General Principles for the Design of the Cross-section - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 5

The outline of the clearance gauge includes areas solely reserved for traffic. Emergency
pavements are provided on each side of the carriageway, which are 1.00 m wide and have
to have clear headroom of 2.25 m. These are separated from the carriageway with kerbs,
normally 7 cm high. Part areas are assigned at a height > 2.25 m above the emergency side
pavements, in which easily deformable furniture elements particularly traffic signs and no-
tices can be located although these are only permitted to approach within 50 cm of the traffic
gauge; jet fans required for ventilation have to be installed in niches or ceiling coves. Easily
deformable light fittings are only permitted to approach within 50 cm of the traffic gauge
at a height of > 3.75 m. If jet fans are located inside the normal structural dimensions, this
results in widenings of the emergency pavements dependent on the diameter of the fans to
be installed [77].
It is often practical to locate traffic signs on the end walls of breakdown bays. In excep-
tional cases, traffic signs can by located down to a minimum of 30 cm from the traffic
gauge at a height > 2.25 m above the emergency pavements; but this does not apply where
a widening of the emergency pavement has been provided for fans. If traffic signs have
to be made with smaller dimensions than stated in the regulations [32], then this has to be
agreed with the authority responsible for traffic management.
Light fittings are permitted to approach within 50 cm of the traffic gauge in exceptional
cases when it can be ensured that a clear headroom of 4.10 m from the top of the emergency
pavement to the underside of the light fitting is maintained at all points. Jet fans with external
diameters ≤ 70 cm are permitted in exceptional cases to be located in the safety margin with
a minimum distance at the side of ≥ 30 cm to the traffic gauge in the upper corners.
Gradient and cross‑slope. According to the RAS-L [75], the gradient in uninhabited ar-
eas running through tunnels should be limited to 4 % if possible and a maximum of 2.5 %
should be the intention, particularly for longer distances. The chimney effect, which also
increases with increasing gradient, normally leads to higher longitudinal flow, which in
case of fire can severely impair the rapid and effective removal of smoke by a ventilation
system. In order to ensure road safety and due to the chimney effect, gradients steeper than
5 % should be avoided in road tunnels in uninhabited areas.
A minimum cross-slope of 2.5 % is specified for straight stretches in order to drain surface
water [76]. Depending on the design speed, the cross-slope may have to be adapted to suit
the curve radius [75]. In addition to these conventional requirements, the cross-slope of
roads in tunnels has special significance in case of an accident. If a fire breaks out, any
leaking flammable liquids have to be drained away as fast as possible, which is ensured
by a steep cross-slope and high-capacity drainage. Slot channels with a capacity of 100 l/s
should therefore be provided, with firestops spaced at max. 50 m [77].
1.2.2 Constructional measures for road safety in tunnels
Breakdown bays. Breakdown bays should be provided where the provision of hard shoul-
ders is not economically justifiable. They are required in tunnels more than 900 m long, and
under special conditions from 600 m (for example ≥ 4,000 HGV · km / bore and day) [77].
The end wall should have an angle of ≤ 1:3 in the travel direction (Fig. 1-4). It can be secured
by suitable passive protection according to RPS [78]. Concrete protection walls should have
an angle ≤ 1:3. In tunnels with two-way traffic, these requirements apply to both end walls.