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I have tried searching for this, but I am unable to find a good answer. There is all sorts of stats about the longest bridge in the world, the highest, the longest span, but I think one of the most difficult things is the depth of the pier.

From my internet search I found a San Francisco article that says the deepest pier in the world is on the Bay Bridge at 242 feet (~74m). This was built in the 1930's, so does this feat of engineering still remain at the deepest, or has it been superseded?

I initially wondered about this because of Boris Johnson refloating the idea of the idea of a road bridge between England and France. At 26 miles, that would not be a world breaking engineering challenge, but it seemed to me the depth of the piers. The Strait of Dover deepest point is 68m, so this does seem like a feasible project from a engineering point of view, but I think the question remains whether it is an efficient use of resource. Particularly when the Channel Tunnel utilisation is only running at 48% or so.

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    $\begingroup$ By depth of pier I assume you mean the depth of water the pier would be submerged in, not the foundation depth of burial into the geological mass. $\endgroup$
    – Fred
    Commented Jan 21, 2018 at 14:22
  • $\begingroup$ @Fred - That is very good point. For example, if the depth from the surface of water to seabed was 30m, but the sea bed was another 30m of sand to the bedrock, the total pier structure would be 60m? I would interested in knowing both. $\endgroup$
    – David
    Commented Jan 22, 2018 at 10:40
  • $\begingroup$ no answer for you , but it stands to reason a really deep pier would be on a bridge of similar length ( you can span 1000m and a large number of deep piers would likely be cost prohibitive ( or well known anyway ). Per wikipedia there are 40-50 bridges in the world in that class to go look at.. $\endgroup$
    – agentp
    Commented Jan 22, 2018 at 14:58
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    $\begingroup$ Perhaps the geology of the seabed also affects the maximum depth that is feasible? $\endgroup$
    – Jonathan R Swift
    Commented Sep 24, 2019 at 7:40
  • $\begingroup$ Geology is critical. One solution is to examine the bedrock holes with a boroscope to evaluate the integrity of the rock. $\endgroup$
    – blacksmith37
    Commented Nov 5, 2021 at 15:33

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It's not a bridge, but if you wish to consider the depth of marine structures generally, a good example is the Troll A platform in the North Sea. From Wikipedia:

"The Troll A platform has an overall height of 472 metres (1,549 ft), weighs 683,600 tons (1.2 million tons with ballast) and has the distinction of being the tallest and heaviest structure ever moved by mankind. The platform stands on the sea floor 303 metres (994 feet) below the surface of the sea ..."

Troll A Platform Norway

As a starting point for bridges you have a list of the highest bridges with the highest bridge being the Duge Bridge in China with a deck height of 565m:

Duge Bridge China

As for marine crossings, there is a bridge (that apparently exceeds San Francisco Oakland Bay Bridge you have referenced) that has received instruction to proceed during 2010 but has since been put on hold. The Bridge of the Horns is a Red Sea crossing at the Island of Perim (300m sea depth) from Yemen to Djibouti that is described by COWI DK:

"The length and navigational clearance required west of Perim will likely mean at least 10km of multi-span suspension bridge, with the remainder being approach bridges. The concrete pylons will have to be supported up to depths of 300m and at the same time tower 400m over the water's surface to support the ultra-long span suspension bridge. Each pylon will be 700m. It will be enormous."

enter image description here

Just for information, Professor TY Lin has proposed a Gibraltar crossing that would have a height of 910m. I could not find specific information on depth, but one would assume that it would exceed 300m (the sea depth there ranges between 300 and 900m). A tunnel was originally considered but has been dismissed as it would be 900m deep, the deepest tunnel in the world is the Eiksund Tunnel at 287m.

Coming back to the underlying purpose of your question; ie Boris Johnson's proposal, I have pointed out these extreme data points to illustrate that the project is well within the bounds of current engineering capability. In my view the most significant challenge of the project would be determining the location and magnitude of the main span required so as not to interfere with shipping vessels as this would likely determine the size of piers and depth of foundations.

(Note that these depths are referenced to the sea bed, they do not include depths that might be required for foundations. Checking the bathymetric survey data at the San Francisco Oakland Bay Bridge crossing does not support a sea bed depth of 74m (looks to be in the range of 20 to 30m), therefore the depth you have quoted (74m) may include foundations; for comparison purposes the bathymetric data at the Strait of Dover yields a sea bed depth of about 50m - as you have indicated an extreme of 68m to the sea bed.)

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    $\begingroup$ Note the OP references the San Francisco Oakland bay bridge, not the golden gate. $\endgroup$
    – agentp
    Commented Jan 22, 2018 at 12:33
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    $\begingroup$ @agentp Corrected now. It looks as though the OPs data may include the depth to foundations as the sea bed is generally only about 30m at that location. $\endgroup$
    – AsymLabs
    Commented Jan 22, 2018 at 12:48
  • $\begingroup$ There are many oil platforms deeper than the 242' Bay bridge. $\endgroup$
    – blacksmith37
    Commented Sep 22, 2019 at 19:11
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The Tasman bridge in Hobart, Tasmania has a pile 265ft below mean water level. This on the western side, I understand, so is not connected to the highest span.

http://www.parliament.tas.gov.au/history/tbstats.htm

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Piles of the Padma Bridge are 128 meter (420 ft) long. Each pier has 6 - 7 piles.

http://www.padmabridge.gov.bd/mainbr.php

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