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| Description |
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Geosynthetic clay liners (GCL) consist of clay material, usually
bentonite, woven between two geotextile layers. This lining mechanism
uses the properties of bentonite (see Bentonite treatments) and
the material to which it is bonded holds the clay in place.
GCLs require soil cover of at least 300mm to provide sufficient
normal force to confine the expansion of the bentonite core layer.
The bentonite swells and extrudes through the geotextile fabric
at the overlap, forming a seal by producing a dense and uniform
clay barrier with the same hydraulic qualities in all parts of
the clay liner (Curtis, 1998).
The GCL selected for installation was Bentofix x750.
| Experience |
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GCLs are used in a range of environmental and containment applications,
particularly when performance is critical and when penetration
risks are present. Examples include landfill cappings and cutoff
walls to contain pollution.
GCL was installed in 200m of the Toolondo Channel at Wimmera Mallee
Water in July 2002.
| Site
preparation |
|
Subgrade preparation required removal of silt and over-excavation
of 400mm to allow for covering of the GCL. Batters were trimmed
to 1:3. Spoil was placed adjacent to the channel to facilitate
backfilling. Loose rubble was removed before rolling of subgrade.
| Installation |
|
A lifting frame fitted to an excavator suspended the GCL roll and
facilitated placement across the channel. The material was pulled
across the channel by a crew of four and up the channel bank as
far as possible. It was then attached to another lifting frame
(constructed for the project with grip plates attached) and a second
excavator. The second excavator then assisted the crew to drag
the material up the bank to the required height. Each length of
liner was overlapped by 300mm, with the top of the overlap on the
downstream side. It took two days to install the liner, and an
additional three days to repair areas ripped by equipment moving
over the liner.
 Figure
1 Installing the GCL.
The
GCL was anchored by placing a minimum of 400mm of backfill over
the material on a berm located 300mm above the high-water mark.
Excavators then covered the liner with 400mm of backfill material.
The recommended minimum backfill is 300mm, but this thickness is
difficult to place and compact. Once the cover had been placed
an excavator compacted the backfill using its tracks but had to
move cautiously to avoid movement and damage to the GCL. On the
sides a 3:1 bank slope was sufficient to maintain the backfill
material on the liner without it slumping.
Figure 2 Burying the GCL.
The end of the GCL was keyed into the channel profile to a depth
of 400mm in a 1.2m wide trench across the channel. Backfill material
was then compacted in the trench.
The site was fenced to prevent access to sheep and cattle, and
signs identifying the liner’s location were installed.
Specialised equipment
Lifting frames were required to hold the GCL roll and to pull the
material to other side of the channel.
Installation issues
- The material was heavy, an difficult and labour
intensive to handle. Its white colour produced significant
sun glare.
- The material tore easily when the compaction equipment
turned. Compaction therefore focused in two separate directions
- up
and down the bank and backwards and forwards along the channel
length
- to avoid the need for turning. Four passes were found to
provide a reasonable compaction to ensure longevity of the
cover. The
supplier recommended a loose covering.
- The subgrade appeared
to settle after the backfill material was placed and caused
the joins to come apart. These had
to be excavated
and additional GCL laid. Wet soil patches also appear to
have caused differential settlement during compaction.
- The
joining overlap of 300mm appeared to be insufficient.
Weather
considerations
Installation in wet conditions is not possible as rain is absorbed
by the bentonite, making the liner heavier and harder to handle.
| Durability |
|
The lifespan of covered GCL is estimated to be 25 years.
| Seepage
reduction |
|
The baseline seepage rate in the Tooloondo Channel was 11.1L/m2/day.
Post-remediation seepage assessments revealed a seepage reduction
of 87%.
| Cost |
|
A breakdown of the costs is presented in table below. The total
cost of lining the channel was $59,532, or $11.91/m2 (2002)
.
Table 1 Breakdown of costs for buried HDPE
|
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Item
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Cost ($)
|
Cost ($/m2)1
|
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Site preparation (excavator, grader, etc.)
|
$10,125 |
$2.03 |
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Materials (inc. delivery)
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$33,447 |
$6.69 |
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Installation (contractor joining)
|
$ 7,228 |
$1.45 |
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Backfilling (excavator, etc.)
|
$ 3,680 |
$0.74 |
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WMW staff wages, on-costs, etc.
|
$ 3,148 |
$0.63 |
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Fencing
|
$ 1,905 |
$9.52/m |
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TOTAL
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$59,532 |
$11.91 |
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1. Cost per square metre based on 5,000m2 of material supplied.
2. Cost per metre of fencing based on 200m section of channel.
| Advantages
and disadvantages |
|
Advantages
- It was possible to order the
exact length of liner required across the channel to
be supplied on one roll. This eliminated the need to ‘cut
and paste’ the liner.
- The material is suitable
for sites where no clay source was available.
Disadvantages
- The material is very heavy, difficult to handle,
and labour intensive to install.
| Related
pages |
|
Flexible membrane lining
techniques
Flexible
membrane materials
Covered liners
High-density
polyethylene (0.75mm)
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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