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# Slab on grade close to the ocean

## Slab on grade close to the ocean

(OP)
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 !

### RE: Slab on grade close to the ocean

Assuming we are talking about a solid slab (no pockets, voids, etc.), it will not float because the unit weight of concrete is greater than the unit weight of water.

### RE: Slab on grade close to the ocean

Okiryu - Don't get confused by equations, go back to basics (Archimedes Principle).
Concrete weighs a nominal 150 lb/ft3 (I use 144 lb/ft3 for flotation purposes).
Seawater, 64 lb/ft3.
Regardless of thickness (or "thinness") the safety factor of concrete floating is 144 lb/ft3 / 64 lb/ft3 = 2.25.
Solid concrete will not float, thickness does not matter.

### RE: Slab on grade close to the ocean

(OP)

Yes, the concrete will not float but the slab will have water tight joints and because of the difference of water head between the bottom of the slab and the maximum tide, I think that there will be an uplift force exerted by the groundwater. Perhaps I did not explain it correctly. In my previous equation, I was trying to equate the weight of the slab against this uplift force. I was rethinking about it and because in this case the slab does not get submerged, do I need to consider the full weight of the concrete and not the bouyant weight?

### RE: Slab on grade close to the ocean

Okiryu - Unless the ground water has positive pressure (artesian) it will not contribute to uplift. I would consider only the submerged weight of concrete (144 lb/ft3 - 64 lb/ft3 = 80 lb/ft3).

If ground water is artesian you would know it; ground water would be flowing up and out of the soil all the time, not just at high tide.

Note that the ground water level is related to tide level... that is strong evidence that ground water is NOT artesian.

### RE: Slab on grade close to the ocean

(OP)
SRE thanks again.

I am confused. I was thinking that since the groundwater is related to the ocean tides, and since the highest tide elevation is higher than the finish floor elevation, during this highest tide, the groundwater will try to push up the slab creating uplift forces. By the way, the slab is enclosed by surrounding walls. I am attaching a crude sketch for reference.

### RE: Slab on grade close to the ocean

Okiryu - I was assuming the slab was at ground elevation. Since the sketch shows the slab is underground and the space above the slab is "air" there will be uplift. Value of uplift can be calculated using Archimedes Principle - Dry weight of everything in the enclosed space, including the concrete slab, versus the weight of displaced water.

### RE: Slab on grade close to the ocean

(OP)
SRE, yes, sorry for the lack of explanation. That's why, I am using the below equation to check the thickness of the slab to resist the uplift force (Assuming that the resistant force is only the weight of the slab.

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

0.30 m is the difference between the bottom of the slab and the highest groundwater elevation. And H is the slab thickness. I am getting high values of H so I was thinking if there are other ways to resist that uplift force.

Thanks again !

### RE: Slab on grade close to the ocean

• check your unit weight and use heaviest aggregate you can get
• thicker slab is the most straight forward way
• you may be able to rely on friction / cohesion along the sides
• a footing can increase the downward load
• anchors might be used as a last resort
search the forum, this has been discussed many times before

### RE: Slab on grade close to the ocean

Based on my experience constructing underground hydraulic plugs , I would be a lot more concerned about the constructability of this design, rather than long term uplift issues. Assuming the local tides change every 12 hours or so , and given your financial concerns related to only 0.75 metres, how will the slab resist infiltration and hydraulic forces , when the concrete is only say 6-8 hours old ...... compressive strength maybe 250 psi???

### RE: Slab on grade close to the ocean

Okiryu - My calcs indicate there is a problem with the formula you are using. I get a thickness 0.133 meters... of course a flotation safety factor is needed, so thickness would increased.

Where I am (coastal South Carolina) height of tides varies daily, roughly correlated with motion of the moon. Also, storm surge (from hurricanes or other wind events) can cause water to be much deeper (say 5+ meters deeper) than a even an extreme predicted high tide (a so-called King Tide). BTW, for these reasons true basements are very rare here.

The project you are considering is really a small scale cofferdam, the floor being the concrete seal, with exactly the same problems and solutions needed.

