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Description
Core trenches (also known as key trenches and cut-off walls)
are vertical subsurface barriers designed to reduce or contain
groundwater flow and limit flow into surrounding soils. Core
trenches are constructed adjacent to the channel banks on one
or both sides of the channel to restrict the lateral seepage
of water from the channel. Core trenches are constructed by excavating
a cut at least 150mm wide, down to an impervious layer. The excavation
is then filled with a core material that is typically one of
the following:
- An earth liner, such as clay or bentonite slurry.
- A hard
liner, such as concrete or sheet piling.
- A geomembrane liner,
such as a vertical synthetic membrane (Sinclair Knight
Merz, 2001).
Implementation of core trenching
technologies is dependent on site conditions, especially the
presence of an impermeable layer,
usually clay or bedrock, to key the base of the barrier. Without
an aquitard at the base of the core trench, groundwater will
flow under the barrier. The resultant seepage damage will be
as severe as it was before the works began, the only difference
being the time delay before seepage is evident (Wimmera Mallee
Water, 1995).
Core trenching can be used in conjunction with additional technologies
such as drains or extraction wells to control groundwater volumes
contained by the wall.
Core trenching is a lower cost seepage reduction method than
installation of a liner across the whole wetted perimeter of
the channel waterway, although obviously it cannot stop seepage
loss through the channel bed. Effectiveness of the method is
dependent on achieving a satisfactory seal between the compacted
material forming the curtain and the underlaying foundation.
It is difficult to align the sheeting and adequately compact
the backfill material to the full depth of the narrow trench.
As a result, core trenching and sheeting is generally less
effective than installation of a full liner for reduction of
seepage, and
it is used mainly as a stopgap means to quickly rectify serious
leakage in short channel lengths.
| Experience |
 |
Core trenches were employed in the United States in a localised
area to reduce seepage from a 120m section of channel. This
project was reported as successful, although it was noted that
the costs
of this method must be compared to alternatives (USBR, 1976).
Core trenches have been employed in the Mulwala channel in
Murray Irrigation Limited with positive results in stabilising
the groundwater
pressure. However, similar vertical membrane works on Berrigan
Main were not successful with failure suspected to be due to
the soil type of the channel banks (Aseervatham and Thompson,
1998)
| Maintenance |
|
Although core trenching is more difficult to construct than
waterway liners, once in place it is not exposed to mechanical
or other
damage.
| Durability |
|
The estimated life is 25 years.
| Seepage
reduction |
|
Experience suggests that there is an initial reduction in seepage
loss for one or two seasons. Seepage then increases as new
seepage paths develop under or around the curtain. For comparison
to
other control methods core trenching is assessed at less than
50% for seepage reduction (Sinclair Knight Merz, 2001).
| Cost |
|
Core trenches have been used for decades for isolating contaminated
groundwater plumes, so the equipment and methodology are readily
available and well known. The cost of core trenching the clay
backfill is estimated at $32/m to $40/m (i.e. $80/m for both
channel banks) (Sinclair Knight Merz, 1998).
Costs depend on:
- Depth of excavation required.
- Presence of stones and boulders.
- The impermeable barrier
to be installed in the trench.
- Other site-specific factors.
| Advantages
and disadvantages |
|
Advantages
- Core trenching is a lower cost seepage reduction
method than installation of a liner across the whole wetted
perimeter of
the channel waterway.
- Although core trenching is more difficult
to construct than waterway liners, once in place it is
less exposed to mechanical
or other damage.
- Core trenching can be constructed when
the channel is in operation.
- Core trenching can be constructed
without significant bank reprofiling.
Disadvantages
- Without an aquitard at the base of the core trench, groundwater
will flow under the barrier. The resultant seepage damage
will be as severe as it was before the works began.
- It is difficult
to align the sheeting and adequately compact the backfill
material to the full depth in the narrow trench.
As a result, core trenching is generally less effective
than installation of a full liner for reduction of seepage
and
it is used mainly as a stopgap means to quickly rectify serious
leakage in short channel lengths.
| Related
pages |
|
Groundwater intervention techniques
Groundwater
pumping
Vegetation
Tile
drains |
|
