic pressure

of soil pore-water

Figure 4 shows the influence of squeezing pressure applied

on the osmotic pressure of soil pore-water for several

compacted Calcigel specimens. According to the figure, all

specimens show the same trend that the osmotic pressure of

soil pore-water decreases by increasing squeezing pressure.

Figure 4 also shows that the squeezing pressures required

to obtain the first drop of soil pore-water are 45, 60, and

70 MPa for the specimens with water content of 60, 48,

and 43%, respectively. It seems that the squeezing pressure

required to obtain the first drop of soil pore-water increases

by decreasing the specimen water content.

Figure 4 also shows that, when applying a squeezing

pressure of 45 MPa for a duration of 5 min, no water is

obtained from the specimens with water content of 48 and

43%. On the other hand, the use of a squeezing pressure of

70 MPa results in underestimation of the osmotic pressure

of specimens with water content of 48 and 60%. Conse-

quently, it is not possible to use a single squeezing pressure

π = 38.54EC1.0489

1

10

100

1000

10000

0.1 1 10 100 1000

Electrical conductivity, EC (mS/cm)

Osm

oti

c p

ress

ure

, π (

kPa)

Experimental result

USDA 1950

Fig. 3 Calibration curve for determining osmotic pressure by means

of electrical conductivity measurement of soil pore-water

20

25

30

35

40

45

50

55

60

40 50 60 70 80 90 100

Squeezing pressure (MPa)

Osm

oti

c p

ress

ure

of

soil

po

re-w

ater

(kP

a)

.

γd=1.02 Mg/m3;w=60%

γd=1.16 Mg/m3;w=48%

γd=1.17 Mg/m3;w=43%

Fig. 4 Squeezing pressure effect on the osmotic pressure of soil

pore-water of compacted Calcigel specimens


123

for the material used in this study and the pressure applied

should be determined experimentally.

Figure 5 shows the squeezing pressure effects on the

osmotic pressure of soil pore-water and cation concentra-

tion of the squeezed soil pore-water of specimens with

water content at liquid limit (i.e. 180% for Calcigel and

400% for Indian bentonite). As shown in Fig. 5, the

osmotic pressure of soil pore-water decreases by increasing

squeezing pressure for both specimens. The result reveals

that it happens not only to the unsaturated specimens

(Fig. 4) but also to the saturated specimens (Fig. 5). This

occurs due to the decrease in the concentration of cation of

the squeezed soil pore-water when increasing squeezing

pressure as shown in the cation concentration versus

squeezing pressure relationship in Fig. 5. The progressive

decay of the osmotic pressure is due to restrictive mem-

brane effect resulting difficulty of cation to diffuse at ele-

vated pressure as also observed by Iyer [17] and Di

Mariano et al. [11]. Di Mariano et al. [11] used the highest

value of the electrical conductivity that best represents the

osmotic suction of their specimen. In order to avoid this

effect, in this discussion, the term of osmotic suction is

used only for the osmotic pressure of soil pore-water

obtained from the first drop of soil pore-water.

According to Fig. 5, from the highest to lowest con-

centration, cations in the pore-water of Calcigel are Ca2?,

Na?, K?, and Mg2?, whereas the cations in the pore-water

of Indian bentonite are Na?, Ca2?, K?, and Mg2?.

Interestingly, in the CEC determination of Calcigel, the K?

cation was not found in the basic cation exchangeable.

Figure 5a also shows that the Na? concentration is higher

than Mg2? concentration in the extracted soil pore-water of

Calcigel. Opposite result has been found in the CEC

determination that the Mg2? concentration is higher than

Na? concentration. For the Indian bentonite, based on the

result of CEC investigation, the Mg2? concentration is

higher than K? concentration. Reverse result has been

found in the soil pore-water as shown in cation concen-

tration versus squeezing pressure of Indian bentonite

(Fig. 5b). The result obtained in this study shows that the

cations observed in the CEC determination are different

from cations in the osmotic suction determination. The

cations obtained in the CEC are immobile cations which

are possibly removed by chemical reaction, whereas the

cations obtained in osmotic suction determination using the

squeezing technique are mobile cations which are free from

the electrical charge of clay surface when water is added to

the soil. Interaction between these cations influences the

surface potential of clay [27]. It is plausible that the cations

obtained from CEC determination are responsible for the

hydration force of soil suction (matric suction component)