### RE: Slab on grade close to the ocean

Buoyant force = length x width x height x 9.81 kn/m3

Height is gw elevation minus bottom of slab elevation

Concrete floor force = length x width x thickness x 24 kn/m 3

If you want to consider wall weight add wall thickness x perimeter x height of wall x 24kn/m3

I use 24kn/m3 for reinforced concrete. You can add dead weight of house on that if you want. Also building below the groundwater table means waterproofing. In my jurisdiction that means two floor slabs separated by bitumen.

### RE: Slab on grade close to the ocean

(OP)
Hi SRE...I made a mistake in my explanation - the 0.30 m difference is from the top of the slab but I can see that you are considering the weight of the slab as the "actual" weight (2310 kg/m3). I was considering the submerged weight of the slab (2310 kg/m3 - 1025 kg/m3 = 1285 kg/m3).

So assuming that the 0.30 m is from the top of the slab, my equation was (and using "simple" values of water and concrete densities):

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

10 kN/m3 * (H+0.30) = (24 kN/m3 - 10 10 kN/m3) * H

H= 0.75 m

So, you suggest that the weight of the slab should be the total weight and not the submerged weight? If so, I got a H of 0.21 m (with the above water and concrete densities).

### RE: Slab on grade close to the ocean

(OP)
I can see that GeoEnvGuy is also considering the total weight of the slab and not the submerged weight... I am a geotech guy so in my mind, if we have submerged soils, when we account for the "effective stress", we use the buoyant weight of the soil.... so I was thinking the same way for the concrete in this particular problem...

### RE: Slab on grade close to the ocean

(OP)
Perhaps I am overthinking this but I think that SRE and GeoEnvGuy may be correct. Since we are not submerging the slab, then the total weight of the slab can be used?

### RE: Slab on grade close to the ocean

Okiryu - Consider an arbitrary 1 meter square area of floor:

Weight of concrete floor = 2310 kg/m3 x H

Weight of water displaced by concrete floor plus the "air" above it = 1025kg/m3 (0.30 + H)

These two quantities have to be equal for the "cofferdam" not to float:

2310 (H) = 1025 (0.30 + H)

Solve for H. I get H = 0.24 meters (floor thickness)

I agree that weight of perimeter walls (and interior walls, if any) can be considered toward overcoming buoyancy... however this can create a new problem. All of the wall weight is applied at the perimeter (or at the location of interior walls), not uniformly across the floor. If the floor is not make thick enough to resist buoyancy with the floor's own weight, this "thin" floor will have to be designed as an up-side-down structural slab to resist the uniformly applied hydrostatic uplift underneath the floor.

www.SlideRuleEra.net

### RE: Slab on grade close to the ocean

(OP)
All, thanks much for the input. I have it more clear know... I was out for a while from this website and realized again how helpful is it ! Thanks again for your assistance !

### RE: Slab on grade close to the ocean

add footings to increase the weight of fill
also, if you dont waterproof well, the tank will fill with water.

### RE: Slab on grade close to the ocean

(OP)
cvg, Yes, we are adding a waterproofing liner under the slab and watertight joints. Thanks for the advice!

### RE: Slab on grade close to the ocean

What you are describing has a whole bunch of design references available on the internet paid for by US taxpayers. google "FEMA Dry Floodproofing" and "FEMA Floodproofing Coastal". There is enough info there to give you some guidance and figure out some problems you didn't know you had.

Based on your questions, I have the concern that you do not have the High Water Level that you need to design for. If this project were in the USA, a low design water elevation is a real smoking gun that would be a slam dunk for any forensic engineer working for an insurance company.

### RE: Slab on grade close to the ocean

(OP)
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 !

### RE: Slab on grade close to the ocean

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.

### RE: Slab on grade close to the ocean

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.

### RE: Slab on grade close to the ocean

(OP)
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 !

### RE: Slab on grade close to the ocean

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.

### RE: Slab on grade close to the ocean

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

### RE: Slab on grade close to the ocean

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

### RE: Slab on grade close to the ocean

(OP)
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 !

### RE: Slab on grade close to the ocean

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.

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