Tremie Concrete for Underwater Installation 1

[JedClampett]

Are there any good articles or texts on tremie concrete? I've looked at ACI 304 (pretty good)and Nawy's "Concrete Construction Handbook"(just OK) which both had some advice. It just seems like there are a lot of engineers who know of tremie installation, but very few who have any real experience.
The contractor says they know what they're doing, but I'm skeptical.

 

[JAE]

From a quick Google Search (the first one is a new publication):

http://www.dfi.org/viewpub.asp?tid=TM-CONCRETE2016

https://publications.csiro.au/rpr/download?pid=csiro:EP112143&dsid=DS1

http://www.bittner-shen.com/publica..._Shafts_the_Key_Issues_and_Case_Histories.pdf

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[BigH]

Tomlinson's book Pile Design and Construction, say the 7th edition, pg 381-382 and 468-89, discusses concrete by tremie. It has a number of practical tips and watch-out fors . . in the pages.

 

[Ron]

JC...tremie concrete has limited application, mostly in foundation and bridge construction. One common variation on tremie application is augered concrete piles, the main difference is that the concrete is placed under pump pressure, not just gravity head.

Advances in concrete technology have allowed several techniques that make tremie placement less necessary. Better pump mixes, better/larger pumps, mix proportioning to better alleviate segregation from drop placement, better set and hydration control and other progress.

Further, tremie placement requires quite a bit of expensive, large equipment (crane, concrete bucket, etc)....plus it is relatively slow!

 

[SlideRuleEra]

Jed - What are the three dimensions of the the planned tremied concrete?

Edit: Also, what are the contractor's proposed means and methods?

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[JedClampett]

The dimensions are about 70 ft. x 120 ft. x 4'-6".
As far as means and methods, they want to use one 150 cu yd. per hour pumper and one spare. One tremie pipe, 5 inches in diameter. Site has limited access and is in a slightly remote location. There's a lot of things they want to do that contradict ACI 304 (five inch pipe, dragging the pipe across the pour, etc.).
I'd prefer multiple chutes pre-positioned in a 15 or 20 foot grid shuffled around while pouring continues. While one is being used, the others could be staged.

 

[hokie66]

I have never heard of placing a slab like that underwater. As others have said, tremie is normally used for deep foundations. It sounds like they just want to pump concrete into water, not actual tremie.

 

[SlideRuleEra]

Jed, you are so right! That contractor does not know what he is doing. In addition to the problems you have noted, the average placement rate for the 150 yd³/hr pump (running full bore, non stop) is just 6 in/hr. Not good, since the idea of tremied concrete is to have a solid mass. The tremie pipe should be embedded at least 2 feet into the concrete that has not had its initial set - that's not going to happen here. Placement in layers is done sometimes, but is not the best practice.

A thickness of 4.5 feet is "thin" for tremied concrete, especially since there is so much volume to be placed. As noted above (pipe embedded 2 ft.) a good bit of the total thickness may be minimally acceptable (especially at the perimeter) even under the best of conditions.

The surface of tremied concrete typically has a slope of about 1:6 (it can range from 1:3 to 1:12). The 15 ft. grid you mentioned is just right to minimize this issue. Dragging the pipe horizontally through the concrete is a no-no. If the concrete could be placed fast enough (not going to happen with one pumper) the pipes could be "leap frogged" around the area (instead of having 30, or so, pipes on a 15 ft. grid.

A few other items:

Concrete mix with high cement content: often 6.5 to 7 bags cement per cubic yard (some may "wash out" during underwater placement).

High slump concrete: typically 8 inches (needed since vibration is not practical).

A temporary plug in the 10 inch diameter (minimum) steel (not aluminum) pipe at the beginning of placement. This is to ensure the first concrete deposited has not segregated.

In the concrete mix, use gravel (rounded) instead of crushed stone. Less flow resistance.

Are there any specs on tremied concrete in the contract?

