Civil Engineering Reference
In-Depth Information
maintenance liability. It would require a roughened surface that would
attract the dirt and dust particles present in the tunnel. This would darken
the wall compromising the lighting and also be more difficult to clean
because of its roughness. As it happens, some tunnels with spray-applied
fire protection have good natural acoustic properties as the fire protection
is left with just such a rough finish. This is the case for the Øresund Tunnel,
which has spray-applied fire protection above the level of the wall cladding
and to the underside of the roof. This was a side benefit, however, and not
the result of a project requirement.
There are two areas where noise has to be considered and, if necessary,
action has to be taken to reduce it. These are at the portals and around any
ventilation exhaust points; assessment of the noise levels at the portals will
form part of the overall EIA. At the portals, the traffic emerges into the
open approaches, which usually have full height retaining walls to reflect
the noise. The geometry of the carriageway and the walls tends to reflect
the noise upwards. In an urban situation, this can cause excessive noise
levels for surrounding buildings.
When planning the tunnel, base level measurements of the ambient noise
levels in the portal areas should be taken. The increase in noise levels due
to the tunnel can then be established. The increase at a typical major road
tunnel portal can be in the order of 10-15 dBA. The environmental author-
ities will set the limits for noise levels around the portal, and if the levels
are too high, there are only a limited number of mitigating measures that
can be taken. If the surrounding buildings are relatively low, then noise
barriers can be erected on top of the approach walls. These will create
a noise shadow reducing the noise levels in the surrounding area. If the
buildings are higher, then the noise barriers will be less effective. Improving
the sound insulation of the building might be possible, although in a mod-
ern building, scope for improvement will be limited. Conversely, the noise
insulation in a modern building may be adequate to provide acceptable
noise levels in the interior despite the increase due to the tunnel. The spread
of noise from a portal and its approaches can also be limited by the addi-
tional ground absorption provided by the soft landscaping often installed
around the approaches.
If these measures do not work, then it might be necessary to extend the
covered length of the tunnel so that the portal is located in a less sen-
sitive position. This was necessary at the Medway Tunnel in the United
Kingdom, where the cut and cover tunnel was extended 250 m to move
the portal away from a development area where the increased noise and air
pollution would have been unacceptable to the developer. In that instance,
the developer of the site contributed to the additional costs as he decided it
was in his interest to do so. An intermediate solution is to use sunscreens
that form a partial barrier over the approaches and may have a degree of
sound-absorbing ability.
