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A buried membrane lining system consists of a flexible membrane covered
with a protective layer of earth or concrete forms.
| Advantages
and disadvantages |
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A cover material provides the following advantages (ICID, 1990):
- Protection from exposure to the elements, including UV radiation,
wind uplift and n turbulent water.
- Protection against damage from vehicles, maintenance equipment,
stock, wildlife, plant growth, fire or vandalism.
- Reduction in temperature variation and consequently lower stresses
caused by thermal expansion.
- Protection from uplift by groundwater, (i.e. through provision
of sufficient down force).
- Reduced risk of human or animal drownings caused by inability
to climb slippery walls of a lined channel.
- Cheaper lining material due to protection by cover.
The disadvantages of provision of this cover include:
- Flatter batter slopes due to cover material.
- Liner cannot be easily inspected or repaired.
- Additional cost due to excavation and replacement of cover
material.
| Installation |
|
The successful installation of a covered membrane is critical to
the long-term success of the liner. The synthetic liner is most susceptible
to damage and hence proper and careful installation is critical to
the success of a seepage reduction program.
The installation of a covered membrane is performed in four main
steps
- Excavation of the channel profile to allow for placement of
the protective cover, and trimming of the batter slope to ensure
stability of the protective cover.
- Preparation of the subgrade to ensure a firm, relatively smooth
surface, free from sharp rocks, roots and other objects that
might puncture the membrane and organic material that may decompose
and cause settlement.
- Installation of the liner, supplied in factory-fabricated sheets
or rolls. The material is placed in the channel profile and joined
to adjacent sheets. Edges of the liner are anchored in trenches
at the top of the batter.
- Placement of the cover material, undertaken as soon as practicable
to reduce risk of damage to the liner and dislodgement by wind.
Several materials are used for covering flexible membrane liners
and providing protection from the adverse effects of exposure
to the elements, such as earth fill and various concrete products.
| Maintenance |
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Following construction, the main threats of damage to the buried
geomembrane come from disturbance of the cover material. Therefore
the main maintenance requirement is regular inspection and repair
of the cover material to ensure that it remains intact to protect
the liner material buried beneath.
The main causes of disruption to the cover material and liner are:
- Channel operation, including desilting by mechanical excavation;
- Invasion of weeds and their removal; and
- Animal traffic or invasion (e.g. yabbies).
Flexible membrane liners are generally repaired with a patch of the
material welded over the location of the damage. Covered liners first
require the removal of the backfill material and location of the
leak before the area can be repaired.
| Performance |
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Field studies indicate that buried plastic membrane liners generally
provide satisfactory seepage control. The reduction in seepage achieved
from installing a covered membrane in different case studies is presented
in the table below.
Table 1 Seepage rates for covered flexible membranes
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Material
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Seepage rate (L/m2/day
and % reduction) with time
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Reference
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Cover is earthen unless otherwise stated
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Before lining
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After
lining
|
2 yr
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3 yr
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4 yr
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5 yr
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10yr+
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PE (0.15mm)
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600
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6-49
92%
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Pohjakas & Rapp, 1967
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PE (0.20mm)
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4.63
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4.03
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Pohjakas & Rapp, 1967
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PE (0.20mm)
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4.93
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3.43
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Pohjakas & Rapp, 1967
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Butyl (0.79mm)
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12.2
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Hickey, 1971
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PVC (0.25mm)
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2.03
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Morrison & Starbuck, 1984
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PVC (0.25mm)
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150
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0.0
100%
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3.03
98%
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Morrison & Starbuck, 1984
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PVC (0.25mm)
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5001
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Morrison & Starbuck, 1984
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PE (0.25mm)
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6.4
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452
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Morrison & Starbuck, 1984
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PVC (0.50mm)
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2.03
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Weimer, 1987
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PE (0.10mm) geocomposite
and 75mm shotcrete cover
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425
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15
96%
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0' 90
79%
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Swihart et al., 1994
Swihart et al., 1999
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VLDPE (0.76mm), geotextile underlay and shotcrete cover
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425
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33
92%
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0' 30
93%
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Swihart et al., 1994
Swihart et al., 1999
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PVC (1mm)
and 76mm grout-filled mattress
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195
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30
85%
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15
92%
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Swihart et al., 1994
Swihart et al., 1999
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FPP (0.50mm)
and precast concrete blocks
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67
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27
60%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and T-shaped precast walls
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37
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25
29%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and precast slabs (with sealant)
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36
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19
47%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and precast slabs (with geotextile)
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40
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6.0
85%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and precast slabs (no geotextile)
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40
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8.0
80%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and in-situ concrete
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55
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1.43
97%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and bricks in mortar
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60
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1.63
97%
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Bodla and Tariq, 1999
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FPP (0.50mm)
and concrete filled mattresses
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24
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2.03
92%
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Bodla and Tariq, 1999
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1. The PVC liner in this case study had suffered
major damage.
2. Increase thought to be due to damage to the lining caused
during inspection in previous year.
3. Within the accuracy of the measurements of channel seepage,
values of less than 5L/m2/day can be regarded as virtually nil
(pers. comm., Ralph Burch, 2001).
| Related
pages |
|
Flexible membrane
lining techniques
Flexible membrane materials
High-density
polyethylene (0.75mm)
Geosynthetic
clay liners
Exposed liners
High-density
polyethylene (2mm exposed)
High-density
polyethylene (1.5mm exposed)
Linear
low-density PE and very low-density PE
(1.5mm)
DamSeal
Unreinforced
polypropylene (1mm)
Unreinforced
polypropylene (0.75mm)
Reinforced
polypropylene (1.1mm)
Butyl
rubber
Asphalt |
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