Massive Buoyant Force on Mat Slab below GWT

[Shanman_]

Hello All,

My team is racking our brains trying to rationalize a scenario with the following:

A 5 story-wood building is being designed over 2 stories of concrete. We have a 24" mat slab in the basement and two levels of PT slab above. The depth of the highest elevated level is 12" and the lower level is 8".

The issue to contend with is that the soils report as well as historical groundwater elevation data put the bottom of the mat at ~13 ft below the groundwater level. Thus, the report recommends to design the mat using Hydrostatic uplift of 800 psf! The mass of the wood superstructure above is let's say is 250 psf for argument's sake. Adding up the area weights, we can say 900 psf dead load for the building.

We are bouncing back and forth on the following points:
- As the nature of the buoyant force is less dynamic and more precise than a lateral hydrostatic load or a live load for example, we are contemplating running it as a fluid load and therefore would be factored at 1.2H as opposed to 1.6H. Any opinions on running it this way?
- Does the uplift on the underside of the mat contribute to additional compression in the columns as well as punching shear experienced in the mat slab? This would mean an additional 720 kips axial load (service level) to columns at 30 ft oc each way. We have folks leaning both ways.
- Since the mass of the mat slab is not enough to adequately resist the hydrostatic forces at the basement level, would the net uplift carry into the upper concrete levels as upward thrusts at the columns until there is no net uplift?

Would appreciate input from anyone, thanks.

 

[Agent666]

The only way I'd personally take the 1.2 factor is if the water cannot get any higher. For example some cutoff drains at that level, or take water table at ground level.

The load in the slab is reacted by the columns, it's not an additional load in the columns going up the building if that's what you are asking. It's no different than any building with bearing loads under a slab.

You need to hold down the slab or remove buoyancy forces until such time that there is sufficient self weight as construction proceeds to stop it popping out if the ground under the load factored buoyancy loads, probably with 0.9 times dead loads. If you have insufficient weight, loads don't go up the columns, your basement floats until equilibrium is achieved.

 

[CANPRO]

- Using 1.2H instead of 1.6H sounds reasonable. I don't know the code specifics that you're working with, but the approach sounds logical.

- I don't think the buoyant force contributes to column loads. The buoyant force can't generate any more load in the column than is provided by the weight of the structure - at that point you have lift off and your building is floating. All this means is that some of the weight of the building is resisted by buoyancy instead of direct bearing on the soil.

- Same as above, I don't think that the uplift forces come into play.

Here is a real simple thought process that I think answers the last two points: 1) the building is a static structure, sum of the forces in the vertical direction is zero. 2) You can cut a horizontal section of the building at any column level and draw a free body diagram of the entire structure - you have the loads produced from the self weight, live load, wind, etc... and this is all resisted by the reactions in the columns at your section cut, the buoyancy doesn't come into play. The columns don't know if they're being supported by soil or a buoyant force.

 

[WinelandV]

You need to hold down the slab or remove buoyancy forces until such time that there is sufficient self weight as construction proceeds to stop it popping out if the ground under the load factored buoyancy loads, probably with 0.9 times dead loads. If you have insufficient weight, loads don't go up the columns, your basement floats until equilibrium is achieved.

You know, I wonder if it'd be possible to "sink" a building into place. Dig the hole at the appropriate size, let the water fill. Lower a precast matt slab into the pool, and start building.

There's probably a thousand or two reasons why this would't work, but it's a fun idea.

 

[Agent666]

Wouldn't say its common in buildings, but certainly cassions in civil/infrastructure construction are used all the time for the likes of bridge piers underwater and so forth.

 

[XR250]

Perhaps a network of helical piers to resist the buoyancy force?

 

[azcats]

The code defines H as groundwater pressure and specifies a load factor of 1.6. Not sure I'd be comfortable (or have any justification for) using 1.2.

My guess is that they intended a stability FOS wrt the buoyancy.

 

[CANPRO]

You know, I wonder if it'd be possible to "sink" a building into place

I believe they did something like this for the Brooklyn Bridge. They sunk sealed formwork to the riverbed and filled the void with compressed air, then workers went into the sealed void to dig out the ground by hand. While the workers were digging, the towers for the bridge were being built above, which pushed the formwork deeper into the river bed. There was an great documentary about this on Netflix. This is also how they discovered the bends (also known as decompression sickness or caisson disease).

 

[Agent666]

The code defines H as groundwater pressure and specifies a load factor of 1.6. Not sure I'd be comfortable (or have any justification for) using 1.2.

In AS/NZS1170.0 (AU/NZ loadings code) we are allowed to use 1.2 based on the water level being defined as follows:-

In reality the annual probability of exceedance criteria is impossible/difficult to evaluate unless you have long term ground water records, taking the water level to ground level is often overly onerous. Otherwise a 1.5 load factor is required due to the higher risk of water level not being known with certainty.

 

[HouseBoy]

I would expect 1.2 factor is acceptable for uplift forces and 1.6 for horizontal forces. Looks like the code language is not specific about this but that would be my inclination.

In a similar situation many years ago, our firm designed the basement mat slab to be 4 ft thick in order to achieve sufficient ballast.

