How did they build harbour walls and harbour arms, the ones that are partially submerged at all times ?

Question

I have been in to plenty of harbours here and abroad on boats and ships and always wondered how they built the deep harbour walls especially the old ones

Answer 1

Natural rocky outcrops were taken advantage of (as was the low tide); and large rocks were piled up, probably using dry-stone walling techniques, topped off with mortared stone.  I expect historically, they had to re-do any part-finished fortifications if a storm interrupted the work.  Many manmade harbours were pretty unstable, and the only ones that survived are those that were eventually built with enough stability to withstand most weathers; and well enough to be repaired when damaged.

Today the stone blocks are replaced with concrete; but it is not a modern invention; the Romans used it - presumably where large rocks weren't readily available.  See this article: http://www.eurekalert.org/pub_releases/2...

Answer 2

They use what's called a cofferdam -

A cofferdam is an enclosure beneath the water constructed to allow water to be displaced by air for the purpose of creating a dry work environment. Commonly used for oil rig construction and repair, bridge and dam work, the cofferdam is usually a welded steel structure that is temporary, typically dismantled after work is completed. Its components consist of sheeting, wales, and cross braces.

The cofferdam is also used on occasion in the shipbuilding and ship repair industry, when it is not practical to put a ship in drydock for repair or alteration. An example of such an application in shipbuilding is when a ship is actually cut in two while still in the water, and a new section of ship is floated in to lengthen the ship. Torch cutting of the hull is done inside a cofferdam attached directly to the hull of the ship, and is then detached before the hull sections are floated apart. The cofferdam is later replaced while the hull sections are welded together again.

See also caisson.

A cofferdam may also refer to an insulating space between two watertight bulkheads or decks within a ship. A cofferdam may be a void (empty) space or a ballast space. Cofferdams are usually employed in this way to ensure oil or other chemicals do not leak into machinery spaces.

Answer 3

They waited till the water levels fell during glacial periods.  Rube!

Answer 4

With a coffin dam they block around a small area with stones  and sand and pump all the water out then build the wall in the dry space then remove the dam and allow the water back in if the harbour is very old before pumps where invented they would get labourers to take the water out by hand.

Answer 5

I always wondered that to, like how did they get the cement to set?  Or was it something else?  or did they lower it all in...

Answer 6

With great difficulty . . . .

Answer 7

Cofferdams as above.

added bit of info: this was only possible once they had fully hydraulic lime and cement - so they would harden underwater over the following months after construction.

A similar process is used for bridge pillars in rivers.

Some harbour walls are made using gabions - cages filled with rocks and lowered into place, stacked up and with a concrete top to them above water. The weight and overlapping cages ( like bricks ) holds the whole thing together.