whereas the cations in the free pore-water are responsible

for the osmotic component of soil suction. According to

Pusch and Yong [34], the sorption from very low water

content is due to the hydration force which is associated

with the matric potential and, beyond this; hydration is

0

100

200

300

0 5 10 15 20

Cat

ion

co

nce

ntr

atio

n, n

(m

g/l)


Ca2+Na+K+Mg2+

20

25

30

35

40

Osm

oti

c p

ress

. of

soil

po

re-w

ater

(k

Pa)

.

Calcigel

w=180%

0

500

1000

1500

0 5 10 15 20 25 30

Cat

ion

co

nce

ntr

atio

n, n

(m

g/l)

.


Na+Ca2+K+Mg2+

250

275

300

325

350

Osm

oti

c p

res.

of

soil

po

re-w

ater

(k

Pa)

Indian Bentonite

w=400%

(b) (a)

Fig. 5 Squeezing pressure effects on the osmotic soil pore-water and cation concentration for (a) Calcigel and (b) Indian Bentonite with water

content at liquid limit


123

ascribed to mechanism represented by diffuse double-layer

which is associated with the osmotic potential.

Figure 6 shows the influence of initial conditions of the

specimens on the squeezing pressure required to obtain the

first drop of soil pore-water and, thus, osmotic suction.

Water content, dry density, and degree of saturation of the

specimens as shown in the figure (refer to Fig. 1).

According to Fig. 6a, the pressure required to squeeze out

the soil pore-water for specimens with low water content is

higher than pressure needed to be applied to specimens

with high water content. The magnitude of the squeezing

pressure used appears to be dependent on the initial water

content of the specimen. It is obvious that drier soil

requires much higher pressure to deform the soil skeleton,

expel air, and squeeze water that is placed in the micro

pores and tightly adsorbed by clay surface. Therefore, it is

clearly shown that the use of single pressure to squeeze soil

pore-water is not possible to be used in this study. Fig-

ure 6a also shows that at a given water content the

squeezing pressure required to obtain the first drop of soil

pore-water for Calcigel specimen is higher than that for

Indian bentonite. The result shows that the ability of Cal-

cigel (Ca–Mg bentonite) to hold water molecules is

stronger than that of Indian bentonite (Na bentonite), which

conforms the findings of Pusch et al. [33].

Figure 6b and c shows the relationship between

squeezing pressures for osmotic suction determination

applied in this study versus dry density and degree of

saturation of the specimens, respectively. According to

Fig. 6b, the squeezing pressure used to squeeze out soil

pore-water for the specimens with high dry density is

higher than pressure needed to be applied to the specimens

with low dry density. Based on Fig. 6c, the squeezing

pressure applied to obtain the first drop of soil-pore water

for osmotic suction determination decreases by increasing

the degree of saturation of specimens. Even the depen-

dency of squeezing pressure on the initial water content of

specimen is higher than the dependency on the initial dry

density and degree of saturation as shown from the their

coefficient of correlation. Thus, it can be concluded that the

R2 = 0.95

R2 = 0.93

20

30

40

50

60

70

80

90

100

25 35 45 55 65

Water content (%)

Sq

uee

zin

g p

ress

ure

(M

Pa)

CalcigelIndian Bentonite

R2 = 0.68

R2 = 0.78

20

30

40

50

60

70

80

90

100

Dry density (Mg/m3)

Sq

uee

zin

g p

ress

ure

(M

Pa)

CalcigelIndian Bentonite

(b)(a)

R2 = 0.66

R2 = 0.71

20

30

40

50

60

70

80

90

100

0.9 1 1.1 1.2 1.3 1.4

50 60 70 80 90 100

Degree of saturation (%)

Sq

uee

zin

g p

ress

ure

(M

Pa) Calcigel

Indian Bentonite

(c)

Fig. 6 The magnitude of squeezing pressure required to obtain the first drop of soil pore-water as a function of (a) water content, (b) dry density,

and (c) degree of saturation


123

squeezing pressure required to obtain the first drop of soil

pore-water is also dependent on the initial dry density and

degree of saturation of specimens.

