Boris Johnson,
the UK foreign secretary, has
proposed building a bridge across the English Channel to
connect England and France, to complement the rail tunnel that opened in 1994.
Leaving aside the political and financial challenge of such a project, would it
even be feasible to build such a long structure over the open sea and one of
the busiest shipping lanes in the world?
For spanning
long distances we use either cable stayed or suspension bridges, because they
are light compared to conventional bridges and provide large areas unobstructed
by pylons.
The longest suspension bridge built to
date is in Kobe, Japan, measuring 3.9 km in total, with clear space between its
pylons of almost 2km.
The longest bridge with a single cable-stayed span built
to date is the Sutong Yangtze River Bridge near Suzhou, China. Its two pylons
are separated by a 1km main span and it is 8km long in total. But it is not a
completely suspended structure, with a number of densely spaced pylons on
either side of the main span.
Similarly,
the longest bridge in the world is the
Danyang–Kunshan Grand Bridge in China. It is mostly a conventional bridge made
from columns sunk into the seabed, although it has a cable-stayed section. The
bridge is 165km long (just over 100 miles) but the short distance between the
columns would not do for the busy Channel.
A bridge over
the English Channel that wouldn’t disrupt shipping would require a span of 22
miles, or 38km in total, which is an order of magnitude larger than anything
else built to date. This presents a considerable civil engineering challenge
for a number of reasons.
Tallest bridge
ever
The channel is
between 40m and 60m deep and some passenger ships are more than 70m tall. So to let ships pass
underneath, the pylons supporting the bridge would have to be around 150m tall.
To support the cables you would have to add pylons above the deck, which would
mean a total pylon height well above 500m. Again, nothing this tall has ever
been built.
Apart from the
challenge of building them, the high pylons would need a very careful
assessment of how the wind would affect them and the attached cables. This
would have to be supported by a vibrational analysis to prevent potential
failure. As the wind hits the pylons and the cables, it causes them to vibrate.
In some instances, this can grow into a phenomenon known as “flutter”, causing
the entire structure to become unstable, as happened to the Tacoma bridge, which collapsed in 1940.
To prevent the
cables from becoming overstressed, you would need a relatively large number of
pylons to hold the bridge up. But again, this conflicts with the requirement of
having unobstructed space on the water for ships to pass through.
The cables would
also have to be very strong and light cables, possibly stronger than the very
expensive carbon fibre composites. To make them we would probably need to
research new materials and find a way to make it affordable.
Another big
unknown quantity is the undersea ground conditions, which would need to be
carefully investigated to see if they would be suitable for the pylons’
foundations. This means we also don’t know how much the foundations would add
to the cost.
Possible
solution
Following the
lead of the Danyang–Kunshan Grand Bridge in China, it may be possible to
combine a cable-supported structure, for the part where a clear sailing space
is needed, with a conventional column bridge style. This might ease problems of
construction and cost, although it may still disrupt shipping routes to a
certain degree.
But the only
thing we can say for sure is that cost of the project would likely run into
billions of pounds. It’s a project that would require a lot of further research
into the properties of cable materials, not to mention a proper feasibility
study.
http://theconversation.com/boris-johnsons-english-channel-bridge-an-engineering-experts-view-90409
