Other structures may be provided within or at the portals of a tunnel are described below.
Emergency or fire-fighting recesses contain fire hydrants, hose reels (sometimes) and special equipment for use by the fire brigade. They are located at intervals along the tunnel length.
They may be combined with the safety recesses, containing emergency telephones and portable fire extinguishers, discussed in Section Safety recesses.
In many tunnels, electrical sub-stations and mechanical, electrical, communications and control equipment may be housed in plant rooms located within the tunnel.
The layout and sizing of plant rooms follow the same principles as for plant rooms in service buildings. For example, adequate space is needed for opening of cabinet doors and access to switchgear. Allowance for cable runs and bend radii is important and can be more problematic compared to external buildings because of tunnel construction and space constraints in the tunnel bore.
Consideration should be given to safe access to tunnel plant rooms. This may be possible during bore closures only. In some tunnels, lay-bys may be provided adjacent to plant rooms to allow maintenance vehicles to stop safely, even without tunnel closure.
Between the exit portal and the neighbouring entry portal of two uni-directionally used tubes, a substantial air recirculation may take place, depending on the local geometry and wind direction. The same problem exists between exit portal and fresh air intake of a semi-transverse ventilation station.
In short tunnels with high self-ventilation this may be of no concern, but in longer tunnels this effect should be reduced. Depending on the circumstances, the splitter wall may need to extend approximately 20 to 40 m from the portal. Further details are given in Section IV.2.3 "Recirculation" of report 1995 05.02.B "Road tunnels emissions, environment, ventilation".