Slab on grade close to the ocean

[Okiryu]

Hi, we have a project to design a slab on grade close to the ocean. Groundwater levels at our site are related to the ocean tides. The ocean's highest water level (HWL) is about 0.3 m higher than the top of the slab. Due to project constraints, at this point, we cannot install subdrains with a sump pit and pump system. So, we have to design the slab to resist the buoyant forces. I am considering the buoyant weight of the concrete in the calculations. Considering only the weight of the slab to resist the buoyant forces, I get a very thick slab (about 0.75 m).

The equation that I am using is :

ɣwater*(H+0.30)=(ɣconcete - ɣwater)* H

where H is the thickness of the slab.

If this equation is correct, we cannot afford for a 0.75 m slab. Are there any other options to deal with this situation?

Thanks !

 

[Okiryu]

darth, HWL was based on tide charts and field observations. I think we have been conservative on that. Also, the FEMA documents are good, thanks !

 

[BigH]

I have a silly question - perhaps I've been on vacation too long.

What is the soil type that you are sitting on? Is it a pervious sand or is it a fairly impermeable fine grained soil?

Have you installed piezometer (standpipe) to measure the fluctuations of the actual groundwater level subject to the tides? Measure the groundwater level "continuously" against the tide level and see what the relationship is.

The point I am positing is with respect to the time it would take for the tide to effect the groundwater level at your structure - is it very quick or very slow - whereas the tide reaches its maximum height but then the tide reverses - does this fluctuation actually occur at your structure? Just a thought.

 

[Ron]

Further to BigH's comments, remember that as water tries to push up on the slab it will meet resistance from the mass of the slab. When this occurs, the water will then try to escape laterally. If you have clean sands or other highly permeable soils, lateral movement of the water will occur. If you have fine grained soils, the fluctuations in tidal action will likely occur faster than the soil can respond permeably.

 

[Okiryu]

Hi BigH and Ron, thanks much for the good input. Soils are sandy clays with gravels (fine grained portion about 50%). We did not have the luxury to install piezometers but I expect that the soils are quite pervious since based on site observations, the water heights at the structure are following the ocean tides (currently, there is not a SOG there, and the site is flooding constantly).

Ron, made a good point, if water will move laterally, it may erode the foundations of the surrounding walls. Perhaps we may need some kind of filter at the outside portion of the foundations.

Thanks !

 

[BigH]

I really do not think putting in a very shallow standpipe - you could probably use a hand auger . . . and then being able to monitor the water levels during 1 or 2 days of tides to get a feel on what is happening . . . is a luxury. It should be very cheap to do - yes, you have to have someone there to take water level readings every hour or so . . . but with the effort given on this page - it would have paid for itself.

 

[MacGruber22]

Is there any reason to provide a shear connection between the slab and walls if the slab is sized to balance the total uplift? Is a waterstop sufficient in this case?

-Mac

 

[MacGruber22]

I believe I answered my own question. No connection is required if/only if the remainder of the building mass can also resist the full buoyant force. Also, assumes there is no unbalanced over-turning moment that develops from local areas of wall that do not have enough mass.

-Mac

 

[Okiryu]

Thanks for your responses. BigH, your suggestion for installing a shallow standpipe is good. I will check this with our management.

MacGruber, we are resisting the uplift with the weight of the slab so connections to the existing structure are not needed.

Thanks again for the input !

 

[saikee119]

I believe thread starter has to bite the bullet this time.

Using density of water=1 kg/m3, concrete=2.4 kg/m3, factor of safety=1, submerge depth=0.3 the the thickness x required is just

(0.3+x)1 = (x)(2.4-1) giving x=0.75m

This is the safest guarantee on the extreme condition when the tide level is 0.3m above the top of the slab the displaced volume of water, 0.3m air space + 0.75m concrete, balances the dead weight of the concrete so no risk of flotation.

The definition of HWL is very important here as it should be the HAT or the highest Astronomical Tide or the highest tide recorded say over 30 or 50 years from a nearby tidal gauge.

Flooding of the slab is prevented by sealing the joint, which can be challenging, and waterproofing the underside of the slab.

Installing shear connection between the new slab and the surrounding wall, which could be existing, will require the slab designed to withstand bending from the upthrust and strengthening along the border that has shear connection. Using dead weight is a much simpler solution.

The only way to reduce the slab thickness is to install any permanent dead load, which is unlikely to be significant, as soon as possible and include it as part of the slab weight.

There will be maintenance problems in future possibly from leakage from existing wall or the new slab. A sound and beefy slab goes along way as a solid base for future rectification work.