| Guidelines
On Design of Noise Barriers
Contents
4.1
Design Consideration
Noise barriers
should be designed so that they require minimal maintenance other
than cleaning. Concrete or masonry walls require little or no maintenance
during the desirable service life of 40 years, but transparent sections
need frequent cleaning and might well need replacing during their
service life. Careful design can prevent the need for on-site modifications
or other damage during construction that might considerably reduce
the life of noise barriers. For example, hammering of panels for
fitting into place could cause damage and should be prohibited.
Therefore, design should be done carefully with due consideration
of the practicability in construction. Plastic panels should incorporate
resistance to the effects of ultra-violet light. Surfaces and joints
should not include dirt or moisture traps or other details liable
to cause rust staining. The effects of weathering on colour and
of rainwash on accumulated surface grime should also be considered.
It may be necessary
to provide access from the protected side for maintenance purposes
and where there is a right of way for pedestrians or cyclists. This
may render a barrier vulnerable to vandalism and the choice of form
and materials should take this factor into account. It may be appropriate
for pedestrian and cycle paths to be lit; where painted surfaces
are required, polyamide based finishes will enable easier removal
of graffiti. Materials for noise barriers should possess good fire
retardant properties and comply with the "Particular Specification
for Noise Barrier" published by Highways Department, February
2001. Though there is no specific requirement of service life, noise
barrier material manufacturer is, however, required to guarantee
for at least 10 years on properties such as colour resistance, stone
impact resistance, aging and corrosion resistance, light transmission,
fire retardant properties etc.
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4.2
Materials and Detailing
In order to
minimise the need for maintenance, attention should be paid to the
selection of materials used in the construction of noise barriers.
The quality of materials used should be appropriate to the location.
For example, barriers built in relatively inaccessible locations
or in areas likely to be subject to extreme weather conditions will
need more durable components than those which can be more easily
maintained or are in relatively sheltered positions. Care should
be taken over design details in order to eliminate possible moisture
traps which would encourage rot or chemical attack. Alloy and metal
fittings should be carefully selected to avoid differences in electrochemical
potential which would accelerate corrosion. Plants selected for
use in conjunction with a noise barrier should generally be of hardy
species which require a low level of maintenance.
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4.3
Cleaning
With the passage
of time, barrier surfaces may become stained by contaminants such
as water-splash from the road surface, airborne grime, bird droppings,
honeydew or sap from overhanging trees. Concrete or masonry noise
barriers may not need cleaning in certain locations as the surfaces
would be washed by rain water and their textured finish may control
staining. Flat surfaces, however, will require regular cleaning
as contamination will be more apparent and will detract from the
appearance of the barrier. High pressure water jets mounted on purpose
built tankers, or hand washing with brushes and low pressure water
are suitable treatments.
The frequency
of cleaning required will depend on the degree of contamination
that occurs. Water splash contamination can be reduced by distancing
the barrier from the edge of the carriageway, although this will
have the drawback of reducing its effectiveness in attenuating the
road traffic noise. Effective road surface drainage will also reduce
splash effects by preventing puddles from forming. Bird dropping
staining can be controlled by the use of design details or chemical
repellents that deter birds from perching on the barrier. A very
thin wire at a height of about 50mm along the top edge of the barrier
will help to prevent birds resting, thus control bird droppings.
Trees and other overhanging vegetation may need trimming or cutting
back to prevent abrasion and marking of the barrier. Transparent
noise barriers will need to be cleaned more frequently than other
types because they will show any contamination more readily or surface
treatments can be used. Proprietary-made self-cleansing panels could
also be considered where its use is justified.
Purpose-made
vehicles fitted with water tanks, hoses, brushes and access platforms
would reduce the cost of cleaning barriers but long lengths of barrier
will be required to justify the necessary investment. In the short
term, access platforms can be used to reach the far sides of barriers
in order to carry out cleaning and other maintenance. Noise barriers
erected near to the carriageway may require lane closures during
maintenance; traffic management will be especially important for
access to any barriers in the central reserve. Their use is not
encouraged, but zero maintenance barriers (self cleaning, impact
resistant) would be appropriate in this location.
Similarly, it
would be difficult to clean the outside of noise barriers erected
on high level structures, as such zero maintenance barriers should
be used.
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4.4
Other Maintenance Tasks
In addition
to cleaning, other maintenance tasks include :
| a) |
Tightening
joints and fixings after initial construction. This should take
place at the end of the construction maintenance period. |
| b) |
Painting
or treatment of metal surfaces. This requirement can be reduced
by using anodized aluminium, galvanized or weathering steel.
But colours may need to be refreshed periodically if they are
an important element in the design. |
| c) |
Periodic
maintenance of planting - weeding, replacement of failed plants
and, if necessary, watering to secure the proper establishment
of the vegetation in the initial period, followed by periodic
thinning, or pruning as necessary. (Barriers composed of living
material retaining earth require a more intensive management
regime.) |
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4.5
Access
The need for
future maintenance should be taken into account when deciding on
the form of a noise barrier. Where it will need to be inspected
from time to time, screen planting should be placed with sufficient
space to permit easy access. Doors or gaps should be provided at
reasonable intervals to give access to either side of the barrier.
Frequent access will be needed to clean both sides of a transparent
noise barrier - on bridges and viaducts this might necessitate the
use of specialized equipment. Working area will be required for
erecting platform or parking of vehicle with hydraulic lifting platform
that can reach both sides of the noise barrier for the carrying
out of maintenance works.
Gates or gaps
should be provided at about 200m intervals to provide access for
the maintenance of both the noise barrier and any planting behind
the noise barrier. Where possible these access points should be
located to provide access to any traffic control and communications
equipment.
Where access
point is to be provided for pedestrian but doors are not practical,
then, another section of parallel barrier should be provided in
front of the access point to avoid degrading of the acoustic performance.
One face of this barrier should be provided with absorptive materials
to avoid multiple reflections between parallel barriers. The length
of this additional barrier should be at least several times of the
width of the gap/opening (3x) or as a rough guide, x + 2y, where
x is the width of the gap and y is the spacing of the two barriers,
whichever is larger. See the figure below for different arrangements
at the opening. The exact length required should be worked out during
detail design stage having considered standard acoustical principles
and practices.
Figure
4.5.1 Arrangement of barriers at opening
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