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SWELLING AND COMPRESSIBILITY CHARACTERISTICS OF SOIL – BENTONITE MIXTURES (Agus Setyo Munt ohar)

Jurusan Teknik Sipil, Fakultas Teknik Sipil dan Perencanaan – Universitas Kristen Petra

http://puslit.petra.ac.id/journals/civil/

93

SWEL LING AND COMP RE SSIBILITY CHARACTER ISTICS OF

SOIL – BEN TONITE MIXTUR ES

Ag u s S e t y o M u n t o h a r

Departm ent of Civil an d E nvironmen tal En gineering, Fa culty of Engineering, University of Malaya,Kuala Lumpur MalaysiaEm ail: am un toha r@hotm ail.com

ABSTRACT

Understanding characteristics of soil mixtures lead to increasing the confidence level before applyingsuch materials in the field. The outcomes of this study can provide insight into the swelling and thecompressibility behavior of soil – bentonite mixtures, between non-swelling materials and swellingma ter ials. A simple swell an d compression labora tory test ha s been conducted for the pur poses of this study. The result of this study indicated that the existence of bentonite in the soil mixturesinfluence the swelling behavior, which follows a hyperbolic curve model. Amount and size of non-

swelling fra ction affected th e swelling an d compr essibility.

Keywords: bent onite, soil mixtur es, swelling, swelling pressu re, compr essibility.

INTRODUCTION The swelling behavior of expansive soils oftencauses unfavorable problems, such as differen-tial settlement and ground heaving. Recently,expansive soils are attracting greater attention.The soil is often designed as soil mixturesrequiring among others low shrinkage a n d

swelling properties, low hydraulic conductivity,and high strength. There are variousapplications where the soils have to beengineered to suit the desired behavior as back-filling (buffer) materials for high-level nuclearwaste [1], and soils barrier for landfill liner [2,3].

Many attempts have been performed in the pastto understand the properties, indices andgeotechnical properties, of the soil mixtures.Previous researchers have concentrated on the

swelling behavior of expansive clays but withlittle attention given to the compressibilitycharacteristics. The soil mixtures are commonlya blend of swelling soils (montmorillonite) withnon-swelling soils (kaolinite, and/or sand) [2, 4].The engineering properties of clay soils, such asthe plastic limit and liquid limit, depend uponthe moisture content of the clays which in turnis at tr ibuta ble to th e am ount of clays with highcation exchange capacity and to the type of inter layer cations. The plast icity index, i.e. th e

N o t e: Discussion is expected before November, 1st 2003. Theproper discussion will be published in “Dimensi TeknikSipil” Volum e 6 Num ber 1 Mar ch 2004.

liquid limit minus the plastic limit, is a commonindicator of an expansive soil. At liquid limit,Na-montmorillonite may have water filmsbetween 100Å and 200Å thick and Ca-montmorillonite may have water films between15Å and 19Å thick [5]. Shrink and swellingbehaviour of montmorillonite soils makeunstable slopes and cause building foundation

problems. Swelling or uplift pressures generatedby montmorillonite may approach values ashigh as 981 kP a [6].

The solution to some of these engineeringproblems may be resolved by understanding thebehaviour of clays composed of single and/ormixtures of clay minerals. This paper presentsthe result of a study in the volume change andcompressibility of soil – bentonite mixtures. Anexperiment is carried out to study swell –deformation characteristics of bentonite mixedwith different amount and types of non-swelling

soil such as coarser fraction (sand) and finerfractions (silt).

LABORATORY EXP ER IMENTS

M a t e r i a l s

The soils used in this study are commercialbentonite clay, kaolin, and fine sand. Theparticle size distributions of the soils are shownin Fig. 1, and the basic properties are listed inTable 1. The percentage of bentonite by weight

is varied between 5% and 100%. The plasticityand potential expansiveness of soil – bentonitemixtures ar e illustrat ed in Fig. 2.

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T a b l e 1 . P h y s ic a l p r o p e r t i e s o f t h e s o i l u s e d .

