Cofferdam in Flowing River 2

TheHayDuke · Dec 7, 2023

Hey All,

I'm currently in the preliminary stages of designing a cofferdam for a water intake in sizable river (approximately 8 m deep and 1 m/s flow rate prior to the cofferdam construction) and I'm wondering if anyone has any good resources for either earth fill cofferdams in flowing water or just simply how to predict the behavior of fine grained material that's end dumped in those conditions.

The reason we're looking at earth fill rather than sheet piles or other structural options is the shallow bedrock and high chance of cobbles or coarse material causing issues. I think also with the remote location in northern Canada earth fill will just be cheaper.

but no one at my company has any formal experience with them in conditions like this. There's a few more senior guys who can give me rough "ehhh that probably wont work" type advice but that only gets you so far.

The currently idea is an initial riprap berm with fine grained material placed against it. Most of what I can see online has the impermeable material placed on the river side of the granular, but without compaction or placement control, I don't know how you could maintain stable slopes. Or how you would prevent it from eroding away before you can armor it. We could place the fines on the interior of the riprap but that drastically increases the amount of riprap needed because we need to maintain a minimum interior space.

So anyways, just wondering if there are any design manuals or papers on the topic that I should read, I cant seem to find much, and what I can find is from the 1960's typically.

thanks in advance!

TigerGuy · Dec 7, 2023

Fine grained material will mostly drift down stream when it is dumped into the water. What about pressure grouting the rip rap?

cvg · Dec 7, 2023

grouted rock berm will be expensive and will leak like a sieve. it would require a lot of pumping

might be easier to construct a forebay with intake structure in the dry. than just connect with a channel to the river. no cofferdam needed.

would also eliminate the problems with sediment and flowing water currents

LittleInch · Dec 8, 2023

Can you sketch this out a bit as it's difficult to grasp what you're trying to achieve.

It sounds like you want to create a water tight coffer dam in an 8m deep river with water flowing at 1m/sec?

That sounds like quite a big ask, but without defining what you want or how much space you have its not easy to say.

Using gabions initially to create your structure would seem like a good start point to at least get some reduction in water speed and then allow gradual build up of material, but 8m deep?? or look into geobags to contain your earth fill and drop them in to create your wall?

But you're going to need 15 to 20m of river bank to create your dam. Have you got that?
What other material have you got (Rock sizes etc and what do you mean by "earth fill". That's very vague.

How long is the dam?
How do you plan to seal the end?
Is this permanent or just to build the intake structure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

TheHayDuke · Dec 8, 2023

LittleInch,

Its a temporary cofferdam just to install the actual water intake and connect it to the rest of the system. The intake is currently about 80 m into the river so the currently total length of cofferdam is just shy of 300 m. Space isnt really an issue, with the larger footprints we've estimated we're not halfway across the river, theres plenty of space on the bank.

Its definitely a big ask, but from what i understand we cant adjust where the intake is to move it closer to the shore as its in the only part of the river deep enough to maintain water flow through dry years. The intake will service a water treatment plant that'll supply a few rural counties.

AS for material, we do have access to a large amount of high plastic clay till, that needs to be excavated anyways for the water treatment ponds. In terms of rock, we have a couple sources for both Class 1 and Class 3 riprap out of the rockies so having material to withstand the flow shouldnt be too much of an issue. Its just combining the two that I'm not sure about.

The cofferdam should only need to be in service about 4-6 months longer than it takes to construct it, so we're hoping to get in and out in a year but its hard to say how feasible that is. One contractor i talked to thats done similar work seemed to think so.

LittleInch · Dec 8, 2023

Why do need to have a coffer dam is my first thought.
Or is this coming from a civil contractor who gets to spend lots of money?

Can't you make up the pipe onshore, make a concrete end inlet structure, drop that in the river and then drag the pipe out to meet it?
Can you get any barges out there?
Then rock dump the pipe?

Or drill the pipe out?

A coffer dam basically 120 odd metres long by 10m? wide to create a passage for a water inlet just sounds nuts. There must be an easier way?

But if not then start at the shore side with your big rocks and just build a causeway for the strength part, then line the outside with your clay either in geo bags or just dump it on top.

And get some big pumps.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

TheHayDuke · Dec 8, 2023

Most of those are questions that were decided before I got roped into this project, there wasn't originally geotech scope but the water resource guys i guess are set on this and the need for a cofferdam of some description to build the intake in the river.

dumping rocks out progressively from the shore is the plan, with a clay liner on the outside. I just dont know how you would manage the immediate or medium term erosion of it. I mean we can throw more rocks for armoring on the outside, but even knowing how long that slope is to ball park the amount of rock needed isnt easy.

Theres a good chance i talk them into outsourcing this to a more specialized consultant but for now im trying to use it as an excuse to learn as much as i can about these interesting design problems.

LittleInch · Dec 8, 2023

Gabions.

I love them.

Also geo bags might work?

Fill them initially with rocks then add clay filled ones on the outside. Work on a slope of 40 odd degrees and you can count them in.

Will make it easier to remove when you need to and reduce quantities lost to the river. The environmentalists will love you.

e.g.

https://trapbag.com/constructing-a-cellular-cofferdam/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

BridgePiers · Jan 22, 2024

Geo bags in 8m deep water though?

I have no idea. I'm just curious.

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