General Principles for the Design of the Cross-section - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 1

1 General Principles for the Design of the Cross-section

1.1 General

The shape and size of the design cross-section derive firstly from the purpose of the tunnel
(rail tunnel, road tunnel, sewer, water tunnel or pressure tunnel for a hydropower station)
and thus the required clearance gauge. Secondly, the dimensions will also be influenced,
as is the alignment, by the geotechnical or structural conditions in the ground to be passed
through; whether earth or water pressure could occur or whether no external loading is
to be expected. Thirdly, the construction process also has an effect on the design of the
cross-section; for a given clearance gauge, the most economic cross-section is that which
can be constructed with the least excavation and support technology and with the optimal
machinery, taking into account the given basic shape.

1.2 Dependence on intended use

1.2.1 Road tunnels
General. The traffic conditions in a road tunnel should in principle correspond to those
in the open air. Road tunnels are, however, special sections of a road and demand strin-
gent requirements for their construction, maintenance and operation. Road tunnels have
to meet particular requirements regarding road safety and operational safety. When the
needs of traffic management are balanced against economy, it is therefore necessary and
justifiable in many cases to limit the speed compared to parts of the road in the open air.
The permitted maximum speed is thus normally limited to 80 km/h in road tunnels, which
inevitably differentiates the traffic flow in tunnels from roads in the open air.
Tunnel cross‑section. Road tunnels with two-way traffic and those with one-way traffic
are fundamentally different. Two-way tunnels normally consist of a single tube with one
lane in each direction. In one-way tunnels, the traffic in each direction is constructionally
separated, for example through the provision of two bores. While in the past each bore was
often laid out with two lanes without a hard shoulder, the changing composition of the traf-
fic and ever increasing traffic loading will also demand three lanes without hard shoulder,
and in exceptional cases even three lanes with a hard shoulder.
The design of the cross-section of road tunnels has to consider road traffic aspects, op-
erational equipment and the tunnel structure. The design of the cross-section of a cut-
and-cover road tunnel is often subject to different constraints from a mined underground
tunnel. Some examples of cross-sections of mined road tunnels are shown in Fig. 1-1.