Tremie Seals and Buoyancy 1

[appot]

This thread got me thinking about the submerged unit weight of concrete:
thread507-419886

Unreinforced, tremie concrete probably weighs about 140 pcf or 140 - 62.4 = 78 pcf submerged.
12' head (from water level outside cofferdam to top of tremie pour) = 12' x 62.4 pcf = 750 psf.
750 psf / 78 pcf = 9.6' of buoyant concrete.

I am not seeing the mechanics behind using the submerged unit weight. I have found several DOT guides that also do not use the submerged unit weight. Could someone guide me in the right direction on this please? I have attached a picture with my though process.

In my image below, my cofferdam has 3 stages.
I) excavation cofferdam, and water inside the cofferdam reaches equilibrium with groundwater
II) tremie concrete is placed
III) cofferdam is dewatered

In stage I, we can probably all agree that Point A is in force equilibrium.

In stage II, I am neglecting the rising water level in the cofferdam due to the addition of new material (concrete).
I have drawn a FBD of the vertical forces on the concrete seal during this stage, which shows hydrostatic pressure at the bottom, and the smaller hydrostatic pressure on the top. The difference in the top and bottom hydrostatic pressures results in the buoyant force. I did not draw it, but the FBD could simply be Wc point down, and a vector Fb pointing up.

In stage III, the hydrostatic force on the top of the seal becomes zero. Now we are left with a hydrostatic force on the bottom, and the weight of the concrete.

 

[SlideRuleEra]

appot - I don't use submerged weight, but go with "Archimedes' Principles" instead. IMHO, much less potential for confusion. The same thing as submerged weight, just another way of looking at it.

In the Stage II diagram, for a cofferdam in open water, the water level rise in a cofferdam as the seal is placed is usually negligible. Since there is no significant differential hydrostatic pressure between inside and outside, the steel sheeting interlocks will "leak like a sieve" - allowing the displaced water to flow out immediately. If the cofferdam is surrounded by soil, as shown in your illustration, may be different.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[appot]

Thank you, this makes sense now.

Most people would know where the top of there mudmat (tremie seal) since it would equal the bottom of footer elevation. The buoyant weight method makes this more apparent.

 

[Guest090822]

It is just a terminology difference, end result is the same.