There’s no doubt in these days of advanced materials and engineering techniques why a bridge can’t be built across the English Channel, but here are a few problems that they would have to overcome to make it a success:
Cost- but that shouldn't be a problem in these days of PFI2. Security - stopping illegal immigrants from simply taking a twenty mile walk across it. Maintenance - apart from who would choose the colour, painting it would be a nightmare, and what happens when you drop your brush? Weather - this is one that I imagine didn't immediately spring to the mind of Boris Johnson.
I thought that I’d use SYNOP observational data from the AWS on the Sandettie lightship, which lies approximately 8 or 9 nautical miles to the northeast of where any bridge might be built to see the kind of wind speeds any bridge would have to cope with.
Figure 3 is an anemograph of the hourly surface winds since the 1st of November 2017 from Sandettie, in what’s been more or less a fairly typical autumn and winter so far across the British Isles. As you can see the English Channel, especially the mid-channel, is a very windy place. The yellow outlined smoothed line in the top chart is a 24 hour moving average of mean wind speed in knots. The mean looks to be about 20 knots for the last 11 weeks or so, with windier spells when the mean varied between 25 and 35 knots. As you can see there were plenty of occasions when the gusts (darker red) were of storm force 10, and on three occasions at hurricane force 12 strength.
If the bridge is to stand clear of shipping it will need to be high and the anemometer on Sandettie can’t be much more than 10 metres above the sea surface, so if anything you might need to add an extra 10 or even 20% to these values to get an idea of what the bridge would have to tolerate. Cars and lorries would have to be almost boxed in to prevent them from being side-swiped by any gale, if they weren’t, I can imagine that the bridge would have to be closed for extended periods during any autumn or winter.
Apparently the longest bridge in the world at the moment is the 26.4 mile long Jiaozhou bay bridge on the southern coast of China’s Shandong Peninsula in northeastern China (fig 4). Chinese workers there toiled at marathon pace to build the bridge in just four years, starting at each side and meeting in the middle. The structure has 5,200 pillars and cost at least $2.3 billion. The bridge is apparently earthquake and typhoon proof, and designed to withstand the impact of a 300,000 ton vessel. I wonder if that’s the team Boris is planning to get to build it?