Swelling Characteristics of Bentonite Clay Mats

Abstract

Bentonite absorbs water to a greater extent than any other ordinary plastic clay and as a consequence it swells depending on the change in its moisture content. This paper aims to give an overview of the swelling characteristic of bentonites with particular observations from that of prehydrated clay mats. How the swelling characteristics vary with the type of clay, water, and the encompassing boundary materials is presented. Accordingly, it examines the swelling response of the clay mat and the pressure exerted when the swelling is both fully and partially restrained. The development of a new apparatus to measure pressures exerted by the clay as a result of the swelling process is also described. A theoretical study is made to postulate how these pressures can be evaluated for field situations. Though bentonites are known to have very high swelling pressures, how this is negated by the compressibility of the composite behaviour with the necessary backfill is demonstrated. Tests show significant differences in swelling behaviour and pressure development when the GCL is in deionized water and sea water environments. Free swell characteristics were similarly affected.. Unique variation of pressure exerted due to swelling in partially restrained conditions was established with experimental observations made during the study. Published (publisher's copy) Peer Reviewed

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Author(s): Diman, Siti F; Wijeyesekera, D.Chitral
Article title: Swelling Characteristics of Bentonite Clay Mats
Year of publication: 2008
Citation: Diman, S.F; Wijeyesekera, D.C. (2008) ‘Swelling Characteristics of
Bentonite Clay Mats’ Proceedings of the AC&T, pp 179 -185.
Link to published version:
http://www.uel.ac.uk/act/proceedings/documents/ACT08.pdf

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
179
SWELLING CHARACTERISTICS OF BENTONITE CLAY MATS
Siti F. Diman & Chitral Wijeyesekera
Built Environment Research Group
u0628081@uel.ac.uk; d.c.wijeyesekera@uel.ac.uk
Abstract: Bentonite absorbs water to a greater extent than any other ordinary plastic clay and as a
consequence it swells depending on the change in its moisture content. This paper aims to give an
overview of the swelling characteristic of bentonites with particular observations from that of
prehydrated clay mats. How the swelling characteristics vary with the type of clay, water, and the
encompassing boundary materials is presented. Accordingly, it examines the swelling response of the
clay mat and the pressure exerted when the swelling is both fully and partially restrained. The
development of a new apparatus to measure pressures exerted by the clay as a result of the swelling
process is also described. A theoretical study is made to postulate how these pressures can be
evaluated for field situations. Though bentonites are known to have very high swelling pressures, how
this is negated by the compressibility of the composite behaviour with the necessary backfill is
demonstrated. Tests show significant differences in swelling behaviour and pressure development
when the GCL is in deionized water and sea water environments. Free swell characteristics were
similarly affected.. Unique variation of pressure exerted due to swelling in partially restrained
conditions was established with experimental observations made during the study.
1. Introduction:
Bentonite is used in the manufacture of
geosynthetic clay liners (GCL), which are
sealing elements and commonly known as
factory-manufactured contaminant / leachate
/ fluid barriers. These consist of a thin layer
(~ 5mm) of either calcium or sodium
bentonite core sandwiched between two
geomembranes or geotextiles. Design
engineers and environmental agencies have
developed a growing interest in the
application of GCLs as an alternative /
preference to compacted clays in cover
systems or as bottom lining of waste
containment facilities. They have also
gained widespread use in variety of sealing
applications, predominantly in hydraulic
engineering, structural water proofing and
groundwater protection. (Wijeyesekera,
2003). In order to meet the demands of an
effective contaminant barrier in hostile
geochemical environments, a factory
controlled prehydration with cation
exchange resisting polymer to an
appropriate moisture condition prior to
installation is always necessary
(McLoughlin, 2004, Wijeyesekera, 2003).
Bentonite is expansive and the dominant
clay mineral is montmorillonite.
Montmorillonite exhibits an extraordinary
potential for volume change with the
increase and decrease of water content in the
clay. Accordingly construction engineers
cautiously stay clear of swelling soils such
as bentonites as they are generally
considered not desirable for foundation of
buildings or geotechnical structures. This
risk depends on the extent of its occurrence
both in terms of area and in particular
thickness. Depending on the thickness of the
deposit there is a high probability of
excessive deformation over their lifetime.
Over time, the swelling increases the rate of
deterioration of the buildings, causing
expensive damage. Hence, an in depth study
of swelling characteristics of expansive
clays is both important and useful for the
design of structures in such soils. The focus
of this paper is on the swelling