As shown in Fig. 6, the lowest water content of speci-

mens used was limited to 30% since there was no soil pore-

water obtained from the specimen with water content less

than 30% in the squeezing technique until pressure of

90 MPa (i.e. the maximum pressure that can be applied in

the compression machine). An attempt has been made to

correlate the squeezing pressure data and a generalized

state parameter, (e/Gs)HSr, suggested by Nagaraj et al.

[30]. Nagaraj et al. [30], neglected the specific gravity (Gs)

of soils by considering that the values are close for some

soils. However, Gs was used in this study in order to

increase the accuracy of the parameter used. The state

parameter was used to account for the effects of pore-water

tension on the degree of saturation in the range of 40–90%.

Figure 7a shows the squeezing pressure versus the gen-

eralized state parameter, (e/Gs)HSr, relationship.

According to Fig. 7, the pressures required to squeeze

out first drop of soil pore-water for both bentonite merge

into one line. The data points are fit with Eq. 2 giving

coefficient of determination (R2) of 0.94 and standard error

of estimate (SEE) of 5. Three different types of bentonites

(i.e. MX80, GMZ, and Calcigel II) were used to verify the

equation. The physical properties and osmotic suction data

of the bentonites are summarized in Table 2. The result

shows that the squeezing pressures of the bentonites are

close to the predicted line as shown in Fig. 7b. Therefore,

Eq. 2 can be used to predict the pressure required to

squeeze out first drop of soil pore-water with the same

pressure increment performed in this study. This provides

an answer to the current problem on the suitable pressure to

be applied in the osmotic suction measurement using

squeezing technique, especially for unsaturated soil

condition.

Psqueeze ¼ 316 exp �0:3871ðe=GSÞffiffiffiffi

Sr

p

� �

ð2Þ

5.2 Total suction, matric suction, and osmotic suction

of compacted bentonite

Figure 8 shows total and matric suction obtained from filter

paper method and osmotic suction obtained from squeezing

technique of compacted Calcigel. Since the contact filter

paper represents only the capillary component of matric

suction, the total suction minus matric suction curve rep-

resents the hydration force and osmotic suction of the

specimen. Figure 8 also shows that at low water content or

high suction (i.e. approximately 50 MPa), the matric suc-

tion obtained from contact filter paper method is the same

as total suction obtained from non contact filter paper

method. This is due to the fact that, at low water content,

the water flows in the specimen mainly in vapor form and,

therefore, the suction measured using contact filter paper

method includes hydration forces, capillary component,

and osmotic component (or total suction). This is close to

the range of matric suction that can be measured using

contact filter paper method as suggested by Ridley and

Wray [36] (i.e. 30–30,000 kPa).

Figure 8 also presents that the magnitude of osmotic

suction of compacted Calcigel shows almost constant (i.e.

R2 = 0.94

0

10

20

30

40

50

60

70

80

90

100

3 4 5 6 7

(e/Gs)(Sr)0.5

Squ

eezi

ng p

ress

ure

(MP

a)

.

Calcigel

Indian Bentonite

prediction

0

10

20

30

40

50

60

70

80

90

100

3 4 5 6 7

(e/Gs)(Sr)0.5

Squ

eezi

ng p

ress

ure

(MP

a)

.

GMZ

MX80

Calcigel II

(b)(a)

Fig. 7 (a) Squeezing pressure versus (e/GS)(Sr)0.5, and (b) verification of Eq. 2 using three different types of bentonite


123

50 kPa) for the range of water content investigated herein.

