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| High-density polyethylene (0.75mm) |
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| Description |
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High-density polyethylene (HDPE), commonly used as a geomembrane,
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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Studies in the United States have installed an HDPE plastic liner
underneath a form of concrete cover (shotcrete, grout-filled mattresses
and roller-compacted concrete) and have shown that this method
has an effectiveness in reducing seepage in the order of 95% (Swihart
and Haynes, 1999). However, in other studies an HDPE plastic liner
underneath 400mm of earth cover was estimated to have an effectiveness
of only 75% in reducing seepage (Sinclair Knight Merz, 1998).
The installation of covered HDPE (0.75mm) was trialed over a length
of 1,200m in Wimmera Mallee Water’s Donald Main Channel.
| Site
preparation |
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Channel reprofiling in the Donald channel Main Channel involved
over-excavation by 400mm and regrading of batters to 1:3. Excavated
material was stockpiled adjacent to the channel for use as backfill
over the liner. Subgrade preparation involved removing all loose
rubble from inside the channel and rolling the subgrade surface.
| Installation |
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Method of installation
The liner in this project was dragged across the channel with the
assistance of a hydraulic powered unroller. Each length of HDPE
was laid across the channel overlapping the previous piece by 100mm.
Both sides of the liner were wiped clean of dust and moisture and
then joined using wedge welding. The joint was tested for faults
using a metal prong.
The liner was anchored into an anchor trench formed by a grader
on the channel bank, and backfilled with 400mm of cover.
Figure
1 Unrolling the HDPE (0.75mm) liner into the channel profile
Figure
2 HDPE (0.75mm) - Welding the seams
Following the testing of the joints, excavators placed 400mm
of local soil over the liner. After a cover of 200mm had
been placed
the excavator gently compacted the fill using the excavator bucket.
The backfill was stable on the 1:3 batter slope. Local soil was
used as the channel experiences non-scouring velocities (generally
less than 0.3m/s). Crushed rock scour protection was not required,
significantly reducing installation costs.
Figure
3 Burying the HDPE (0.75mm) liner
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.
The site was fenced to prevent access to sheep and cattle, and
signs identifying the liner’s location were installed.
Specialised equipment
A hydraulic unroller was used to assist the crews in placing the
liner across the channel.
Installation issues
- There were five
noticeable bends in the 1,200m long channel, and from past experience
it was estimated that
an additional 4m
of length of material would be required per bend. Additional
length was quoted in the tender estimates to allow for these
bends.
- The use of the hydraulic unroller required access along
the channel bank. The original placement of spoil from the
channel
bed did
not allow for this, so the spoil had to be double-handled
at certain locations.
- Concern was raised over the quality of
the thermal seams as dust, exacerbated by the wind, was fouling
the liner prior
to
seaming.
A dry clean cloth was used to clean the area. There was
no testing of the seams apart from prodding with a metal prong.
Future specifications
for this type of installation should include non-destructive
testing of the field seams and repair if necessary. This
could easily be
accomplished by air testing of double-wedge thermal seams
or vacuum box testing.
- Due to the wind direction the contractor
shingled the downstream panel of HDPE over the upstream panel.
To reduce
the risk
of water movement lifting the downstream panel and putting
stress
on the
seam, specifications should stipulate that the upstream
panel should be laid over the downstream panel.
Weather
considerations
Installation times increase substantially in windy conditions
as the material is more difficult to handle.
Welding cannot take place
in wet conditions.
| Durability |
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It has been estimated that a well-installed buried HDPE liner
has an estimated life of 25 years (Sinclair Knight Merz,
1998). However,
some researchers have used a design life of 50 years for
both buried and concrete-covered HDPE geomembranes.
The supplier advised that the 0.75mm HDPE material has a
10-year warranty.
| Seepage
reduction |
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The baseline seepage rate at the Donald Main Channel ranged
from 30-50L/m2/day. Post-remediation seepage assessments
were undertaken
in March 2003 and revealed a seepage reduction of between
87 and 92% (reduced to 3.8L/m2/day).
| Cost |
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A breakdown of the costs is presented in the table below.
The total cost of $161,130 (2001), or $6.62/m2,
is based on a supplied
surface
area of 24,346m2.
Table 1 Breakdown of costs for buried HDPE
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Item
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Cost ($)
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Cost ($/m2)1
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Site preparation (excavator, grader, etc.)
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$30,938.50
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$1.27
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Materials (inc. delivery)
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$54,778.00
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$2.25
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Installation (contractor joining)
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$22,566.18
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$0.93
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Backfilling (excavator, etc.)
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$26,599.50
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$1.09
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WMW staff wages, on-costs, etc.
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$12,154.30
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$0.51
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Fencing
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$10,822.50
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$9/m
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Miscellaneous
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$3,270.40
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$0.13
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TOTAL
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$161,129.88
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$6.62
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1. Cost per square metre based on supplied material of 24,346m2.
The costs are indicative of costs faced
for a commercial application in early 2001.
These costs are exclusive of GST.
The price of HDPE (0.75mm) material increased to approximately
$2.75/m2 in November 2001, which is more than
the $2.25/m2 indicated by the costs incurred for
this installation. Using this price, the total installation
cost would have increased
to approximately $7.10/m2.
| Advantages
and disadvantages |
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Advantages
- The HDPE liner can be bonded to concrete structures
by dyna-bolting the liner using teflon strips as the interface
between the concrete and the liner material.
Disadvantages
- Subgrade preparation is critical, as the HDPE liner
at 0.75mm is very susceptible to puncture. All protrusions,
such as large clods, rocks or sharp objects must
be removed
from the substrate, and the backfill must also be
free of this material.
- Installation requires specialised
skills and equipment to weld the seams.
| Related
pages |
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Flexible membrane lining
techniques
Flexible
membrane materials
Covered liners
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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