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| High-density polyethylene (2mm exposed) |
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| Description |
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High-density polyethylene (HDPE), commonly used as a geomembrane
and is similar to the material used in black polyethylene pipes.
It has a broad chemical resistance and excellent UV resistance,
but has a lack of flexibility and can develop brittle stress cracking
at low stresses if not properly formulated.
Since HDPE is a very stiff material, it cannot be prefabricated
into panels. Instead it is delivered to the site in rolls, usually
up to 6m wide (9m widths are manufactured in the United States)
and all the seaming is done on-site. The HDPE liner is rolled out
and laid across the channel with overlapping pieces to enable weld
joining.
| Experience |
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Sections of exposed HDPE of 1.5mm thickness were installed on the
bottom of channel sections only as part of the United States Deschutes
trials (Swihart and Haynes, 1999). This liner immediately encountered
problems due to the formation of ‘whales’ where the
material had lifted from the substrate due to the build-up of pressure
underneath. Several unsuccessful attempts were made to repair the
sections. Unfortunately the cause of the ‘whales’ was
never resolved, although volcanic gases were suspected.
In Australia 2.0mm HDPE exposed liner has been used for channel
remediation in Queensland by Sun Water but no formal literature
covering performance is available.
The IAL project trialed 200m of 2.0mm exposed HDPE at the Finley
Main Channel by Murray Irrigation Limited in 2003.
| Site
preparation |
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Moderate subgrade preparation was required to reshape the channel
profile and remove sharp stones, debris and vegetation.
| Installation |
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Method of installation
The HDPE was supplied in 6.5m wide rolls of approximately 200m
in length. To allow easier installation, reduce labour requirements
on site, and reduce the amount of joining in wet and dirty site
conditions, some workshop prefabrication was undertaken. Approximately
four water authority personnel and three from the supplier were
required to install the material, which included welding sections
together.
The 200m section was laid and welded in approximately two days.
The liner was anchored in trenches along the channel bank and the
end treatment utilised geosynthetic clay lining (GCL).
Figure 1 Laying the HDPE
 Figure
2 Completed channel section
Specialised equipment
A spool bar was used to lift the HDPE material and provide the
means for laying it across the channel profile. A lifting frame
was used to place the GCL for the end treatment. At the downstream
end of the trial section the HDPE liner was joined to an existing
membrane, and the upstream end was trenched into the channel profile.
Installation issues
- Site conditions were less than ideal for the installation,
with the channel bed being quite wet due to 50mm of rainfall
in the preceding two weeks. In addition to this, groundwater
intrusion necessitated regular pumping to dewater the channel.
Water ponded in the bottom of the liner and caused difficulties
during welding. Effort was required in order to keep joins
clean for welding.
- HDPE off-cuts were used as drop sheets under
each join to keep mud and water from fouling the welding
process. This meant
that slightly less material was available for lining so only
160m (instead of the full 200m) was completed.
- Access to the
site was restricted due to wet conditions. A fence on one
side of the channel also limited access. Only
vehicles with tracks could access the site.
Weather
considerations
Wind conditions during installation were calm. Windier conditions
would cause significantly greater installation problems, particularly
with large sections of material.
| Durability |
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The lifespan of exposed HDPE is estimated to be 20-40 years.
| Seepage
reduction |
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The baseline seepage rate at the trial site was 5.2L/m2/day. A
post-remediation pondage test is yet to be undertaken.
| Cost |
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A breakdown of costs is presented in the table below. The total
lining cost of $43,632 (2003), or $12.12/m2, is based on supplied
surface area of 3,600m2. Despite only 160m of liner being installed
instead of 200m (due to site conditions requiring drop sets HDPE
drop sheets beneath the liner to keep the joins clean), the cost
per square metre is considered representative of the installation
costs for the full 200m (3,600m2) long section. This is because
MIL estimate that material and labour requirements for installing
200m in good conditions would be similar to installing 160m in
poor conditions as experienced at this site.
Table 1 Breakdown of costs for HDPE (2mm)
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Item
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Cost ($)
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Cost ($/m2)1
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Site preparation (excavator, grader, etc.)
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$3,322 |
$0.92 |
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Materials (inc. delivery)
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$20,000 |
$5.55 |
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Installation (contractor joining)
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$1,400 |
$0.39 |
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Backfilling (excavator, etc.)
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$12,000 |
$3.33 |
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WMW staff wages, on-costs, etc.
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$3,950 |
$1.10 |
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Fencing
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$1,710 |
$8.55/m |
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Miscellaneous
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$1,250 |
$0.35 |
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TOTAL
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$43,632 |
$12.12 |
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1. Cost per square metre based on 3,600m2 of material supplied.
2. Cost per metre of fencing based on 200m section of channel.
Cost for materials and installation are specific to this job. Costs
for materials and transport would be less in larger applications.
Other costs would be expected to reduce marginally as economies
of scale in earth works, installation, utilisation of staff, and
fencing are achieved. These costs are exclusive of GST.
| Advantages
and disadvantages |
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Advantages
- Despite some unfavourable conditions in the form
of a wet subgrade, ease of installation of the HDPE was
considered to be quite good.
Disadvantages
- HDPE is difficult to install under windy conditions,
as this makes it difficult to lay the material and can
cause the 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 |
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Flexible membrane lining
techniques
Flexible
membrane materials
Covered liners
High-density
polyethylene (0.75mm)
Geosynthetic
clay liners
Exposed liners
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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