Soilsdescription

Sand%

Silt%

Clay%

D50

µm

LiquidLimit

PlasticLimit

Linear shrinkage

Activity,PI/CF

Bentonite a 5.4 2.4 73.20.62 307.3% 45.4% 17.4% 3.6Kaolin 4.3 75.6 19.9 4.1 72.3% 39.8% 6.6% 1.6Sand b 71.5 4.0 0.0 820 NP - - -

Note: a

Wyoming bentonite,b

Mining sand (24.5% Gravel), NP = non-plastic,PI = plasticity Index, CF = Clay fraction

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10Particle size (mm)

P e r c e n t p a s s i n g

Bentonite

Kaolin

Sand

Clay Silt Sand Gravel

Figure 1. Particle size distribu tion of soi ls.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Liquid Limits, LL (%)

P l a s t i c i t y I n d e x ,

P I ( %

Kaolin-Benonite

Sand-Bentonite

A-LineU-Line

MH/OH

ML/OL

CL/OL

CH/OH

CL-ML

0

20

40

60

80

100

0 10 20 30 40 50 60 70

Clay content (%)

P l a s t i c i t y I n d e x ( %

Very HighHigh

Medium

Low

Low

A = 0.5

A = 2.0

Swelling Potential

Figure 2. Potential expansiveness and plasticity chart of soil-bentonite mixtures

The kaolin comprised predominantly silt(75.6%), whereas 73.2% of the bentonite is clay.The sand comprised 35.6% coarse, 23.6%medium, and 12.3% fine size. Uniformitycoefficient (Cu) and coefficient of curvature (Cc)are 9.08 and 1.19 respectively. Fur ther , thesand can be classified into well-graded sandwith gravel (SW).

T e s t s S c h e m e

A conventional oedometer apparatus was usedfor determination of the swelling andcompressibility of soil mixtures. The calculatedquan tities of soil mixtur es, at optimum moistur econtent (OMC), were transferred to consoli-dation ring of 50 mm internal diameter and 20mm height. All the soil mixtures were com-pacted statically in the specially constructedmould (Fig. 3) to their maximum dry density(MDD) using the hydraulic jack. The com-pressive force was applied to the upper pluguntil the flange is in contact with the barrel of

the mould. The moisture content and drydensity of the specimens after moulded wasdetermined as a contr ol to their OMC and MDD.The results ar e presented in Table 2.

The specimen was positioned in the loading

frame with a seating load of 3.89 kPa. The soilsamples were then inundated with distil ledwater and allowed to swell until they reachedequilibrium values of swelling. At this point astandard consolidation test is conducted byapplying incremental loads starting with 14 kPaand ending with 1600 kPa. The pressurerequired to revert the specimen to its initialheight was determined a s th e swelling pressure.

45.2

φ85.

5.0

15.0

φ63.5

φ63.3

φ 85.0

25.2

15.0

φ 85.0

Upper plug

Mould

Bottomplaten

Oedometer

ring φ5020.0

φ63.3

Extruder plunger

A A

φ63 .

Section A – A

φ50.0

Bottom platen

Mould, t = 3 mm

Oedometer ring

Figure 3. Static compaction mould for swelling

specimens.

T a b l e 2 . C o m p a c t i o n P r o p e r t i e s S o il M ix t u r e s .

Soil Code KB1 KB2 KB3 KB4 SB1 SB2 SB3

OMC (%) 32.4 30.6 30.7 31.5 18.2 17.9 16.5Moisture *(%) 32.43 30.20 31.03 31.24 18.53 18.10 16.64

Variability (%) 0.09 1.30 1.07 0.82 1.80 1.11 0.84MDD (Mg/m3) 1.28 1.29 1.30 1.31 1.72 1.67 1.60

Dry density*(Mg/m3)

1.275 1.294 1.308 1.314 1.719 1.662 1.610

Variability (%) 0.39 0.31 0.61 0.30 0.05 0.47 0.62

Note: *After molded; OMC : optimum moisture content; MDD:maximum dry density

C la y m i n e r a l o gy

The clay minerals have been determined by X-ray diffraction test as shown in Fig. 4. Thekaolin consists of mainly kaolinite mineralswhich is obviously at peak 12.7 Å, 24 Å, and38.43 Å. The other mineral, illite was also

detected interlayer with montmorillonite andkaolinite at 5.01 Å, 4.46 Å and 4.37 Å. The mica-

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like clay minerals, phlogopite (brown mica) wasobserved at 8.89o. The montmorillonite mineralwas found strongly in bentonite at basaltspacing of 5.75 Å, 17.25 Å, and 19.98 Å. Quartzand ill i te mineral was strongly appeared atbasalt spacing of 3.36 Å and 3.19 Å respectively.

The quartz was also found at 4.28 Å, 2.46 Å, and2.29 Å in the low intensities. Mixed layerbetween illite and montmorillonite was found at4.52 Å of basalt spa cing.[4].

5 10 15 20 25 30 35 40

Diffraction Angle (2 θθ)

M

Q

Q

K

I

K

I

Bentonite

Kaolin

I/M

I/M/K KK

K

I/M

I

K/M Q Q

Note: I: Illite, K: Kaolinite, M: Montmorillonite, Q: Quartz

Figure 4. X-Ray diffr action of kaolin and bentoni te soils.