The data were similar to the osmotic suction of specimen at

liquid limit (i.e. 35 kPa) as shown in Fig. 5. According to

Fig. 8, the osmotic suction values obtained from squeezing

technique are lower than the values of the total suction

minus matric suction curves. The result proves that the

total suction minus matric suction data obtained using filter

paper method represents the osmotic suction and hydration

forces.

Figure 9 shows the total, matric, and osmotic suction of

compacted Indian bentonite. Like Calcigel, the total suc-

tion and matric suction were measured using contact and

non-contact filter paper method, respectively, and the

osmotic suction was obtained using squeezing technique.

According to Fig. 9, the osmotic suction values decrease

by increasing water content of specimen. The figure also

shows that the magnitudes of osmotic suction of Indian

bentonite are very high (i.e. almost a half of total suction

values) and much higher than matric suction (i.e. capillary

component) in the range of water content used in this

study. Also at liquid limit as shown in Fig. 5, the Indian

bentonite has osmotic suction of 325 kPa which is much

higher than that of Calcigel. The result reveals that the

osmotic suction gives significant contribution to the mag-

nitude of total suction and influences the behavior of the

Indian bentonite. Therefore, at the range of water content

used in this study, the Indian bentonite is sensitive to the

change in the osmotic component of soil suction.

Figure 9 also shows that the total suction minus matric

suction due to capillary component is higher than magni-

tude of osmotic suction. This supports the previous finding

(i.e. for Calcigel specimen) that the total suction minus

matric suction obtained from filter paper method is sum of

Table 2 Summaries of osmotic suction investigations of soils

Properties Calcigel Indian

Bentonite

GMZ MX80 Calcigel

II

FEBEXa Boom

ClaybRegina

ClaycBearpaw

ShaledMorden

Shaled

Specific gravity 2.8 2.85 2.71 2.65 2.75 2.7 2.67 2.83 2.83 2.69

Liquid limit (%) 180 400 276 411 90 102 67 78 103.6 67

Plastic limit (%) 56 34 37 37 42 53 40 30.6 29.8 29

Plasticitas Index 124 366 239 374 48 49 27 47.8 73.8 38

Montmorillonite content (%) 50–60 – – 75f 45 92 15 20 45 0

Clay content (%) 40 83 60 – – 68 50–60 64.9 48 45

Fine content (%) 100 100 98 – – 92 90–100 97.8 – –

Total Ss (m2/g) 500 400 421 526 400 725 52.5 53e 266 153

CEC (meq/100 g) 49 62 68 73f 66 102 34 31.7e 65.3 68

Basic cations exchange Ca–Mg Na Na–Ca Na Ca Ca–Mg – Cae Na–Ca –

Surface charge density (leq/m2) 0.98 1.55 1.62 1.39 1.65 1.41 6.48 5.98 2.45 4.83

Osmotic suction (kPa) 44–57 1943–3659 79–96 1080 45–54 319–579 443 187–202 567.97 827

Main cations in free pore-water Ca Na – – – Na Na, Ca Ca Na Na–K

Psqueeze (MPa) 50–90 30–80 20–50 26–65 35–65 60 10 34.5 4.83 4.83

a ENRESA [14]b Romero [37]c Krahn and Fredlund [18]d Morgenstern and Balasubramonian [28]e Barbour and Fredlund [7]f Muller-Vonmoos and Kahr [29]

10

100

1000

10000

100000

1000000

0 10 20 30 40 50 60

Water Content (%)

Su

ctio

n (

kPa)

Total suctionBestfit (Total suction)Matric suction due to capillary pressureBestfit (Matric suction due to capillary pressure)Osmotic suction from experimental result

Total suction - Matric suction due to capillary pressure

Fig. 8 Total suction, matric suction, and osmotic suction of

compacted Calcigel


123

osmotic suction and hydration forces. From these results, it

can be concluded that the osmotic suction should be

determined independently.

5.3 Osmotic suction and swelling pressure

of compacted bentonite

Figure 10 shows the osmotic suction versus swelling

pressure of compacted bentonite used in this study.