 

[WARose]

[blue](OP)[/blue]

A 5 story-wood building is being designed over 2 stories of concrete. We have a 24" mat slab in the basement and two levels of PT slab above. The depth of the highest elevated level is 12" and the lower level is 8".

The issue to contend with is that the soils report as well as historical groundwater elevation data put the bottom of the mat at ~13 ft below the groundwater level. Thus, the report recommends to design the mat using Hydrostatic uplift of 800 psf!

I don't really have an opinion on the load factor deal.....but I have to question the logic of that uplift number because it seem like the guy is telling you to run with that alone. I don't know the soil type or the (complete) situation....but I would think that if you have soil over that footing.....you could consider the saturated soil weight resisting the uplift. Or if this is a basement, and it isn't water tight.....you could consider some sort of equalization of water pressure during a flood. I think FEMA discussed this in one of their publications.

 

[HouseBoy]

WAR
For the water pressure to "equalize" it would be necessary for the water to reach the same level inside the basement as outside. I assume that is going to be unworkable.
The basement/foundation is going to act like a boat, even if it is a leaky boat. So long as the water is squirting in, the upward pressure is there to lift the "boat".
The FEMA design guidance that I am familiar with requires that the water be able to flow freely into and out of a building. It's not a leaky basement situation.
I would think that some amount of soil weight could be attributed to resisting the upward pressure but that is a little dubious to if the soil is truly saturated as I would guess the "friction" between soil particles would be reduced.

 

[Agent666]

You'd use the effective weight of saturated soils (wt soil - wt water) for any soil over footings. Don't forget you may also have water pressure above the footings acting downwards in this situation as well to equate things out a little.

 

[WARose]

For the water pressure to "equalize" it would be necessary for the water to reach the same level inside the basement as outside. I assume that is going to be unworkable.

I'm not talking complete equalization.........but unless we are talking a sealed basement.....that uplift seems excessive.

The question I would have for the OP is: if you do have to have the foundation 13' down (for frost depth I would assume).....why have a "basement" at all at that depth? Why not bury the footings/mat and have a slab on grade (higher up) to pick up the building's equipment (if necessary)?

 

[HouseBoy]

A66
For all the water pressure acting above the footings (I assume you mean acting downward on the tops of any footings that are projecting outward from the foundation wall) there would also be a slightly greater force acting upward on that portion of the footing so... no help.
Remember, this thing is acting like a boat. You need to achieve sufficient dead load to counteract the 800 pst "upward" force. OP writes that he effectively has 250 psf so he needs 550 psf effective downward load. That's about 3.5 extra ft of concrete thickness for the mat slab.

A correction to my earlier post about the water level inside equaling the water level outside, it would be that less about 4 ft.
Still doesn't see workable to have a basement "nearly" full of water.

Looking at the OP, is it correct to understand that you have a 24" mat foundation slab, and a 12" PT and 8" PT concrete slabs along with 5 floor of wood frame building? And, would that roughly be 500 psf or so of concrete plus maybe a little weight for the wood floors?
So really, do you only need an additional 300 psf of dead load? Maybe that's just another 2 feet of concrete thickness.

 

[Agent666]

Yeah I know its slightly different pressures top and bottom, all I'm saying is don't forget the benefits of the load on top of footing projections if you're working out the upwards load on the bottom in terms of area. You're far better simply considering the volume of water displaced to work out the total buoyancy force for global stability. But for the localised design of the mat spanning between columns you're basing it on the pressure on the underside.

 

[ggcdn]

One thing to consider is that if the factored hydrostatic force requires an 'effective' GWT that is above grade, it is kind of nonsensical to be designing for that load, in my opinion. Then you might consider capping your uplift force by the full basement depth x 62.4psf/ft

 

[Mad Mike]

A practical comment- if the groundwater level and uplift forces are such a major consideration in the design, make sure they are accurate and if necessary, have them reviewed by an independent geotechnical firm, run some additional field tests etc.

Before designing for those uplift forces, see if you can't find a way of reducing or eliminating them using drainage.

All the best,
Mike

 

[WARose]

To address the OP's other questions........

- Does the uplift on the underside of the mat contribute to additional compression in the columns as well as punching shear experienced in the mat slab? This would mean an additional 720 kips axial load (service level) to columns at 30 ft oc each way. We have folks leaning both ways.

- Since the mass of the mat slab is not enough to adequately resist the hydrostatic forces at the basement level, would the net uplift carry into the upper concrete levels as upward thrusts at the columns until there is no net uplift?

I don't think it would cause a increase in net compression in the columns. The direction of the forces are opposed to each other.....so I don't think that would happen.

In fact, if you were to have the proper safety factors against this (I'd think 1.5 minimum)......I would think this would be impossible.

 

[HouseBoy]

The way that the hydrostatic uplift forces will "transfer" to the interior columns is if the mat slab/footing is not stiff enough to lift the whole building.
I think stiffness of the mat slab is a critical concern. Presumably the perimeter /basement walls will be extremely stiff so... if the mat slab is not stiff enough, it will cause uplift for interior columns.

OP does not provide dimensions so it's difficult to say how likely this problem would be.