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In recent years synthetic fabric form works pneumatically
filled with concrete grout have been used quite successfully
for erosion
protection and seepage prevention (Kraatz, 1977). Grout-injected
mattresses or grouted fabric mats consist of two layers of a woven
or punched geotextile fabric, connected together at regular intervals.
When a cement grout is pumped into the space between the layers
of fabric, the result is a concrete filled ‘quilt’ of
some 75mm thickness (Stevenson, 1999).
The main advantage of this lining is that it can be installed in
operating channels without dewatering. Other advantages include (Swihart,
et al. 1994):
- They can conform to a non-uniform cross-section.
- They
require minimum subgrade preparation.
- They eliminate the
need for an anchor trench.
- They ensure a uniform thickness
of concrete.
However, by themselves they rely entirely
on the integrity of the grout mattress. The geotextile facing
on the grout
blanket erodes
after a few years and exposes the concrete ‘pillows’ which
are then susceptible to erosion. It can be difficult to properly
fill the grout mattress if the grout batch is unusually stiff
or the pump cannot manage the necessary pressure (Swihart et
al.,
1994).
They have been demonstrated to provide excellent service after
seven years, with minor maintenance requirements to seal transverse
cracks
(Swihart and Haynes, 1999). The grout mattresses achieved a 98%
(6L/m2/day) reduction in seepage initially after installation.
However, after
7 years of service, grout filled mats provided only 70% reduction
in seepage with a seepage rate of 90L/m2/day. The outer fabric
of the grout mattress was starting to showing deterioration and
in locations
some of the grout pillows or bricks were missing (Swihart and
Haynes, 1999).
Joints are still advised in this system, although at approximately
twice the spacing recommended for plain concrete. Drying shrinkage
is minimised, as a lot of water escapes the weave of the fabric
during filling, leaving quite a dry mortar mix in the mattress
(Stevenson,
1999).
Although the placement of a grout-injected mattress can tolerate
less rigorous foundation preparation than placed concrete, the
smoother and more even the foundation, the easier and neater
the job (Stevenson,
1999). The mattresses can be laid out over the channel profile
and joined together with zips, welds or sewn seams, depending
on type
and manufacture. They must then be anchored temporarily in place,
allowing for the fact the mattresses tend to tighten and shrink
as they are filled (Stevenson, 1999).
For durability, the mattresses must be anchored into the ground
at upstream and downstream ends. This is accomplished by turning
them
down into trenches dug across the channel and up the sides, then
concreting them in place. The edges of the mattresses along the
tops of the channel walls can be similarly anchored, or they
can merely
be left flat to form an apron along each side of the channel
and held in place by their own weight when filled (Stevenson,
1999).
Successful mattress filling depends on a continuous supply of
grout being available, so the operation must be organised with
sufficient
labour, materials or mixing facilities. Premixed concrete suppliers
can be employed to mix and deliver the grout in quantity. It
is also possible to use concrete that has been mixed with fibre
reinforcement.
The cost is estimated to be of the order of $25/m2 to line a
channel with 75mm grouted fabric mat (Swihart et al., 1994).
| Related
pages |
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Hard
surface lining techniques
Concrete
Shotcrete
Soil-cement lining
Flumes and pipes
Tiles and bricks
Asphalt |
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