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| Earthen lining techniques |
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Description
Earthen lining is a method of channel construction or remediation
where a property of an in-situ or imported soil is employed as the
main method of seepage reduction. With the advancements in soil engineering
and earthmoving equipment, earthen lining is one of the most common
types of channel remediation used in Australia and overseas. It has
been widely used for long-term remediation and has more of a proven
performance history than other lining methods.
Compacted earthen linings are recognised as a relatively inexpensive
and effective way of reducing seepage if required materials, equipment
and labour are readily available and the scale of the project is
large enough to fully utilise heavy earthmoving equipment. Locally
available material is usually considered first for channel remediation.
(Swihart and Rutenbeck, 2001)
There are a wide variety of earthen lining processes, with subtle
variations and combinations including:
- Soil additives
- In-situ or imported materials
- Thicknesses on slopes and beds
- Subgrade preparation
- Construction method
- Degree of compaction.
Earthen linings discussed in the literature review include:
- In-situ compacted earth
- Compacted clay mixtures
- Loosely placed earth
- Soil mixed with stabilising additives such as resins, chemicals,
asphalts, petrochemicals, and cement
- Bentonite.
Earthen material is often used in combination with other materials
to provide a suitable liner. This section discusses lining combinations
where the properties of the earthen material are the primary cause
of seepage reduction.
Earthen channel lining construction involves a variety of earthwork
problems. Because it often involves long distances, various subgrade
materials and conditions can be encountered, all of which must be
treated differently. Requirements for subgrade preparation and the
ease of construction are different, and costs of installing earthen
lining can vary considerably.
| Soil properties |
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Physical properties of earthen lining materials, whether in-situ
or imported, are important. Soils must be low in permeability, free
from shrinkage and swelling, and should have good stability and erosion
resistant properties for use in side slopes (Swihart and Rutenbeck,
2001).
Soil types have been ranked according to soil properties, erosion
resistance, and use in compacted linings.
The soil properties are:
- Permeability – provides an indication of effectiveness
at seepage reduction
- Shearing strength – provides indication of bank stability
- Compacted density –ability to reduce void space through
compaction.
Erosion protection of earthen linings may be necessary depending
on material properties and channel operation. Linings constructed
of silty or sandy materials with little coarse gravel are susceptible
to scouring. The cost of reducing the velocity using a larger section
should be compared with the cost of protecting a higher-velocity
smaller section with a gravel cover.
Table 1 Ranking of important
physical properties of soils and their uses for channel lining
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Major
divisions of soils
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Typical
names of soil groups
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Group
symbols
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Soil properties*
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Suitability for canals**
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Perme-
ability
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Shearing
strength
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Compacted
density
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Erosion
resistance
|
Compacted
earthen linings
|
|
Coarse
grained soils
Gravels |
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Clean gravels
|
Well-graded
gravels, gravel-sand mixtures, little or no fines
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GW
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14
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16
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15
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2
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-
|
|
|
Poorly
graded gravels, gravel-sand mixtures, little or no fines
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GP
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16
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14
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8
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3
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-
|
|
Silty
gravels, poorly graded gravel-sand-silt mixtures
|
GM
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12
|
10
|
12
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5
|
6
|
|
| Gravels with fines |
Clayey
gravels, poorly graded gravel-sand-clay mixtures
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GC
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6
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8
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11
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4
|
2
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| |
Gravel
with sand-clay binder
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GW-GC
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8
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13
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16
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1
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1
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| Sands |
Well-graded
sands, gravelly sands, little or no fines
|
SW
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13
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15
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13
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8
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-
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|
| Clean sands |
Poorly
graded sands, gravelly sands, little or no fines
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SP
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15
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11
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7
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9
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-
|
|
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Silty
sands, poorly graded sand-silt mixtures
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SM
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11
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9
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10
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10
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7#
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| Sands with fines |
Clayey
sands, poorly graded sand-clay mixtures
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SC
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5
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7
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9
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7
|
4
|
| |
Sand
with clay binder
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SW-SC
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7
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12
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14
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6
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3
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| Fine grained soils |
|
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Inorganic
silts and very fine sands, rock flour, silty or clayey fine
sands with slight plasticity
|
ML
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10
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5
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5
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-
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8#
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| Silts and clays (liquid limit < 50) |
Inorganic
clays of low to medium plasticity, gravelly clays, sandy
clays, silty clays, lean clays
|
CL
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3
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6
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6
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11
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5
|
| |
Organic
silts and organic silty clays of low plasticity
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OL
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4
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2
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3
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-
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9#
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|
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Inorganic
silt, micaceous or diatomaceous fine sandy or silty soils,
elastic silts
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MH
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9
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3
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2
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-
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-
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| Silts and clays (liquid limit > 50) |
Inorganic
clays of high plasticity, fat clays
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CH
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1
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4
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4
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12
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10#
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| |
Organic
clays of medium to high plasticity
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OH
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2
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1
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1
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-
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-
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| Highly organic soils |
|
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Peat
and other highly organic soils
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Pt
|
|
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Note:
* Numbers provided for soil properties indicate the order of increasing
values for the physical property named (1 = best, 16 = worst).
** Numbers provided for suitability indicate relative suitability
(1 = best, 16 = worst).
# Denotes that erosion is a critical consideration when constructing
canals.
| Subgrade
preparation |
|
The preparation of subgrade is an important consideration in earthen
lining because it can limit the degree of compaction that can be
achieved using compaction equipment in the channel, especially on
side slopes (Jones, 1989). Subgrade can form part of the lining.
Piping of the earthen lining material may occur:
- If the subgrade consist of sands, gravels or fractured rock
- If the earthen lining material contains fine-grained cohesionless
materials.
Piping can be avoided by placing a geotextile or sandy gravel filter
layer between the subgrade and the earthen lining.
Subgrade soils that are silty, dry and low-density are subject to
subsidence, making development of stable embankments difficult. This
can be assisted by ripping or scarifying followed by compaction.
| Cost
of earthen linings |
|
The cost of earthen linings varies considerably, according to the
prevailing conditions of the project. The most important factors
influencing the unit cost of earthen linings are the following (Kraatz,
1977):
- Size of job: It is important to have economies of scale on
projects requiring heavy earthmoving equipment.
- Size of channel: Usually the larger the channel profile, the
lower the unit cost when earthmoving equipment is used.
- Quality of material: If the local material is unacceptable
and must be improved, mixing with hauled soils will increase
costs. If material is highly permeable, thicker lining might
be required to achieve desired reduction in seepage, therefore
increasing costs.
- Source of material: Transport is expensive, so proximity of
the channel to the material source is a critical factor in determining
the cost of the lining. Suitable materials removed during channel
excavation are obviously the least expensive.
Other factors affecting the cost of earthen linings include:
- Ease of site access for haul vehicles
- Weather conditions
- Optimum natural moisture content for compaction
- Subgrade preparation required.
| Related
pages |
|
Compacted earthen liners
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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