Tremie Slab

[ron9876]

Looking at a cofferdam with a tremie slab. Heavy uplift on slab. Will have tiedowns to resist the uplift. How do you check the capacity of the slab in bending and shear if it is unreinforced?

 

[bridgebuster]

I've never seen anyone check for shear and bending.

 

[ron9876]

But if it is 4.5' deep and the uplift is 1,550 psf it would seem that stresses should be checked.

 

[slickdeals]

I assume you are designing the tremie slab thickness for the hydrostatic due to the water table onsite during construction and providing tie downs for the design flood?

Is 1550 psf your net uplift? A 4.5' slab is good for 405 psf including the 0.6 factor.

You can always use ACI Chapter 22.

 

[BAretired]

slickdeals,

Is Chapter 22 the provisions for Plain Concrete Design? If so, that would seem appropriate.

BA

 

[slickdeals]

@BA: Yes, Chapter 22 is for Structural Plain Concrete.

 

[ron9876]

But I don't think plain concrete applies to concrete in continuous tension right. The 1550 is gross uplift.

 

[hokie66]

If the slab is in bending and shear, you have to check the stresses. If you don't want to check the stresses, provide enough tiedowns so that bending doesn't matter.

 

[BAretired]

I have never designed concrete placed by tremie. Is it out of the question to reinforce such a slab?

BA

 

[slickdeals]

The system works such that concrete is placed in standing water, displacing the water. It would be rather difficult to have reinforcing laid out under water when placing the tremie. I am no expert on tremie slab, just had one that was designed (not yet constructed due to the economy).

 

[BAretired]

@slick,
I agree it would be difficult but if the reinforcement could be held in place underwater, perhaps it is possible.

BA

 

[dhengr]

I’ve not done much design of tremie seals themselves, but I have worked around them. It has been an awful long time ago though, and as I recall it involved a good understanding of concrete design and placing, and some serious seat of the pants engineering and some ingenious construction methods and considerable construction experience. There just wasn’t much of a cookbook (I mean code, with recipe) at the time, but it wasn’t a new method either. No contractor would have bid using Slick’s .6 margin of safety on this temporary structural element, but they did try to figure out how to incorporate the tremie seal into the final found. design so that it wasn’t wasted material. I would assume one could find much better guidance on the subject today. As Hokie suggested properly placed tiedowns, lead to a bunch of overlapping pullout cones and the bending starts to go away, there was no appendix “D” back them. When they drove the sheet piling, and excavated the interior to a greater depth, they would use a water jet to clean the sheet piling of mud and junk and then did consider bond btwn. the conc. and the sheet piling vs. piling pull-out as part of their hold down force, again no cookbook values that I know of, just basic sheet piling design. The seal can be and was often was reinforced; cages were assembled, out of the water and then lowered and hung from the cofferdam bracing above. The bottom and top 6-12" of conc. were not considered for serious design, only as a lighter DL, because of uncertain conc. quality, otherwise good conc. strengths, bond values, curing, etc. were achieved. The top surface was cleaned of laitance and any pouring irregularities as soon as the conc. started to set up, with a water jet; if you waited to long, days of jack hammer work was involved. This all involved a lot of diver’s time.

 

[PEinc]

If you need to install tiedown anchors, the tremie isn't thick enough to let you dewater in order to install the tiedown anchors. Since the tremie alone is not thick enough to resist the uplift, the tiedown anchors will need to be installed underwater after the excavation is made but before the tremie concrete and resteel are placed. If you try to drill tiedown anchors through an already poured and hardened tremie seal, you will cause major leakage and possibly soil loss up through the drill holes in the tremie concrete unless you know how to seal off the drill holes and maintain positive pressure in the drill holes. Good luck.

Are you able to just install the sheet piling deep enough below bottom of footing so that you can excavate and dewater to the bottom of the footing without using a tremie seal and without causing a quick condition?

If the excavation is being made to top of bedrock, you could drive the sheet piling to rock, excavate (possibly in the wet) and install multiple tiers of bracing, excavate in the wet to rock, install tiedown anchor (dowels?), then pour the tremie seal. And yes, the tremie seal concrete will need to be reinforced unless it is think and massive enough to have minimal shear and bending stresses.

http://www.PeirceEngineering.com

 

[ron9876]

PEinc thanks for the input. The tremie slab thickness is limited because of rock at the bottom of the excavation. We are in South Florida and the rock has solution holes in this area so we can't dewater until the tremie has the capacity to resist the bouyant force.

Will have to install rock anchors or augercast piles, excavate in the wet and place the tremie. I have included reinforcing steel in the preliminary design.