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
180
characteristics and the swelling pressure
development of Bentonite clay mats under
different conditions of volume constraint
and geochemical environments. Though
these clay liners / mats adequately serve the
purpose of being barriers to moisture and
leachate movement, engineers are
apprehensive that under certain construction
circumstances it would be undesirable as
they may exert high swell pressures.
Many researchers have proposed and
introduced different methods, theories and
models to explain the swelling properties
and swelling pressure of GCL. However,
little attention has been paid so far in the
study of the influence of the pressures
exerted on structures when the clay swelling
is partially or fully restrained and how this
pressure is accommodated in field situations.
2. Objectives of the research
The objectives of the research described in
this paper are;
• to study the swelling behaviour and the
swelling pressure of bentonite clays,
• to observe the significance of
volumetric strain on the swell, in order
to establish a model for the relationship
between pressure and volumetric strain,
and
• thus to estimate the magnitude of
pressures developed during unrestrained
and restrained swelling of the bentonite
clay mat in order to develop a sound
explanation of the effects of swelling
pressure in clay mat - structure
interaction
3. Definitions
3.1 Free swell (of montmorillonite)
“Free swell” is defined as the volume
occupied by 2gms of the dry clay when
allowed to fully and freely swell in
deionised water (figure 3).
Swelling occurs when clays are allowed free
access of water. Since clays possess very
high suction within the soil skeleton, these
will draw water into the voids causing the
volume change in the voids and the soil can
then wells and eventually disintegrate.
The clay mineral montmorillonite is derived
from pyrophyllite through isomorphic
replacement of aluminium by magnesium in
the octahedral layer (Figure 1) and its ideal
chemical formula can be written as ;
n(H2O)cxSi8Al4-xMgxO20(OH)4
The octahedral layer is composed of
magnesium and aluminium coordinated in
octahedral with oxygen atoms or hydroxyl
groups. The silica tetrahedral is
interconnected in a silica sheet structure.
Thus the crystallographic structure of
montmorillonite is much like a sandwiched
deck of cards. Sodium ions located between
these platelets allow water to hydrate the
clay mineral in an absorption reaction that
results in its swelling characteristics. Hence
when placed in water, these cards or clay
platelets shift apart. Montmorillonite attracts
water to its negative charged face elctro
Figure 1 Generalized structure of
montmorillonite perpendicular to the c- axis,
(Komine & Ogata 1996)

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
181
Figure 2 Swelling mechanism of
montmorillonite (Komine & Ogata 1996)
magnetically to hold the water in place.
During hydration, a confined layer of dry
bentonite (containing montmorillonite)
changes into a dense monolithic mass with
no discernable individual particles (Figure
2). This unique characteristic makes the
montmorillonite capable of absorbing even
up to 7 to 10 times its own weight in water,
and consequently swelling up to 18 times its
dry volume. reduction in the amount of
swell when the clay mat is in sea water.
The free swell measurements observed in
this study for the clay mat are shown in
Figures 4 and 5. There is a distinct
environment. The rate of swelling of the
clay mat is also faster in the deionised water
(Figure 5).
3.2 Swelling pressure
“Swelling pressure” is the pressure required
to bring the soil back to its original volume
after the soil is allowed to swell without
surcharge (see Figure 3). However the term
“swelling pressure” is not used or defined
consistently throughout the literature.
According to O’Conner and Taylor (1994)
the definition used is often dependent on
what kind of test method is being applied.
Brackley (1973) developed three methods
for determining the swelling pressure. Each
of the three satisfied his definition of
swelling pressure but each test also
produced a considerably different value.
The magnitude of swelling pressure depends
on the degree of confinement of the soil;
higher degrees of confinement lead to
increase in swell pressure. Table 1 (from
Day, 2001) shows Johnson’s various
definitions of swelling pressure in
decreasing order of confinement.
It can be seen from Figure 3, that for a given
volumetric strain (v), the pressure (P)
exerted by the clay due to the ingress of
water can be proposed to be represented by a
function in the form
P = f (v) (1)
The f (v) depends on the type of clay and the
types of water used. The relevance of this
function has been investigated in this study.
4. Instrumentation for swell
pressure measurement
For this study, new instrumentation was
developed and tested. Figure 6 shows a bank
of 5 cells where the pressure developed in a
clay mat under varying volumetric
constraint conditions can be observed.
Central to this instrumentation is the use of a
“flexiforce” transducer to measure the force

