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| Linear low-density PE and very low-density
PE (1.5mm) |
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Pages in this section include:
| Description |
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Linear low-density polyethylene (LLDPE) and very low-density polyethylene
(VLDPE) have been introduced to address the shortcomings of HDPE
in terms of flexibility. These less crystalline forms of polyethylene
are more flexible and less brittle. However, some degree of chemical
and UV exposure resistance is sacrificed as a result, and these
materials are generally not recommended for exposed applications
without UV protection. To improve long-term exposed life, the upper
surface of the LLDPE was UV stabilised using hindered amine light
stabilisers (HALS). While more flexible than HDPE, LLDPE is not
as flexible as PVC.
| Experience |
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Common LLDPE applications include landfill caps, pond and channel
liners, tank liners and sewage processing ponds.
LLDPE (1.5mm) was installed in a 200m section of the Dahwilly Main
Channel by Murray Irrigation Limited in July 2002.
| Site
preparation |
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Subgrade preparation required:
- Channel reprofiling to achieve consistent batter slopes
and minimal protrusions.
- Silt and clod removal.
Figure 1 Completed channel preparation
| Installation |
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Method of installation
Approximately two and a half to three days were required for the
200m installation. This included one day where only four of the
six crew members were available and conditions were less than ideal
due to strong winds.
The liner material was supplied in large rolls approximately 3.5-4m
wide. To lay the LLDPE across the channel profile four of the crew
were required to pull the material from the roll across the channel.
Two additional crew members were required to weld sections together.
The liner was anchored in trenches along the channel bank and the
end treatment utilised utilised geosynthetic clay lining (GCL).
Figure 2 Laying the LLDPE
Specialised equipment
A spool bar was used to hold the material roll while dragging the
liner across the channel. Specialised welding equipment was also
used.
Installation issues
Steeper batters made access difficult when pulling the liner across
the channel. However, significant earthworks would have been required
to flatten the batters.
Where the channel subgrade was moist, greater care was needed to
keep joins clean, including laying of drop sheets to ensure welding
conditions were suitable.
Weather
considerations
Wind was a significant problem during installation, and slowed
the process significantly. This caused difficulty in laying the
material, and often caused sheets to move during the welding process.
| Durability |
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Without UV stabilisation an exposed liner will start to degrade
after 7-10 years. The supplier estimates a useful life of 20 years
for a UV-stabilised product.
| Seepage
reduction |
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The baseline seepage rate at the LLDPE site was 7.7L/m2/day. A
pondage test conducted in 2003 showed a seepage reduction of 94%
(to 0.5L/m2/day).
| Cost |
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A breakdown of the costs is presented in the table below. The total
lining cost of $49,622 (2002), or $16.54/m2, is based on a supplied
surface area of 3,000m2.
Table
1 Breakdown of costs for LLDPE
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Item
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Cost ($)
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Cost ($/m2)1
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Site preparation (excavator, grader, etc.)
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$3,806 |
$1.27 |
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Materials (inc. delivery)
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$24,498 |
$8.17 |
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Installation (contractor joining)
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$2,500 |
$0.83 |
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Backfilling (excavator, etc.)
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$9,908 |
$3.30 |
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WMW staff wages, on-costs, etc.
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$7,523 |
$2.51 |
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Fencing
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$1,138 |
$0.38 |
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Miscellaneous
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$249 |
$5.69/m |
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TOTAL
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$49,622 |
$16.54 |
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1. Cost per square metre based on 3,000m2 of material supplied.
2. Cost per metre of fencing based on 200m section of channel.
Costs for materials and installation are indicative of a commercial
application for a longer section of channel. Other costs would
be expected to reduce marginally as economies of scale in earth
works, delivery, utilisation of staff, and fencing are achieved.
These costs are exclusive of GST.
| Advantages
and disadvantages |
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Advantages
- Despite unfavourable conditions in the form of wind
and a wet subgrade, ease of installation of the LLDPE
was considered to be quite good.
Disadvantages
- LLDPE is difficult to install under windy conditions,
as this makes it difficult to lay and can cause sheets
to move during welding.
- Wet subgrade conditions can interfere
with the welding process.
- Exposed installations require
fencing to prevent risk of damage by animals.
| 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)
DamSeal
Unreinforced
polypropylene (1mm)
Unreinforced
polypropylene (0.75mm)
Reinforced
polypropylene (1.1mm)
Butyl
rubber
Asphalt |
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