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[JedClampett]

SlideRuleEra, there are specifications. But long story short, I included tremie concrete and installation as a "just in case" so I didn't really study it. It's the owner's specification and it's not horrible, but leans heavily on ACI 304.
The contractor is full of confidence, from their one installation, which I haven't seen. As I said, I'm worried about delivery, pumping, placing and everything else. And I know for certain that whatever crap they place, they won't take out, even if you could remove it with a spoon.

 

[hokie66]

Just curious. How deep is the water, and how do they place the reinforcement?

 

[JedClampett]

I'm guessing 30 feet deep for the water.
No reinforcing. Concrete is a cofferdam plug, but will be used to resist uplift and sliding for the completed structure.

 

[SlideRuleEra]

Jed - Of course I don't know the details of the project, but the info presented is disturbing. A cofferdam seal has several "jobs". One, as you mentioned, is to resist uplift. If the cofferdam is dewatered completely (even temporarily), my "back of the envelope" calcs indicate that the seal needs to be at least 10 ft. thick.

IMHO, what this overconfident contractor is proposing guarantees that the seal will be little more than a thick layer of gravel, with little or no cement - certainly not anything that would pass for concrete. The concrete is exiting a 5" hose moving straight down, located just above the bottom. Velocity is over 8 ft/sec (based on 150 yd³/hr). I expect cement in the fresh mix will be washed out quickly.

The second purpose of a seal is to resist artesian water pressure after cofferdam dewatering. Gravel won't do that.

Third, depending on sheet pile embedment and cofferdam internal bracing, the seal resists high horizontal hydrostatic forces after dewatering. Not a job for gravel.

A depth of 30 ft. is a major cofferdam, in my book. Please read this link:

Design Loads and Construction of Tremie Sealed Cofferdams

I hope that I am misinterpreting the situation, but have been around cofferdams my entire life... they are dangerous. I'm aware of multiple fatalities since cofferdam failure is often both sudden and catastrophic.

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[bridgebuster]

I agree with SRE about the thickness of the seal. 4'-6" seems too thin for 30' of head, unless there's a lot of piles. On a recent bridge project by my firm, the piers had a bottom of footing that was about 28' below MHW. The seal was almost 15' thick (the contractor's engineer was overly conservative; I think 11 to 12' would have worked). Meanwhile, our plans showed a 5' thick seal for bidding purposes. Opps, $3.5 Million change order.

What happened was someone used the attached chart without realizing it wasn't applicable in this situation.

 

[JedClampett]

When the tremie is placed, there will be significant head. But after that, the area surrounding the cofferdam will be dewatered using well points to a reasonable level (much, much lower). The dewatering, cofferdam and plug were engineered by others. The other engineers seem to have their act together. They checked the concrete plug compared to the dewatering planned.
I'm not worried about the plug if it gets installed correctly. SlideRuleEra, if you're right, and the concrete is truly horrible, the contractor will be reinstalling as necessary. I am worried that the result will be somewhere in between gravel and good concrete. Plus, it's one of those things that if the project goes south, nobody looks good, no matter how many times we raised a red flag.

 

[SlideRuleEra]

Jed - The exterior well points will certainly help minimize the uplift issue. Well points are capable of drawing water down about 25 ft. at the location of the point itself - its horizontal zone of influence is limited by soil properties. There could still be about 10 ft., or so, of hydraulic head to cause artesian flooding if the seal is not competent.

Is it possible to have the contractor perform a small-scale test of his proposed method? IMHO, that would be money well spent. For everyone's benefit, need to do everything possible to prevent having to remove an incompetent cofferdam seal. Taking out 1400 yd³ of thick, porous, weak concrete is a big deal. Especially since the removal will have to be performed underwater (even with the exterior well points in full operations). The excavation could be dewatered, but to do this deep-well submersible pumps and big bucks will be needed.

This overconfident contractor reminds me of a favorite Mark Twain quotation:

It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.

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