According to the figure, the magnitudes of osmotic suction

are less than the swelling pressure and show constant

values by increasing swelling pressure for Calcigel ben-

tonite. An independent measurement of osmotic suction

was performed to the compacted Calcigel specimen after

swelling pressure test. The magnitude of osmotic suction

obtained was 43 kPa which is almost the same as the

osmotic suction of as-prepared specimens. The result

reveals that the osmotic suction gives insignificant effects

on the swelling pressure of compacted Calcigel. This is

different from Indian bentonite. According to Fig. 10, the

magnitudes of osmotic suction are higher than swelling

pressure values and shows tendency to increase by

increasing swelling pressure. However, there was no

information obtained in this study on the magnitude of

pressure that resulted due to the osmotic suction of the

soils.

The results can be explained qualitatively by means of

osmotic efficiency. Osmotic efficiency is the degree to

which the clay behaves as a perfect semipermeable mem-

brane. Osmotic efficiency of a soil is strongly dependent on

pore fluid chemistry, pore fluid concentration, void ratio,

and interparticle spacing [7]. Considering the cation type

(i.e. calcium type) and its concentration which is very low

in the free pore-water of Calcigel, the material does not

behave as a semi-permeable membrane. It means that there

is no water flow from reservoir to the specimen in order to

balance different concentration of cation between soil pore-

water and distilled water in the reservoir. The water flows

to the specimen mainly in order to fulfill the matric suction

components (i.e. hydration force and capillary component).

Compared to the Indian bentonite which has sodium-type

cation in the soil pore-water with very high concentration,

it is believed that the presence of large difference in con-

centration of cation between soil pore-water and distilled

water in reservoir results in water flow from reservoir to the

specimen. Therefore, the water flows in order to fulfill the

hydration force in between elementary layers and capillary

pressure and to balance the different concentration between

the soil pore-water and water in the reservoir which is

negligible in case of Calcigel. Sketch of water and cation

movements in the bentonites used is presented in Fig. 11.

As mentioned above, the high-osmotic suction would

delay the swelling potential (or swelling strain) develop-

ment to reach maximum swelling [35] and consolidation

process [12] due to migration of the cation. In this study,

the effect of osmotic suction on the swelling pressure

development is investigated by comparing the swelling

pressure development of compacted Calcigel and Indian

bentonite with similar initial conditions (i.e. water content

of 30%, dry density of 1.27–1.3 Mg/m3, and degree of

saturation of 70%). Figure 12 shows the swelling pressure

development of compacted Calcigel and Indian bentonite.

In order to investigate the rate of swelling pressure

development more clearly, the data are plotted as normal-

ized swelling pressure (i.e. swelling pressure (Ps) divided

by maximum swelling pressure (Ps max)) versus square root

of elapsed time. The slope of Ps/Ps max versus t0.5 curve

100

1000

10000

100000

20 30 40 50 60 70

Mixture Water Content (%)

Su

ctio

n (

kPa)

Total suctionBestfit (Total suction)Matric suction due to capillary pressure Bestfit (Matric suction due to capillary pressure)Osmotic suction from experimental result

Total suction-matric suction due to capillary pressure

Fig. 9 Total suction, matric suction, and osmotic suction of

compacted Indian Bentonite

0

500

1000

1500

2000

2500

3000

3500

0 200 400 600 800 1000

Swelling pressure (kPa)

Osm

otic

su

ctio

n (k

Pa)

Indian BentoniteCalcigel

Equality line

Fig. 10 Osmotic suction versus swelling pressure of compacted

bentonite


123

represents the rate of swelling pressure development at

earlier stage (i.e. mostly up to 60% of the maximum

swelling pressure) [2].

