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| Description |
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Bentonite is a natural clay formed by weathering of volcanic ash.
Its chief constituent is the clay mineral montmorillonite. Bentonite
is distinguished from other clays by its extreme fineness, highly
absorbent nature and curious property of swelling in water.
There are generally two classes of bentonite: high-swelling and
low-swelling. The high-swelling variety, sodium bentonite, absorbs
nearly five times its weight in water and at full saturation occupied
a volume 15 times its dry bulk; on drying it returns to its original
volume. This swelling-drying property is reversible over an infinite
number of times in pure water, giving bentonite its water-sealing
properties. Low-swelling bentonite has a wet bulk 1.5-7 times its
dry bulk, and therefore a larger volume of low-swelling bentonite
is required to achieve a desired reduction in seepage.
Bentonite deposits vary greatly in the content of montmorillonite
and therefore their expansion characteristics. Care should be exercised
in the evaluation of local clays and in relating membrane thickness
to their properties.
Bentonite is a versatile and useful material that has been applied
in channel seepage remediation in many different ways. Bentonite
is used as:
- A soil additive where existing or available soils
are inadequate to protect against channel seepage (refer to Modified
soil earthen
linings).
- A sediment sealant when it is applied as a waterborne
material for deposition on the wetted perimeter (refer to
Soil sealants).
- A buried membrane by itself, which is similar
to using bentonite as an imported material in a compacted
liner (refer
to Compaction
of imported material).
- A geosynthetic clay liner (GCL), where
the bentonite is woven into a geotextile to be used as a
covered flexible
membrane
liner (refer to Geosynthetic clay liners).
| Bentonite
used for soil modification |
|
Sometimes referred to as the mixed blanket technique, this commonly
involves mixing bentonite with between 150mm and 300mm of soil
and application in a continuous thoroughly compacted layer. Seepage
reduction of around 90% can be obtained. To achieve this it is
recommended that clay be placed in layers of 150mm and compacted
using vibrating or sheep-foot rollers. Sufficient compaction may
not be achieved using rubber-tyred tracked machinery alone. Compaction
testing during liner placement helps ensure that required compaction
is achieved.
| Bentonite
used as a sealant |
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Colloidal bentonite in channel water has been considered as a rapid,
low-cost sealing method. In theory, the small bentonite particles
should be carried to a considerable depth in the channel bed materials
where seepage was occurring, swell upon becoming saturated, and
plug the soil voids. However, tests have shown that bentonite forms
a surface coating on the soil which contracts upon drying and does
not re-form to produce an effective seal after drying.
One laboratory study concluded that waterborne bentonite can form
a lasting seal, particularly in sands free of silt and clay before
sealing. The study also concluded that bentonite was an economical
option if suitable bentonite concentrations are used and if the
bentonite source is within an economical shipping distance of the
sealing site (Rollins, 1967). However, subsequent field studies
have concluded that waterborne bentonite seals were short-lived,
lasting only until the seals had dried once or twice (Rollins and
Dylla, 1970).
In separate trials, bentonite has been mixed with a polyphosphate
dispersing agent and applied to channel water that has been temporarily
ponded for 24 hours (USBR, 1976). Instead of penetrating the soil,
the dispersed bentonite generally formed a thin coating on the
wetted perimeter of the channel. There was an indication from preliminary
seepage tests that this coating resulted in a seal immediately
after application. However, the thin seal was subject to shrinkage
upon drying and to erosion from flowing water, and the bentonite
was not effective in reducing the seepage for more than a few months.
| Bentonite
used as a membrane or soil blanket |
|
This technique involves laying a blanket of pure bentonite to a
depth of 15-50mm. This layer is then covered with between 150–300mm
of topsoil or gravel and then thoroughly compacted (similar to
compaction of an imported material described in Put
link here).
This method has been used in channel lining for many years and
was introduced to Australia in the 1960s (ICID, 1967). Seepage
reduction of around 95% has been reported.
| Injected
bentonite membrane |
|
This technique has been proposed by Thiess Services in Rocklea,
Queensland. It involves injecting a bentonite product below the
channel surface in either a hydrated or dry state using an injection
plough developed by International Groundwater Technology. The bentonite
product is injected approximately 200mm below the surface and forms
a sealing layer that has an estimated permeability of 1 x 10-8cm/sec.
Injected in a hydrated state, the sealant is approximately 25mm
thick, while the dry state product is initially 3-5mm thick but
swells to 15-20mm thick on hydration. Estimated application costs
are $12-15/m2 depending on site conditions, mobilisation cost,
site location and the area to be treated.
| Performance |
|
Bentonite linings have often lost their effectiveness after only
a few years of service. A common cause of the failure is the decomposing
effect of wetting and drying and other weathering factors (Kraatz,
1977). However, if suitable materials are available from local
deposits and the material can be used in its ‘original’ condition,
it may be lower cost than other types of lining.
In 1940, a bentonite membrane lining was placed in a channel in
Montana, USA. The membrane was only 13mm thick, with a soil cover
of 130mm. After 7 years of operation, seepage losses from the channel
were just as high as they had been before the channel was lined.
This failure is believed to have been due to the use of too thin
a bentonite membrane, with perhaps an inadequate cover (ICID, 1967).
Successful bentonite membrane or blanket trials were reported in
Rollins and Dylla (1970), where uncovered bentonite blankets lasted
5 seasons, while buried bentonite blankets retained 60% effectiveness
after 6 years.
An adequate bentonite lining was also constructed in Montana at
approximately the same time that comprised of a 50mm bentonite
layer with 300mm cover. This channel was determined to be operating
effectively after 20 years without any noticeable deterioration
of cover (ICID, 1967). Considerable seepage through the porous
gravel subgrade formation prior to lining was significantly reduced
(USBR, 1976).
If a satisfactory lining is applied, the estimated life of the
lining is 30 years and estimated effectiveness for seepage control
is 70% (Sinclair Knight Merz, 1998).
| Cost |
|
The cost of placing a bentonite/clay mixture is relatively high
cost compared to other seepage control methods. This is due to
the high cost of bentonite ($340/tonne) and the large amount of
earthworks operations necessary to prepare the channel, mix the
bentonite with earth and carefully place the material on the inside
of the channel (Sinclair Knight Merz, 1998). The high cost of bentonite
makes this lining method more expensive than alternative compacted
earthen lining methods. The estimated cost to apply a clay bentonite
lining is $14-21/m2 (Sinclair Knight Merz, 1998).
| Related
pages |
|
Earthen lining techniques
Compacted earthen liners
Clay
lining example: Channel 12
Clay
lining example: Waranga Western Channel
Other
Australian examples
Channel bank remodelling
Loose earthen linings
Modified soil earthen linings
Soil sealants |
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