Civil Engineering Reference
In-Depth Information
Tidal flow
lane
Two-lane
highway
Two-lane
highway
Cycle
way
Subway
Subway
Figure 4.4
Multilane Second Benelux Tunnel.
immersed tunnel compartment, so dual carriageway roads need the con-
ventional two-cell cross section. For more than three lanes, structural
issues come into play. As the cells become wider, the roof spans become
longer and structural requirements dictate that they become thicker or
alternatively require transverse prestressing. Prestressing is possible but is
an additional construction complication and requires careful detailing to
ensure the durability requirements of a major piece of transportation infra-
structure are met. This leads to a four-lane carriageway normally being
better accommodated in two bores of two lanes each. The overall area of
the cross section will be increased because of the additional verges and hard
shoulders, but the cross section will work better structurally and can be
more economic overall. This was the layout chosen for the Drecht Tunnel
in the Netherlands. The arrangement does, however, have implications for
traffic approaching the tunnel from the dual four-lane carriageway. Drivers
often make unsafe last-minute lane changes to take the other bore. In this
respect, the more costly structural arrangement may be preferred on safety
grounds and this risk should be considered in the choice of cross section.
Modern safety requirements call for a separate bore to enable people
to safely exit the tunnel in an emergency. In some older tunnels, this was
achieved by evacuating people through the central wall into the adjacent
bore carrying the other carriageway and may still be acceptable in shorter
tunnels, but now a separate escape bore is generally preferred. Typically,
these pedestrian escape bores are about 1.2-1.5 m wide; a typical example
is shown in Figure 4.5. To be effective, they must be kept clear of equip-
ment, and doors giving access to the bore must not cause an obstruction.
The pressure in the escape bore has to be higher than the adjacent traffic
bore so that smoke and flames are not drawn into it when a door is opened.
This escape bore can also be used to carry the tunnel services either
above or below the pedestrian space. It is not always straightforward to
combine the evacuation and services requirements, particularly in long tun-
nels where large transformer substations are required at intervals along the
tunnel and the size of these acts as a blockage to the escape bore. A satis-
factory evacuation strategy may be to use the separate central bore for