According to Fig. 12, the increase in swelling pressure

with time of both specimens was rapid at earlier stage of the

test (i.e. up to Ps/Ps max about 0.6 which also means up to

60% of the maximum swelling pressure of the specimens

tested). At Ps/Ps max higher than 0.6, compacted Calcigel

reached a maximum swelling pressure faster than Indian

bentonite. Since suction in Calcigel is mainly developed by

matric suction (i.e. hydration force and capillary compo-

nent), the Calcigel absorbed water mainly to dissipate the

matric suction. The Indian bentonite which has high

osmotic component at corresponding water content used

(i.e. 30%) absorbed water to dissipate the matric suction and

osmotic suction. The swelling pressure development was

rapid at Ps/Ps max less than 0.5 as result of matric suction

dissipation. At this point, calculated from water absorbed by

the specimen during the swelling pressure test, the degree of

saturation of the specimen is 91%. After matric suction

dissipation, overlapping of absorbed cations on the negative

clay surface behaves like a semi-permeable membrane [27],

thus resulting in water flow from reservoir to the soil in

response to the osmotic gradient. This gives rise to increase

in swelling pressure. However, since there is no true semi-

permeable membrane developed in soil, very high salt

concentration differences between the soil pore-water and

water in reservoir cause diffusion of dissolved salts from the

soil pore-water to the reservoir. According to Barbour and

Fredlund [7], at the same dry density the Ca2? soil has lower

osmotic efficiency compared to Na? soil. However, since

Indian bentonite has higher salt concentration in the soil

pore-water, the osmotic efficiency of this soil is being small

and becomes more permeable to the cation diffusion. This

was indicated by increasing the electric conductivity of

water in the reservoir from 1 9 10-2 mS/cm (i.e. EC of

distilled water used in this study) to 3.5 mS/cm after

5 months swelling pressure test. This opposite motion

apparently slows down the rate of swelling pressure

development. The small rate of migration of the cations out

of the soil might be another reason of this delay as found in

the osmotic consolidation process reported by Di Miao [12].

From degree of saturation data, it can be found that osmotic

suction plays a role in the swelling pressure of Indian

bentonite at nearly saturated conditions.

5.4 Osmotic suction of soils

Table 2 summarizes the osmotic suction data obtained in

this study and collected from literature for different types

of soils. Based on Unified Soil Classification System

(USCS), all soils in Table 2 are highly plastic clays. The

osmotic suction data were measured using the same

squeezing pressure technique.

Not true semipermeable membrane

Soil pore-water reservoir

Indian bentoniteCalcigel

Soil pore-water reservoir

High concentrationLow concentration

(1) (2)

(3)

processes: (1) water movement to balance matric suction component(2) water movement to balance matric and osmotic suction components(3) cation difusion due to high concentration of soil pore-water

cationcation cationcationWater moleculeWater molecule

Fig. 11 Sketch of water and cation movements in Calcigel and Indian bentonite

0,0

0,2

0,4

0,6

0,8

1,0

0 10 20 30 40 50 60

Time square, t0.5 (hours0.5)

No

rmal

ized

sw

ellin

g p

ress

ure

(Ps/

Ps

max

)

CalcigelIndian bentonite

γd=1.27 Mg/m3

w=30%γd=1.29 Mg/m3

w=30%

Fig. 12 Swelling pressure development of compacted Calcigel and

Indian bentonite


123

The data presented in Table 2 are then divided into

commercial bentonite soil (i.e. Calcigel, Indian bentonite,

and FEBEX) and natural soil (i.e. Boom clay, Regina clay,

Bearpaw shale, and Morden shale). According to

Table 7.2, FEBEX bentonite is calcium-magnesium ben-

tonite. However, the dominant cation in the extracted soil-

pore water is sodium. This supports the previous finding

that the cations observed in the CEC determination are

different from cations in the soil pore-water. Data in

Table 2 also shows that soil with Na? dominant in the soil

pore-water (i.e. Indian bentonite, FEBEX, Boom clay,

Bearpaw shale, and Morden Shale) shows higher osmotic

suction than the soils with Ca? in the soil pore-water (i.e.

Calcigel and Regina clay).

In order to investigate the relationship between physico-

chemical properties of soil more clearly, the data were

plotted in Fig. 13. Figure 13a, b, c, and d show the osmotic

suction versus specific surface area (Ss), liquid limit (LL),

cation exchange capacity (CEC), and surface charge den-

sity (i.e. ratio of CEC and Ss) relationships, respectively.

