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| Modified soil earthen linings |
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Description
If available soils are below standard requirements for slope stability,
erosion resistance, shrinking and swelling characteristics and permeability,
their natural deficiencies can be overcome by treatment with small
quantities (normally 1-6%) of additives. A range of materials can
be used as additives, blended directly into the soil material (either
in-situ or imported) to improve soil properties. If the soil modification
occurs in-situ, the treatment is generally restricted to a layer
of up to approximately 300mm within the earthen lining.
Many substances have been trialed as soil additives to stabilise
or seal channels and laterals. These include:
- Asphalts
- Chemicals such as sodium silicate and calcium chloride
- Lime
- Petrochemicals
- Cement
- Specially treated resins
- Resin-cement
- Swelling clays
- Organic matter
- A combination of these materials.
Cement added in proportions that create a hard impermeable liner
is referred to as soil-cement, discussed in Soil-cement lining. Cement
added to enhance the soil properties to decrease seepage from channels
is discussed below.
Treatments using of swelling clays generally involve bentonite, which
is used in a variety of ways in addition to being used as an additive
to modify the soil characteristics. See Bentonite treatments.
Little information is available on the effectiveness and economy
of various materials used for channel linings. The use of chemically
stabilised soils for channel linings is still rather experimental,
as soil reactions vary. Some case studies in the USA. using asphalt
proved satisfactory results soon after construction, but no information
is available on long-term performance.
| Resins |
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Specially treated resins tend to waterproof soil and have been used
in powder form as a stabilising agent for roads, added to soils containing
considerable clay. Resins have been used experimentally in channel
linings (USBR, 1976). The amount of resin required to stabilise the
soil depends on the characteristics of the soil but normally ranges
from 1-3%. Because the resin renders the soil water-repellent, water
must be added to the soil before the resin is added. Linings of this
type are mixed and compacted in the same manner as Soil-cement lining,
but no moist curing is required (USBR, 1976).
Several test sections in channels in the US indicated that after
5 years of use resin-cement soil mixture deteriorated badly, indicating
unsatisfactory serviceability (USBR, 1976).
| Chemicals |
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Sodium silicate, in combination with sodium and calcium chloride
has been used to stabilise sandy soils in deep excavations to improve
the bearing power of the soils. The chemicals solidify the soil particles
to a solid mass that is hard and impervious.
However, this method is not regarded as suitable for channel use
because of the high cost of the chemicals and because treated soil
has not proved very resistant to wetting and drying cycles, as well
as freezing and thawing action (USBR, 1976).
| Physical
stabilisation |
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The stability of either clayey or granular soils may be improved,
and the permeability decreased, by combining these soils in proper
proportions. This operation, termed ‘mechanical stabilisation’ in
highway construction, may be accomplished at little expense by mixing
soils in-situ with discs and blades. Compaction of the soil mix adds
to its serviceability (USBR, 1976).
| Organic
matter |
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Gleization is an anaerobic biochemical process that occurs in a flooded
soil. Liners of organic matter may therefore be effective. In Russia,
under anaerobic conditions, the metabolic activities of certain anaerobes
have been shown to effect a chemical reduction process in soil, which
results in increased dispersity and plasticity, and decreased soil
permeability (McConkey, et al, 1990).
Although the rate of gleization is proportional to the amount of
organic material available, gleization occurs even with virtually
no decomposable materials in the soil or water. The soil-sealing
from gleization is believed to be due primarily to the clogging of
the pores with ‘slime’ or ‘gums’ that results
from decomposition.
An organic matter liner can be installed by applying the organic
material with a forage harvester and subsequently covering the organic
matter with a 100mm layer of topsoil using a backhoe.
The gleization method has been found to be promising in trials involving
the remediation of seepage from earthen reservoirs (Nicholaichuk,
1978). The texture of the overburden covering the organic matter,
in this case straw, did not affect the gleization process and is
therefore only required in a thickness sufficient to protect the
organic liner from erosion. Wheat, barley and oat straw, and manure
can be used to promote the gleization process (McConkey, et al, 1990).
Oat straw was the least effective; partially decayed manure reduced
the seepage rate more rapidly than straw; but eventually both resulted
in the same level of seepage reduction. The reduction in seepage
rate is not as rapid as that achieved using materials such as sodium
carbonate and bentonite. Organic matter has been used successfully
in combination with sodium carbonate and bentonite (McConkey, et
al, 1990).
In a field study a channel was treated with an enhanced gleization
method using a buried straw layer and sodium carbonate mixed with
the soil. The sodium carbonate provided only short-term seepage control.
For flooding periods of 12 days or less a buried straw layer reduced
seepage from a small irrigation channel by 30% but seepage rates
were still unacceptably high. Seepage rates increased following the
winter period and after a drying interval of more than eight days.
Enhanced gleization even when combined with soil incorporated sodium
carbonate did not provide satisfactory long-term seepage control
for intermittently flooded channels (McConkey et al, 1990).
Lining with buried straw is not cost-effectiveness compared to other
liners such as covered plastic membranes (McConkey et al, 1990).
| Cement,
asphalt and lime |
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Cement, asphalt, and lime are standard soil stabilising agents for
highway purposes. These materials have also been used for channel
lining purposes, affecting shrinkage and expansion properties of
the soil. After application and mixing the layer is compacted in
the normal manner.
Soil with cement added in small quantities, is called ‘cement-modified’ soil,
in contrast to soil-cement (Soil-cement lining), which contains cement
in larger proportions. When 2-6% cement is used with plastic fine-grained
soils, the soil particles become flocculated to form small conglomerate
masses of new soil with reduced volume change characteristics. When
such treatment is applied to fine-grained soils that already have
low permeabilities, the stability of the soil is improved.
The costs of lining a channel with a mixture of clay and cement,
or clay and lime are approximately $22/m2 and $20/m2 respectively
(Aseervatham and Thompson, 1998). These costs were achieved on sites
that required sealing of the channel bottom only.
A lining of a clay-cement modified soil installed by Murray Irrigation
Limited showed positive results in stabilising groundwater pressure.
However, similar channels lined with clay-lime modified soil did
not show positive results. This has been attributed to the specific
characteristics of the site, which was downstream of a regulator,
resulting in significant erosion of the liner. It was also suspected
that the slope of the channel was too steep, which contributed to
erosive channel velocities (Aseervatham and Thompson, 1998).
In field studies, test sections having higher concentrations of cement
and asphalt have shown less erosion than other treated sections and
the control sections.
Studies have been conducted on the use of combinations of asphalt
with bentonite. Initial tests were successful, with the material
performing well under laboratory conditions (Sommerfeldt, 1977).
Table 1 Seepage rates for modified soil
earthen linings
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| Material |
Seepage rate (L/m2/day and % reduction) with
time |
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| Lining |
Before lining |
|
After Lining |
|
4 years |
| Cement (11%, 75mm) |
290 |
43 |
85% |
61 |
79% |
| Cement (15.5%, 75mm) |
290 |
9 |
97% |
18 |
94% |
| Cement (17.5%, 75mm) |
290 |
21 |
93% |
34 |
88% |
|
Source: USBR, 1977.
| 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
Bentonite treatments
Soil sealants |
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