RE SULT AND DISCUSSION

S w e l li n g C h a r a c t e r i s t i c s

The swell potential was defined as thepercentage swell of a laterally confined sample,which has soaked under a surcharge pressure of 3.89 kPa after being compacted to maximumdensity at optimum moisture content accordingto the compaction test. The results obtained arepresented in Fig. 5 in the form of percentage of swell versus time with varying percentage of bentonite content. The swell is expressed as apercenta ge increase in sample height.

It has been observed that for all the mixtures,increase in swelling with log time is slowinitially, increases steeply, an d t hen reaches anasymptotic value. The time required t o reach a nasymptotic value varies considerably, dependingupon the percentage of bentonite and the typeand amount of non-swelling fraction. Themaximum amount of swelling generallyincreases with increasing bent onite content. F ig.

5 depict, even at the same percentage of bentonite, that considerable differences exist in

the nature of time-swell relationship. At lowbentonite content, the rate of swelling is veryslow with sand but increases gradually withdecrease in the particle size of the non-swellingfraction.

Fig. 6 replots the time-swell relationship aspercentage of the maximum swell. Here, thepercent swell at a particular t ime is calculatedas the ratio of amount of swell of the mixture atthat t ime to the total swell and is denoted as apercentage. This is observed that for sand –bentonite mixtures, the rate of swelling is veryslow and follow a similar swell path. In otherhand, i t was observed that the swelling almostcompleted within 1440 min (±24 hrs) for kaolinmixtur es for all percenta ge of bent onite.

Elapsed Time (min)

0.1 1 10 100 1000 10000

S w e l l ( % )

0

10

20

30

40

50

5%

10%

20%

30%

10%

30%

50%

Percent Bentonite:

Kaolin-Bentonite

(KB)

Sand-Bentonite

(SB)

Figure 5. Swelling behaviour (as percent of initial height)of soil-bentonite mixtures

Elapsed Time (min)

0.1 1 10 100 1000 10000

P e r c e n t a g e o f T o t a l S w e l l ( % )

0

20

40

60

80

100

5%

10%

20%

30%10%

30%

50%

Percent Bentonite:

(KB)

(SB)

Figure 6. Swelling behaviour (as percent of total swell) of soil-bentonite mixtures

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There is very limited reference about the rate of swelling. However, the rate of swelling may beapproached as similar as in the coefficient of consolidation determination as given inEquation 1a.

cv =50

2

2

t

H .T

avgv

(1a)

If T v and H av g are taken as constants and cv issymbolized with cs, then the equation can beexpressed as follows:

cs ≈ 50

1

t (1b)

Th e t 50 is the time required to achieve 50% of

swell. For all mixtu res 50% of swell are a chievedwithin 100 min (±1 hr, 40 min) and 1000 min(±16 hrs, 40 min) for kaolin and sand mixturesrespectively.

Dakshanamurthy [8] noticed two stages of swelling. In the first stage of hydration of dryclay particles, water is adsorbed in successivemonolayers on the surface of montmorilloniteclay apart which referred to as interlayer orintercrystalline swelling. The second phase of swelling is due to double-layer repulsion. Largevolume changes accompany this stage of swelling. These occurrences can be approachedcloser as illustrated in Fig. 7, which the curvecan be divided into three phases. Initial swellingis generally less than 10% of the total swelling.This is essentially due to swelling of thebentonite clay particles within the voids of thecoar ser non-swelling fra ction.

0

20

40

60

80

100

0.1 1 10 100 1000 10000 100000

Elapsed Time (min)

P e r c e n t a g e o f T o t a l S w e l l ( %

Primary

swe l l

Secondary

swe l l

End of

Primary swell

10% Bentonite - 90% Kaolin

200 min

Figure 7. Typical time-percent swell of total swell cur ve

Pr imary swelling develops when th e void can nolonger accommodate further clay particleswelling. It occurs at a faster rate. After theprimary swelling was completed, slow continuedswelling occurs. It is observed that the end of primary swelling of kaolin-bentonite mixturesare varies within 200 – 1000 minutes. Ingeneral, the time needed for completion theprimary swelling increase associated withincrease in bentonite content. But, this is notappeared in the sand-bentonite mixtures.

Furthermore, the swelling mechanism of compacted expansive clay can be illustrated bythe model as given in Fig. 8. The volume of swelling clay particles such as montmorilloniteincrease by absorbing water int o the interlayers

of montmorillonite, and the void in thecompacted expansive clay are filled by thisvolume.

