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If suitable earthen material is available near the site of construction,
or is in-situ and within the channel prism, a lining of compacted
earth is an inexpensive and efficient means of controlling seepage.
This type of lining, especially a thick compacted lining, has proved
better than other types of earthen linings and has been used extensively.
Compaction reduces soil pore sizes by displacing air and water.
Reduction in void size increases the density, compressive strength
and shear strength of the soil and reduces permeability. This is
accompanied by a reduction in volume and settlement of the surface.
Proper compaction is essential to increase the stability and frost
resistance (where required) and to decrease erosion and seepage
losses.
Compaction can generally be achieved by one of four principal means
(McCann and Kelly, 1998)
- Pressure – The static weight of a smooth drum or
tyre rollers compresses the upper levels of the material.
- Kneading – The
concentrated contact area of a sheepsfoot roller kneads material
in its plastic state, but only in the
upper levels of the material.
- Vibration – The oscillation
drum of a vibratory roller re-orientates the particles and
reduces the void spaces, but
only in material within 400mm of the surface.
- Impact – The
high-intensity compression and shear waves generated deep in
soil by dynamic action of an impact roller
compacts soil to effective depths of 2.5m or more
Low permeability of the soil in a compacted state is of prime
importance in seepage reduction. The water loss of a proposed
lining can be
estimated from the results of laboratory permeability tests on
compacted soil specimens, the thickness of the proposed lining
and the water depth. Maximum density and optimum moisture content
should be determined in the laboratory before using soils for
compaction.
Compacted earthen lining can be effective. A well-compacted earthen
lining can be highly impermeable, reducing seepage losses to
20L/m2/day, and is almost comparable to good concrete lining
(ICID, 1967).
Usually the thickness and material components are planned so
that seepage loss is less than 30L/m2/day. Losses up to 60L/m2/day
are
tolerated if natural subgrade soils are very pervious and better
lining materials are not economically available (Kraatz, 1977).
| In-situ
compaction |
 |
The simplest and least expensive compacted earthen lining involves
compaction of the existing channel bed and slopes in-situ using
appropriate compaction equipment. Compaction improves the impermeability
of the existing material and achieves varying degrees of seepage
remediation depending on the properties of the in-situ soils.
| Compaction
of imported material |
|
Compacted liners, also called compacted clay liners, generally
consist of an imported mixture of clay and soil with lower permeability
and better erosion resistance than the in-situ soils. The imported
material is compacted to an appropriate thickness within the
channel to further improve the soil characteristics.
The main issues to be considered in the use of a compacted clay
liner are (Sinclair Knight Merz, 1998):
- Availability of suitable clay close to the site
- Effect of
weed growth, wind and water erosion
- Future maintenance needs
and costs
- Avoidance of damage by machinery or animals
- Conditioning of
clay to required moisture content.
Clay lining
is cost-effective if sufficient clay is available near the
channel. Transport of large quantities over long distances
is not economical.
It is important to source quality clays for use in channel
banks. Suitable materials are (Neville and Chant, 1998):
- Soils with 60% clay content, slightly dispersive, with a
coarse grain to allow high bulk density to be achieved
- Dispersive
soils with more than 60% clay content, to which lime and
coarse grain material is added
- Soils with low permeability and
shrinkage characteristics.
To
be adequate for lining, compacted material:
- Must have very low permeability (10-9m/s is often quoted)
- Should
be free from excessive shrinking or swelling
- Should have low
dispersion properties
- Should resist erosion from flowing water
or wave action
- Should have good slope stability characteristics.
A well-graded
sand and gravel with a clay binder is considered the best material
for a compacted earthen lining, followed by
clayey gravel soils and sand with clay binder. Silty or sandy
clay soils
with low plasticity are not considered suitable. Fat clays
(i.e. inorganic clays of high plasticity) may not be suitable
(because
of swelling and shrinking due to wetting and drying), unless
the lining is protected by a gravel-sand cover (refer to
Table 1 Ranking
of important physical properties of soils and their uses for
channel lining). Suitable soils can be blended on site from
different sources
to produce the optimum mix.
Material not suitable for side slopes might be used advantageously
for lining the bottom of the channel (Sally, 1965).
A compacted earthen lining is constructed by compacting successive
layers of imported material, to a thickness of 150mm. Slope
sections are normally 500mm to 1.0m thick depending on the
size of the
channel section, and bottom linings are commonly from 300 to
300mm thick
(USBR, 1998). A typical cross-section is presented in the figure
below.
Figure 1 Typical earth-lined section
(USBR, 1998)
Channels with seepage problems often have
high localised groundwater levels, and consequently
soft subgrade conditions. Laying and compaction
of earthen liners in these conditions can
be difficult, due to poor machinery access.
This can be overcome using a geotextile between
the subgrade and the liner to support compaction
equipment.
Small channels cannot accommodate heavy equipment
for placing and compacting lining material.
