Civil Engineering Reference
In-Depth Information
the sun will be low in the sky shining directly at the drivers. These cir-
cumstances will require very bright entrance threshold lighting levels. To
reduce the lighting requirement in these circumstances, sunscreens are
often provided over the road at the entrance portal to assist the drivers.
These, however, do have maintenance considerations. In winter condi-
tions, they are susceptible to a buildup of ice and snow, which could
fall onto the traffic below. For this reason, they are not favored by some
authorities, although they can reduce the amount of entrance threshold
lighting required.
The lighting levels in the entrance and exit threshold zones are usu-
ally controlled automatically using photo sensors to measure the ambient
external light levels and adjust the tunnel lighting accordingly. In design-
ing the lighting system for an immersed tunnel, it must be remembered
that although the tunnel will generally operate with one-way traffic in each
bore, under maintenance or exceptional circumstances, it may be necessary
to operate with two-way traffic in one bore. Speed restrictions may have to
be introduced as drivers entering the tunnel the wrong way would be faced
with the shorter exit length lighting threshold. In practice, this is unlikely
to be a major disadvantage as strict speed limits are likely to be in force if
the tunnel is being operated in contraflow mode.
Lighting in the tunnel is provided by luminaires in the tunnel roof above
the traffic lanes. The lengths of the threshold and transition zones depend
generally on the design traffic speed, and the associated safe stopping dis-
tance, as well as, possibly, the orientation of the tunnel. The alignment of
the luminaires is parallel to the direction of the tunnel so that their light
shines transversely on to the tunnel walls and gives a uniform level of light-
ing across the tunnel.
The interior finishes of the tunnel play a part and the walls should have a
high reflectance value and be a relatively light color as this will reduce the
number of luminaires required, with consequently lower capital cost and
energy consumption. For example, in the United Kingdom, the wall reflec-
tance is required to be at least 0.6, whereas a plain concrete finish is only
0.3. In immersed tunnels, where space is at a premium, the provision of a
cladding system would require more internal space and, consequently, be
more expensive, so the concrete walls are often simply painted. The paint
system must provide the required reflectance value, but not give a glossy
finish as this would result in problems with glare from tail lights being
reflected off the wall, particularly in curved tunnels.
There is a wide variety of choice for the luminaires themselves and the
technology is improving all the time. The choice should be made on a
whole-life costing, taking into account capital cost, replacement cost, and
the traffic management associated with this, as well as the energy consump-
tion. LED lighting systems are now being implemented in tunnels, which is
leading to further economies.