Void Swelling

clay (+interlayer

water )

Nonswelling clay/soil Swelling clay

Before water

uptake After water

uptake During water

uptake

Figure 8. The model of the swelling deformation of compacted expansive clay

After water uptake, the compacted expansiveclay can swell at a consta nt vertical pressur e asin the swelling deformat ion test . The volume of compacted clay increases as the volume of swelling clay particles increases until theswelling pressure of the clay particles equals the

vertical pressu re (see Fig. 8).

M a x i m u m S w e l li n g

The maximum swelling can be predicted usinghyperbolic model (Eq. 2) and calculated by Eq. 3[9, 10].

( )( )

( )bt a

t

h

t dht S

o

v +== (2)

( )bbt a

limt limSt

vt

maxv

11 =

+==

∞→∞→

ε(3)

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where t is time from the start of waterinundation, S v(t) is vertical swelling at time (t),and a and b are constants determined by fit t ingprocedures. The calculated maximum swelling ispresented in Table 3 and the curves of thehyperbolic are also shown in Fig. 9. This is

observed that the swelling take longer time toreach th e maximum swelling.

T a b le 3 . S w e ll in g , s w el li n g p r e s su r e a n d c om -

pr e s s ib i l ity ind ex

SoilMix

PercentBentonite

SP(%)

Hyperbolicconstants (Eq. 3)

Max.Swell (%)a

Ps (kPa) Csv (10-2)

a b

5 21.92 4.475 0.0449982 22.22 114 26.99Kaolin: 10 29.96 3.199 0.0324435 30.82 325 25.58

20 31.75 2.194 0.0316716 31.57 356 29.1430 32.65 8.299 0.0293099 34.37 420 39.36

Fine 10 8.68 167.25 0.1007024 9.93 165 7.50Sand: 30 36.96 31.815 0.0244295 40.93 580 32.38

50 41.96 37.675 0.0205627 48.63 660 31.48

Note: SP: swelling potential (measured for 10 days observation), Ps:swelling pressure. a Predicted using Eq. 3, Csv : Coefficient of

Compressibility (after swell)

Elapsed Time (min)

0 5000 10000 15000 20000

S w

e l l ( % )

0

10

20

30

40

50

10% (SB)

5% (KB)

10% (KB)

20% (KB)30% (KB)

30% (SB)

50% (SB)Number in percent

indicates bentonite content

Figure 9. Time-swell hyperboli c relationship f or estima-tion of maximum swelling

S w e l li n g P r e s s u r e

Swell pressure is defined here as the pressurerequired to compress the specimen, that hasbeen soaked and completed the swell under 3.89kPa pressure, back into its originalconfiguration (before swell). The swell pressurewas measured by the pre-swelled method forsimplicity; however, there is experimentalevidence that different methods gave similarresu lts [11, 12] especially at h igh density.

Figure 10 shows the change in sample height(%) and pressure (kPa) curves for soil –bentonite mixtures, which is determined from

oedometer test. At least two essential values canbe attracted from this results i.e. compressibilityan d swelling pressu re of compacted soil.

Log Pressure (kPa)

10 100 1000

P e r c e n t c h a n g e i n s a m p l e h e i g h t

-20

-10

0

10

20

30

40

505%

10%

20%30%

10%

30%

50%

E x p a n s i o n

C o m p r e s s i o n

Percent Bentonite:

(KB)

(SB)

Figure 10. Percent change in s ample height versusloading applied pressure

The figure shows that the swell pressureincrease with increasing bentonite content.Sand – bentonite mixtures exhibit greaterswelling pressure than other mixtures. It isperhaps caused by the greater initial drydensity and lower water content when thespecimen was compacted. This is in agreementwith E l-Sohby and Rabba [12].

Mathew and Rao [13] indicated that byincreasing the valence of exchangeable cationsin homoionized clay, the overall compression inthe system is reduced and the pre-consolidationpressure ( pc) is increased. The equilibrium voidratio at any applied pressure is a direct functionof the repulsive forces arising from theinteraction of adjacent diffuse double layers andpore fluid. As the valence of exchangeablecations in the clay increased, there is areduction in the diffuse double –layer thickness

and in the magnitude of the repulsive forces.These finally result in a lower equilibrium voidratio at any given pressure until higherpressures are reached [14].

C o m p r e s s i b il it y Ch a r a c t e r i s t i c s

The virgin compressibility is defined as thelinear part of the volumetric strain (εv ) vs. logpressure ( p ) curve under virgin loading path.The curves of soil-bentonite mixtures (Fig. 10)show that the virgin compressibili ty charac-teristics ar e not evident u p to a vertical pressureof 25 and 50 kPa. Moreover, the coefficient of compressibility (C sv) is defined as the slope of

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