According to Fig. 13a, there is no relationship between

specific surface area and osmotic suction for commercial

bentonite whereas, for natural soils, the osmotic suction

tends to increase by increasing specific surface area for

wide range of osmotic suction. This is due to the fact that

the specific surface area does not influence the behavior of

the soil which has different physical, chemical, and

mineralogical properties. Cerato [10] found that there is no

strong global trend between specific surface area, liquid

limit, and cation exchange capacity due to a wide range of

sites and geologic deposits of the specimens used.

According to Fig. 13b, there is no relationship between

osmotic suction and liquid limit. This is different from

result reported by Sridharan et al. [42], Di Miao [12], and

Sridharan [41] that, for a soil type mixed with any salt

solutions, the liquid limit of soil decreases by increasing

salt concentration in the soil pore-water. This difference is

possible due to the variation of the physico-chemical

properties and mineralogy of the soil in Table 2.

According to Fig. 13c, there is no relationship between

osmotic suction and CEC for commercial bentonites

whereas, for natural soils, the osmotic suction data tend to

increase by increasing CEC. This supports previous finding

that the cations obtained in the CEC determination can be

different from the cations in the soil pore-water. Sridharan

[41] reported that the physical and mechanical properties of

montmorillonite soils such as liquid limit and swelling

potential were not influenced by the total CEC of the soils

other than only by the amount of exchangeable sodium

cation in the CEC. This relationship cannot be observed in

this study due to lack of exchangeable sodium data from

literature. The last figure (i.e. Fig. 13d) reveals that

osmotic suction data of natural soils show almost constant

values by increasing the surface charge density of soils. For

10

100

1000

10000

0 200 400 600 800

Specific surface area (m2/g)

Osm

otic

suc

tion

(kP

a)

BentonitesOther soils

Calcigel

FEBEX

Indian bentonite

(a)

MX80

GMZ

Calcigel II

10

100

1000

10000

0 100 200 300 400 500

Liquid limit (%)

Osm

otic

suc

tion

(kP

a) BentonitesOther soils

Calcigel

FEBEX

Indian bentonite

(b)Calcigel II

GMZ

MX80

10

100

1000

10000

0 25 50 75 100 125 150

Cation exchange capacity (meq/100g)

Osm

otic

suc

tion

(kP

a) Bentonites

Other soils

Calcigel

FEBEX

Indian bentonite

(c)Calcigel II

GMZ

MX80

10

100

1000

10000

0 1 2 3 4 5 6 7

Surface charge density (meq/m2)

Osm

otic

suc

tion

(kP

a) Bentonites

Other soils

Calcigel

FEBEX

Indian bentonite

(d)Calcigel II

GMZ

MX80

Fig. 13 Osmotic suction of soils as a function of (a) specific surface area, (b) liquid limit, (c) cation exchange capacity, and (d) surface charge

density


123

commercial bentonite soils, the osmotic suction data

increase in the narrow range of surface charge density.

Based on data in Fig. 13, it is concluded that there is no

unique relationship between osmotic suction and physico-

chemical properties of soil. For natural soil, the osmotic

suction might be dependent on the origin and surrounding

environment, whereas, for commercial bentonite, the

osmotic suction might be influenced by the production

process. Therefore, it is recommended to directly deter-

mine the osmotic suction of soils studied.

5.5 Temperature effects on suction of soil pore-water

Assume that the basic cations in the clay surface such as

Na?, Ca2?, Mg2?, and K? combine with Cl-, the salt

solution in the soil pore-water are NaCl, CaCl2, MgCl2, and

KCl. Figure 14a, b, c, and d show the temperature effects

on the total suction of NaCl, CaCl2, MgCl2, and KCl cal-

culated from osmotic coefficient or relative humidity data

of molal salt solution reported by Pitzer and Pelper [32],

DOW [13], Wang et al. [46], and Archer [4], respectively.