Clay liners remain the most common method
for seepage reduction in Australia. Clay
liners have been used by Wimmera Mallee Water
(1995) in protecting against channel seepage
in sections of channel excavated in soil.
In India, some clays used to line channels
are claimed to be more impervious than brick
and cement concrete (Sally, 1965).
Performance of compacted earthen linings
Changes can occur in compacted earthen linings
due to physical weathering in combination
with channel operation. Weathering can cause
a decrease in density and increase in permeability,
decreasing the effectiveness of the lining.
Wetting and drying of soil can also affect
the soil properties.
Freezing and thawing in colder areas can
also cause changes in performance (Kraatz,
1977), although, studies indicate that some
linings exhibit no significant change in
density. The greatest decrease in density
was approximately 7%, which was concentrated
near the lining surface. This lining still
achieved reduced seepage rates of 10 to 30L/m2/day
(USBR, 1976).
Field and laboratory tests on typical soils
in selected US compacted earthen channel
linings demonstrate long-term physical changes.
The unit weight, a property affecting permeability,
was found to vary significantly from one
test site to the other and from year to year.
The unit weight tended to decrease from the
top of the lining to the bottom, although
the reverse also occurs. The changes in unit
weight were largely attributed to frost action.
There was less change in unit weight in soils
that did not have a significant gravel fraction.
Loss of unit weight was attributed to wetting
and drying in some instances. It was concluded
that soil changes found did not significantly
affect the expected performance of the channels
in controlling seepage (Jones, 1987).
Seepage reduction
A number of studies have attempted to quantify
seepage reduction due to installation of
compacted earthen linings and results are
summarised in the table below. A properly
installed clay liner is estimated to have
a seepage reduction effectiveness of 70%
with an estimated life of 30 years (Sinclair
Knight Merz, 1998).
Table 1 Seepage rates for earthen lining
|
|
|
Material
|
Seepage rate (L/m2/day) with time
|
Reference
|
|
|
|
Natural soil
|
Lining |
Pre-lining |
Post-lining |
4 yrs |
5 yrs |
6 yrs |
10yrs |
Seepage Reduction |
|
|
|
Lean clay and sand
|
Compacted clay, soil and gravel cover
|
1338
|
27
|
|
|
|
55
|
98-96%
|
Pohjakas et al, 1967
|
|
|
Clay & sand over silt and gravel
|
Compacted clay (500mm)
|
610
|
30
|
76
|
|
|
|
95 -88%
|
Pohjakas et al, 1967
|
|
|
Clay and silt
|
Compacted clay (300mm)
|
354
|
30
|
|
79
|
|
|
92-78%
|
Pohjakas et al, 1967
|
|
|
Clay, silt and sand
|
Compacted clay (500mm)
|
125
|
40
|
|
|
73
|
|
68-42%
|
Pohjakas et al, 1967
|
|
|
Not specified
|
Lean clay lining (300mm)
|
122
|
21
|
|
|
|
|
83%
|
USBR, 1963
|
|
|
Not specified
|
Compacted clayey limestone material (500mm)
|
671
|
85
|
|
|
|
|
87%
|
ICID, 1957
|
|
|
Not specified
|
Compacted (300 to 900mm)
|
122
|
21
|
|
|
|
|
83%
|
USBR, 1977
|
|
Costs of compacted earthen linings
Factors that influence the unit cost of compacted
earthen linings include (Kraatz, 1977):
- Size of job
- Source of materials
- Weather conditions
- Mixing requirements
- Subgrade preparation
- Cover materials.
Assuming suitable clay is available, the
estimated costs in (1998 dollars) for the
various stages are:
- Excavate clay from borrow, cart and
compact $10-15/m2
- Excavate and shape channel
prior to placement $4/m2 (if required)
- Place
earth layer over clay $4/m2 (if required)
- Final
trim and dispose of excess material
$10/m of channel length
Therefore the estimated total cost for clay
lining a channel is in the order of $10/m2
to $17/m2 (Sinclair Knight Merz, 1998;
Aseervatham and Thompson, 1998).
Maintenance of compacted earthen linings
Properly constructed channels require little
maintenance in the short term. They should
be inspected regularly for erosion, dispersion,
shrinkage cracks, and damage. Stock should
be prevented from accessing channel banks,
as this can cause significant damage, particularly
below the water line.
Advantages and disadvantages
Advantages
- Clay lining is a standard and straightforward
technique for channel remediation, in
which water authorities have expertise
and equipment.
Disadvantages
- Suitable clay material may not be
available at a reasonable cost.
- The
clay lining may be subject to yabby
damage .
- Laboratory tests are required
to ensure compaction standards are
maintained
during
installation.
| Related
pages |
|
Earthen lining techniques
Clay lining example: Channel 12
Clay lining example: Waranga Western
Channel
Other Australian examples
Channel bank lining
Loose earthen linings
Bentonite treatments
Modified soil earthen linings
Soil sealants |
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