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
182
Figure 3.1 – Variation of the pressure exerted by a swelling soil with volumetric strain
Table 1 – Further definitions of swelling pressure (From Day, 2001)
Method Definition Remark
A
(ASTM
D4546)
pressure required to bring soil back to the
original volume after the soil is allowed to
swell completely without surcharge
(except for a small seating pressure)
May lead to larger pressures because the method
incorporates hysteresis that tends to overcome
specimen disturbance
B pressure applied to the soil so that neither
swell or compression takes place on
inundation; a specimen may be confined and
pressure inferred from deflection of the
confining vessel
a null test in which measured swell pressure are
influenced by apparatus stiffness; apparatus of
higher stiffness leads to less expansion on
swelling; large swell pressure can be relieved
with small specimen extension; therefore stiffer
apparatus can provide improved control over
one-dimensional charges and can lead to
improved measurements of swell pressure.
C
(ASTM
D4546)
pressure necessary to permit no change in
volume upon inundation when initially under
applied pressure equal to the overburden
pressure; various loads are applied to the soil
after inundation to maintain no volume
change
must be corrected for specimen disturbance; one
dimensional concolidometr swell tests are
influenced by lateral skin friction especially in
tests conducted on stiff clays or shales
D pressure required for preventing volume
expansion in soil in contact with water;
various loads are applied to the soil after
inundation to maintain no volume change
Requires correction of swell pressure similar to
method C above but a standard correction
procedure is not available
Free swell
Swelling pressure

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
183
3.3 EQUILIBRIUM PRESSURES
Figure 4 Free swelling of clay mat in sea
water and deionised water.
0
2
4
6
8
10
12
14
0 1 2 4 5 25264648
Time (hours)
Swelling (mm)
Salt water Deionised w ater
Figure 5 Time dependent development of
the free swelling of clay mat in sea water
and deionised water.
Figure 6 New swell pressure instrumentation
developed by the clay sample subjected to
the required testing conditions. The
apparatus can permit known magnitudes of
vertical swell strain. The samples are
contained within a stainless steel ring and
this generally confines the sample laterally.
The upper and lower porous plates on the
clay sample allows free access of the water
in which the cell is immersed.
Figure 7 illustrates a typical swell pressure
development with time, demonstrating again
the marked effect of sea water and in this
case reducing the swelling pressure (as per
the strict definition of no volume strain)
Swelling Pressure of Rawmat Type P
0
2
4
6
8
10
12
14
16
0 50 100 150 200
Time (hr)
Swell ing Pressure (N/m m2 )
Deioniz ed water
salt water
Figure 7 Time dependent development of
the swell pressure of GCL Rawmat type P in
sea water and deionised water.
4.1 Influence of the covering geotextiles
It is interesting to note the repeatability of
results (± 5%) with the two deionised water
tests as shown in Figure 8. This could be
further improved with care been given
during the sample preparation and
placement stage. The swelling pressure
observed for the claymat core only is 410
kPa and 180 kPa in deionised water and sea
water respectively. Swelling pressure
observations made with the entire clay mat
including the covering geotextiles gave
lower swelling pressures depending on the
type and thickness of the geofabric.
The tests after a few days.
In deionised
water
In sea
water

