Page 344 - 360.revista de Alta Velocidad - Nº 5

Sepúlveda Abad, Silvia. Lastra de la Rubia, Emilia.

The main disadvantages are the higher costs of construction and operation.

Switching from dual track sections to single track tunnel sections involves the following
aerodynamic considerations.

• Due to the existence of a smaller cross-section in the case of single track tunnels, the
pressure fluctuations are greater.

• However, interference with other trains is lower, which implies better compliance with the
required comfort criteria.

• In relation to micro - pressure waves (sonic boom), the probabilities of occurrence increase
due to a smaller section.

• On the other hand, the presence of slab track instead of a ballasted superstructure, which
is very common nowadays given its lower maintenance cost, increases the probability of the
sonic boom phenomenon.

Since 2005 - 2006 the existence of the sonic boom phenomenon in Europe has been observed
when the lines are put in service or when the speed increases. Until then it had not been
observed due to the preference of double-track tunnels versus single-track tunnels.

• In Germany in 2005, this phenomenon was observed on the Nuremberg - Ingolstadt line in
the first start - up tests in several bitube tunnels of about 7.2 and 7.7km respectively.

• Here in Spain, in the High Speed line Madrid – Valencia it has been detected in the tunnel
of La Cabrera, bitube and about 7km in length. Also in the LAV Madrid - Valladolid / Medina
del Campo this effect has been detected in the bitube tunnel of San Pedro of about 8.7km
in length.

• In Japan, in 1975, the presence of this phenomenon was detected in the tests of the Okayama
- Hakata line of the San - Yo section of the Shinkansen. Until that time, in Japan there was
not any legislation limiting the phenomenon.

6.2 Decription of the sonic boom phenomenon

As it was mentioned above, when a high-speed train enters a tunnel, there is an overpressure
and depression in head and tail that propagate along the same at the speed of sound, reflecting
each time they reach the portals of the tunnel.
At the outlet portal of the tunnel, most of the pressure wave is reflected as an expansion wave
propagating in the opposite direction towards the inlet portal. However, part of this wave is
emitted in all directions outside the tunnel in the form of impulse-type micro-pressure waves,
which generate a detonating sound in the vicinity of the outlet of negative environmental
effects.
Thus, the phenomenon of micro - pressure waves has three phases:

• Generation of the compression wave by the entrance of the train in the tunnel. The shape
of the compression wave generated depends on different parameters such as: areas of the
train and tunnel, geometry of the nose of the train and speed of the train among others.
• Propagation of the wave through the tunnel. The propagation of this wave occurs at the

342 360.revista de alta velocidad