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| Hard surface lining techniques |
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Pages in this section
include:
Hard surface lining techniques use linings such as concrete, mortar,
soil-cement, brick and stone to form a hard impermeable surface.
Channel replacement in the form of pipes and flumes is also discussed
here, but asphalt is discussed with flexible membranes (refer to Asphalt).
Refer to Table 1 Channel seepage remediation decision
matrix for a summary of hard surface lining techniques.
| Channel
prism |
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The cost of the hard surface lining material is relatively high,
and reduction in cost can be achieved through designing an efficient
channel section with the smallest cross-sectional area (i.e. smallest
perimeter for given area), while still maintaining slope stability
on the channel sides and channel carrying capacity. The steepness
of the side slopes is a major limitation of hard surface linings
as the lining may be subject to slippage, and sideslopes are typically
limited to 1:1.5.
| Foundation |
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A sound foundation is important to reduce cracking and danger of
failure due to settlement of the subgrade. Natural in-situ soils
of low density should be thoroughly compacted or removed and replaced
with suitable material (refer to Table
1 Ranking of important physical properties of soils and their uses
for channel lining and Earthen
lining techniques for soil properties and description of compaction
techniques). Compaction of the embankments is generally recommended
at least to the height of the lining. Compaction should occur after
stripping of unsuitable material.
The main problem with hard surface liners is their high dependency
on the integrity of the foundation material. Relatively small settlements
or movements of the foundations can cause leakage, which can affect
the integrity of the foundation (for example washing material away),
causing further movement of the liner and a compounding of the leakage
problem. Such processes can be difficult to detect and rectify even
with close inspection procedures.
Because they expand when wet, clays are usually hazardous to hard-surface
linings and should be avoided in foundations.
| Under-drainage |
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Where a hard-surface channel lining is to be applied in an area where
groundwater is likely to rise above the bottom of the lining, drains
must be provided underneath or alongside the channel to relieve any
hydrostatic pressure that might cause uplift and damage of the lining.
Concrete linings are susceptible to rupture by outside hydrostatic
or other pressures. They can generally withstand only a small amount
of cracking to relieve external hydrostatic pressure without significant
damage. Drainage to relieve the outside hydrostatic pressure is generally
worth the additional cost (Swihart and Rutenbeck, 2001).
There are two common types of artificial drainage installations:
- Under-drains - A drainpipe is placed in gravel envelopes or
geocomposite sheet drains. These longitudinal drains are connected
either to transverse cross drains that discharge water below
the channel or to pump pits. Underdrains sometimes connect to
outlet boxes on the floor of the channel, which are usually equipped
with one-way flap valves to relieve any external pressure that
is greater than the water pressure on the upper surface of the
channel base, but prevent backflow. Unfortunately these flap
valves often become plugged with debris or are damaged by maintenance
equipment removing sediment from the channel (Swihart and Rutenbeck,
2001).
- Flap valves - A permeable gravel blanket of selected material
or sand and gravel pockets is drained into the channel at frequent
intervals by flap valves in the invert.
Both the under-drain system and the connected flap-valve-type drain
must be encased in filter material or geotextile to prevent piping
in the subgrade material into the pipe or through the valve.
Drainage is also recommended in areas that are susceptible to frost
heave if the area is not free draining. Frost heave is generally
not encountered in Australia.
| Joints |
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Due to the rigid nature of hard surface liners, cracks result from
expansion and contraction due to changes in moisture levels and temperature.
Preventing such cracking is impractical, so efforts are directed
towards confining cracks to selected locations by creating weakened
places, which develop into contraction joints along the channel,
which must be sealed. Cracks are initiated at these locations and
random cracking is minimised, which in turns minimises leakage and
consequent damage such loss of foundation soils (Stevenson, 1999;
Swihart and Rutenbeck, 2001). More information on types of joints
in concrete lining is provided in Concrete.
| Hazards |
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Weeds are a potential hazard to hard linings, which they can penetrate,
especially when the lining is installed with minimum thickness. Seepage
can occur either through fractures caused by the weeds or through
root casts when the weeds die. Treatment of the subgrade with a soil
sterilant when linings are to be placed in areas already weed infested
or in old channels where weeds are growing, was advisable until recently.
The use of soil sterilants is no longer advised due to environmental
concerns, and the weeds need to be removed by hand or machinery.
Gypsum is another hazard to hard surface linings. Water in contact
with gypsum will dissolve salts in the soil, in time creating cavities,
which may result in collapse of the hard surface.
| Performance |
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The following table summarises the performance of different hard
surface liners that have been tested in various case studies. Hard
surface linings can perform as an effective seepage remediation measure
if they are installed correctly. However, their lifespan is variable
and dependent on proper installation and ongoing inspection and maintenance.
Table 1 Seepage rates for
hard liners
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Material
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Seepage rate
(L/m2/day
and % reduction)
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Reference
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Before lining
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After lining
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Unreinforced concrete (90mm)
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-
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21
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USBR, 1977
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Unreinforced concrete (100mm)
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-
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152
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USBR, 1977
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Concrete blocks
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131
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61 (53%)
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USBR, 1977
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Shotcrete (40mm)
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-
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9
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USBR, 1977
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Concrete
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-
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11-67
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Kosichchenko, 1991
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In-situ concrete (76mm)
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67
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18 (73%)
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Bodla & Tariq, 1999
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T-shaped precast 51mm without joint sealant
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37
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25 (32%)
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Bodla & Tariq, 1999
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T-shaped precast 51mm with joint sealant
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37
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3.4 (91%)
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Bodla & Tariq, 1999
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Parabolic precast concrete with sealed joints
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69
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11 (84%)
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Bodla & Tariq, 1999
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| Related
pages |
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Concrete
Shotcrete
Grouted fabric mats
Soil-cement lining
Flumes and pipes
Tiles and bricks
Asphalt |
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