From relative humidity data, the total suction of salt

solution was calculated using Kelvin equation, that is, the

thermodynamic relationship between total suction and

relative humidity of the vapor space in the soil [39] (Eq. 3)

and the total suction from osmotic coefficient data was

calculated using Eq. 4 suggested by Lang [19].

st ¼ �RT

Mw 1=qwð Þ lnRH

100

� �

ð3Þ

where st is total suction in kPa, R the universal gas constant

(i.e. 8.31432 J/mol K), T absolute temperature in Kelvin,

Mw the molecular weight of water (i.e. 18.016 kg/kmol),

qw the unit weight water in kg/m3 as a function of

temperature, and RH relative humidity.

st ¼ 2mRT/ ð4Þ

where m is the molal salt solution (mol/Kg), / osmotic

coefficient. The change in water density due to change in

temperature has been considered in the calculation.

100

1000

10000

100000

NaCl concentration (molal)

Suc

tion

(kP

a)

20°C

80°C

Solubility line

(a)100

1000

10000

100000

1000000

0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 14

CaCl2 concentration (molal)

Suc

tion

(kP

a)

20°C

80°C

Solubility at 20°C

Solubility at 80°C

(b)

100

1000

10000

100000

1000000

0 1 2 3 4 5 6 7

MgCl2 concentration (molal)

Suc

tion

(kP

a)

100°C

25°C

Solubility line

(c)100

1000

10000

100000

0 1 2 3 4 5 6 7

KCl concentration (molal)

Suc

tion

(kP

a)

80°C

20°C

Solubility line

(d)

Fig. 14 Temperature effects on suction of salt solution (a) sodium chloride, (b) calcium chloride, (c) magnesium chloride, and (d) potassium

chloride


123

According to Fig. 14, changes in temperature mainly

influence the suction of the salt solution for suction higher

than 1,000 kPa; it can be stated that the osmotic suction of

Calcigel is not influenced by temperature. This is because

the osmotic suction of Calcigel obtained is constant at

50 kPa for the range of water content investigated in this

study. On the contrary, the osmotic suction of Indian

bentonite is possibly influenced by temperature since the

osmotic suction obtained is higher than 1,000 kPa for the

range of water content considered in this study.

6 Conclusion

The osmotic suction of highly plastic clays has been pre-

sented. It can be concluded that, in the squeezing pressure

technique, osmotic suction of highly plastic clay is

obtained from the first drop of extracted soil pore-water.

The squeezing pressure applied is more dependent on the

initial water content of the specimens and less dependent

on the initial dry density and degree of saturation. The

pressure required to obtain the first drop of highly

plastic clays can be estimated from empirical relationship

between squeezing pressure versus a state parameter (i.e.

(e/Gs)HSr).

For the two highly plastic clays used in this study, the

subtraction of total and matric suction is higher than

osmotic suction obtained from squeezing technique and

represents the sum of osmotic suction and hydration force

of soils.

In the swelling pressure measurement, there is no effect

of osmotic suction on the magnitude of swelling pressure

of Calcigel. But, for Indian Bentonite, the magnitude of

swelling pressure is apparently influenced by the osmotic

suction values. High osmotic suction in the Indian ben-

tonite causes delay to reach equilibrium (or maximum

swelling pressure) in the constant volume of swelling

pressure measurement.

There is no unique relationship between the osmotic

suction of soils and its physico-chemical properties.

Therefore, independent measurements of osmotic suction

should be performed in order to study the response of soil

to the change in surrounding environment.

Considering temperature effect on the suction of salt

solutions, temperature influences the suction of salt solu-

tion at suction higher than 1,000 kPa. The osmotic suction

of Calcigel is not influenced by temperature, whereas

the osmotic suction of Indian Clay is influenced by

temperature.

Acknowledgment The final assistance provided by Bundesminis-

terium fur Bildung und Forschung (BMBF), Germany through

research grant 02C0881 for this research is gratefully acknowledged.

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Osmotic suction of highly plastic clays

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