|
|
 |
|
|
Pages in this section include:
Flexible membrane liners require a material for the flexible membrane
that does not need protection from potential sources of damage in
the channel. The material must maintain low permeability for effective
seepage reduction.
Advantages
Potential advantages of using exposed membrane linings in comparison
to Covered liners are (Swihart
and Rutenbeck, 2001):
- Channel side slopes can be increased to 1:1.5 or even 1:1 from
1:2 to 1:3 (ICID, 1990) with subsequent cost saving in reduced
excavation and right of way. Reinforcement of the liner can increase
the tolerable steepness of the side slopes even further.
- Improvement of the channel roughness and hence increase of
carrying capacity of the channel or allowing for a reduction
in the cross-sectional area while maintaining the carrying capacity.
- Elimination of the need for protective earth covering on side
slopes.
- Undetected damage to the membrane liner is minimised.
- Replacement of the liner (if required) is easier.
Disadvantages
Exposed flexible membranes should be made of materials that can resist
the following adverse conditions for sufficient time to be useful
(Swihart and Rutenbeck, 2001):
- Wind and water forces
- Vandalism
- Groundwater uplift pressure in areas of high groundwater levels
- Animal damage
- UV surface degradation
- Abrasion.
Exposed membranes have a smooth, slippery surface, which when wet
is difficult for animals and humans to climb if caught in the channel.
Exposed liners are better suited for channels that continuously carry
water, as the water cover discourages vandalism and provides some
protection from physical forces. Wind uplift may damage or remove
the lining in unwatered systems.
| Installation |
 |
Flexible exposed liners, even though of sturdier material than liners
for covered applications, are susceptible to damage during installation,
so careful installation is critical to the success of the seepage
reduction program.
The installation of an exposed liner is performed in the following
steps:
- 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;
- Placement of liner material (supplied in factory-fabricated
sheets or rolls) in the channel profile and joining to adjacent
sheets.
- Anchoring the liner along its edge in trenches at the top of
the batter.
- Anchoring the liner at both ends.
End treatment
The exposed end sections of liner material need to be anchored to
prevent them from lifting and to protect the material during construction
and removal of pondage test embankments.
Water authorities and suppliers recommend burying a sufficient length
of liner (at least 1m, to be determined by the material supplier)
under approximately 500mm depth of cover material.
Figure 1 Channel cross-section showing end
treatment
Clay should be carefully selected and compacted at close to optimum
moisture content to effectively anchor the liner. Compaction can
be difficult at the upper wedge of the end treatment, due to the
shallow depth. Crushed rock should be placed instead of clay where
the depth is less than 300mm (to prevent scouring) as illustrated
in Figure 1 Channel cross-section showing end treatment.
Given the low-friction characteristics of the flexible membrane liners,
compaction above the end treatment may force clay out of the compaction
area. It may be necessary to protect the liner from damage during
compaction, and additional material may be required to increase friction in
the compaction area.
Geosynthetic clay liners (GCL) were
considered for protection of the liner from clay compaction operations,
provide improved friction properties, to provide an effective seal
by swelling under water. However GCL are relatively expensive and
difficult to use without the correct handling equipment that may
not be available for small applications.
After consideration of various materials, a geotextile material was
recommended to improve the friction properties, assist with anchoring
and provide material protection from earthworks.
| Maintenance |
|
Regular inspections of exposed liners are required to ensure that
any damage is repaired and the integrity of the liner is maintained.
Risks include:
- Debris in the channel
- Wind and water uplift
- Rainfall damage (e.g. washout of anchor trenches)
- Animal traffic
- Attack by rodents, insects and yabbies
- Weeds and root penetration
- Vandalism
- Fire
- Maintenance equipment and vehicles
- Theft.
Annual inspections include looking for holes, tears, slippage, weathering
or items washed downstream. Tears allow vegetation growth that can cause
further damage to the liner. Silt build-up does not cause problems
and fills surface irregularities, smoothing the bed of the channel.
Desilting using heavy equipment is not appropriate due to risk of damage
to the liner. Sludge pumps are more appropriate if silt removal is
necessary.
Regular inspections during the channel’s operating period should
be conducted to assess the liner’s condition and identify any locations
of leakage through the liner.
Flexible membrane liners are generally repaired with a patch of the
material welded over the location of the damage.
| Performance |
|
The performance of exposed flexible membrane liners has found satisfactory
as a method of seepage control, but generally on a short-term basis
(Sinclair Knight Merz, 1998). Several case studies of uncovered membranes in
seepage remediation have concluded that its effectiveness is limited
by the durability of the material in exposed conditions (USBR, 1977). The
properties of the lining material dictate the service life of this
lining option, and the service life varies depending on the material used.
Seepage reduction
The effectiveness of exposed geomembranes in reducing seepage is
estimated at approximately 90% (Swihart and Haynes, 1999). This is
slightly
lower than when cover is provided, due to the potential for mechanical
damage (animal traffic, equipment damage and vandalism). Actual seepage
results
from trial sections installed during the Deschutes canal-lining demonstration
project are presented in Table 1 Seepage rates for exposed
flexible membranes.
One case study investigated the relationship between thickness of an
exposed PE membrane and seepage reduction to identify a minimum material
thickness requirement. The study found that a seepage rate of 690L/m2/day
was reduced to:
- 100L/m2/day (86%) for a PE thickness of 0.4mm;
- 58L/m2/day (92%) for a PE thickness of 1mm; and
- 52L/m2/day (93%) for a PE thickness of 2mm.
An acceptable thickness for this was considered to be 1mm (Kraatz,
1977).
Table 1 Seepage rates for exposed flexible membranes
|
|
Material
|
Seepage rate (L/m2/day and % reduction)
|
|
| |
Pre-lining
|
Post-lining
|
2 yr
|
5 yr
|
6yr
|
|
HDPE (2mm)
|
425
|
0
|
|
0-30
93%
|
|
|
Geolam
(PVC/geotextile composite)
|
~300
|
0
|
|
30-60
80%
|
12
|
|
Hypalon (1.15mm)
|
~300
|
3
|
|
0-15
90%
|
12
|
|
Terra Tuff (0.91mm)
(Hypalon/geotextile composite)
|
~300
|
36
88%
|
|
0-15
90%
|
12
|
|
VLDPE (1.5mm)
(with grout mattress on slopes)
|
~300
|
21
93%
|
Abandoned1
|
|
|
|
HDPE (1.5mm)
(with grout mattress on slopes)
|
~300
|
21
93%
|
Abandoned1
|
|
|
|
1 This channel encountered problems causing
failure, which are discussed below.
Source: Swihart and Haynes, 1999.
|
Durability |
|
Flexible membranes are quite watertight when new but continued effectiveness
depends on chemical and mechanical resistance. Most materials are
subject to deterioration when exposed over a period of time to UV
radiation. The effects of aging and exposure include loss of strength,
flexibility and elasticity. The design life is predicted to be approximately
20-40 years depending on the UV resistance and thickness of the membrane.
To achieve this design life the exposed membrane requires regular
maintenance to patch and repair tears (Swihart and Haynes, 1999).
Installations in the Emerald Irrigation Area in Queensland identified
potential failure mechanisms of exposed liners on the batter slopes.
The liner material tended to creep down the batter as it heated and
expanded. Failures were noted between 4 and 7 years as the material
thickness and strength declined with elongation.
| Related
pages |
|
Flexible membrane lining techniques
Flexible membrane materials
Covered liners
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 |
|