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
184
-100
-50
0
50
100
150
200
250
300
350
400
450
500
0 10203040 5060
Time (hours)
Pressure developed (KPa)
Deionised w ater 1 Salt water Deio nis ed w at er 2
Poly. (Deionised w ater 1) Poly. (Salt w ater) Poly. (Deionised w ater 2)
Figure 8 Time dependent development of
swelling pressure for the clay mat in
deionised water (x2) and sea water.
The sea water persistently reduced the
swelling pressure in both cases GCL core
with or without covering fabric.
4.2 Influence of permitted vertical strain
on the development of the pressure.
Tests were carried out with the core samples
being permitted controlled vertical swells of
0.25mm, 0.50 mm and 1.00 mm. The results
are shown in Figure 9 and compared with
the pressure development when no vertical
swell was permitted. Reader’s attention is
drawn to the delay time for the pressures to
be developed.
-50
0
50
100
150
200
0 1020304050 607080
Time (hours)
Pressure obse rved (KPa)
1.00mm 0.50mm 0.25mm comparison
Poly. (1.00mm) Poly. (0.50mm) Poly. (0.25mm) Poly. (comparison)
Figure 9 Swell pressure development with
permitted levels of vertical strain.
This is as one would expect the clay core
would need to “free swell” by the level of
permitted vertical swell. The resulting levels
of pressures developed are reduced with the
magnitude of the permitted swell. This
implies and also explains that any void
space / compressibility of the covering
geotextiles will reduce the pressure
experienced due to swelling. Accordingly
for an equilibrium pressure of P; the
corresponding deformation of the fabric will
be given by the following equation;
f
f
EA
tP
v,n deformatio = …………. (2)
This must comply with the P = f(v) for the
clay sample ( figure 3).
4.3 Influence of permitted lateral strain
on the development of the pressure.
Further tests were done to investigate how
the clay core would develop the pressure if
it was permitted to swell laterally. In this
instance the clay cores were cut to slightly
reduced diameters of 48, 46 and 44mm. The
diameter of the sample ring is 50mm.
The initiation of the development of
pressure appears to be at the same time of
0.5 hours.
-100
-50
0
50
100
150
200
00.5 11.52 3 4 5 6 7
Time (hours)
Pressure change
(kPa)
44mm diam . 46mm. diam. 48m m. diam. 50 mm. diam
Figure 10 Swell pressure development with
permitted levels of lateral deformation

Advances in Computing and Technology,
The School of Computing and Technology 3rd Annual Conference, 2008
185
This is a consequence of the vertical swell.
Beyond a permitted 4% lateral strain (see 46
and 44 mm graphs) there is a negative
change in vertical pressure indicating a
continued softening of the clay due to
ingress of water and subsequent lateral
squeezing to fill the void of the permitted
lateral strain. This happens in the field
always during installation as there is no
lateral confinement provided to the clay
liner and the clay laterally squeezes from the
edges of the clay mat in response to ingress
of water. The clay squeezing will invariably
occur at the free edges and near the
overlaps. Far from being detrimental this
lateral squeezing of the clay further
promotes the sealing at the overlap. These
tests further demonstrated that the clay mats
when permitted to swell laterally (not
restrained horizontally) the maximum
pressures developed are very much less than
when the samples were confined laterally.
5. CONCLUSIONS
The swelling pressures of bentonite clay
mats can be in the order of 400 kPa.
However the tests described in this paper
has demonstrated that it is significantly
decreased with salinity and with any
provision to swell (vertically or laterally).
Thus the tests described in this paper have
helped to conclude the following;
• Clays swell when a fluid is absorbed
• Salt water reduces the swelling and
swelling pressures in bentonite clay
mats and GCL
• When load is applied, the swelling
reduces and the swelling pressure
reduces.
• When samples of clays were permitted to
swell vertically / laterally the swelling
pressures it produced was much lower than
the swelling pressure which is observed in
samples with no permitted volumetric
change.
5. References
Day R. W. 2001. Soil testing manual.
Procedures, Classification Data, and
sampling Practices Komine H., Ogata N.
1996. Prediction for swelling characteristics
of compacted Bentonite Canadian.
Geotechnical. Journal vol.33, pp11-17
O’Connor K. and Taylor R.N..1994. The
swelling pressure of compacted clayey fill.
Project report 72.p.7
McLoughlin, M. 2004. The Influence of
Mineralogy and Microstructure on the
Contaminant Migration through
Geosynthetic Clay Liners. PhD Thesis,
University of East London, Department of
Civil Engineering, UK.
Wijeyesekera D.C. 2003. Use and
performance of bentonite in GCLs. An
International Conference on Geo-
Environmental Engineering. Keynote paper
pp.27-46