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# Floodproof by compaction

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## Floodproof by compaction

(OP)
I have a building that will be in the floodplain.... FF will be 8' below flood elevation!!

While we are dealing with 8' of water pressure on the walls we are also dealing with a large bouyancy pressure... or are we?

The geotechnical engineer came up with a theory that if we compact the soil underneath the building so that the flood water penetrates it so slowly that bouyancy never has a chance to develop.  He theorizes it would take weeks of standing floodwater for the bouyancy force to develop which is much longer than a flood would last.

Has anyone used this rationale to "floodproof" a building?

Any help is appreciated.

### RE: Floodproof by compaction

at some depth below your building's footprint there is likely a ground water table. Below that depth the effective stress includes "bouyancy." Being in a flood plain (i.e., next to a drainage), it's likely that the ground water table is not that deep (maybe it's within 5 ft or so).

Along comes the flood.  Water is stacked up on the ground surface and adjacent to your building.  Let's say there's 8 ft of water just sitting on the ground.  How long does it take to fully saturate 5 ft of unsaturated ground?

Well, somewhere in this problem lies D'Arcy's law.  We don't know the soil permeability, but we do know that 5 ft of soil only has about 1 or 1 1/2 ft of voids and we know that from time zero (i.e., if you could just wish the water in place) the hydraulic gradient is REALLY HIGH.  I mean if you have 8 ft of water head in instant 1 that's 8 ft of head acting on a minescule "length."

I'd never buy into this concept; well unless I'm really missing something.

I would worry about your building "popping" off it's footings though.  I could imagine you'd need at least 5 stories above flood stage (or tie-downs) to resist the bouyancy of the building.

f-d

¡papá gordo ain't no madre flaca!

### RE: Floodproof by compaction

Extra deep frost walls as cutoff walls may be easier to build and more predictable in behavior.

Will the interior of the building flood? Wouldn't that negate the bouyancy and wall pressure?

Could you build it like beach houses in storm surge areas, with  ground floors designed to withstand flooding and breakaway walls that allow the floodwaters to pass through without overloading the support columns?

Maybe the tyranny of Murphy is the penalty for hubris.  - http://xkcd.com/319/

### RE: Floodproof by compaction

how deep is the floodwater and is the finished floor below the ground? If you are talking about a basement, I would recommend wet floodproofing over dry.

FEMA discourages dry floodproofing for flood depth of 3 feet or greater. 8 feet of flooding is not likely to be approved, regardless of what your geotech thinks. Either way, your local floodplain manager would need to approve this and you should ask them if they have ever approved such a thing.

#### Quote (FEMA P-312, Homeowner's Guide to Retrofitting):

When water rises against a wall, it pushes laterally against the wall. As the depth of water increases, so does this force, as indicated by the arrows in Figure 7-5. Tests performed by the U.S. Army Corps of Engineers (USACE) have shown that, in general, the maximum allowable flood depth for masonry and masonry veneer walls is approximately 3 feet. In these tests, walls exposed to greater depths of water either collapsed or suffered serious structural damage.

Although definitive testing has not been carried out for conventional frame walls without masonry veneer, it is generally accepted that they are difficult to seal, weaker than masonry and masonry walls, and thus likely to fail at lower water depths.

Hydrostatic pressure is exerted not only by floodwater, but also by soils saturated by floodwaters. As a result, basement walls can be subjected to pressures much greater than that from 3 feet of water alone (Figure 7-6). These pressures can easily cause basement walls to buckle inward or collapse (Figure 2-8). For this reason, dry floodproofing in basements is strongly discouraged.

### RE: Floodproof by compaction

(OP)
This will be a commercial building in the floodplain, slab on grade, no basement.  Flood elevation is 8 feet above finish floor.  Exterior walls will be poured in place concrete and waterproofed.  There will be a stop log system used to seal windows and doors upon warning of a coming flood.

We have the lateral hydrostatic pressure on the walls covered... it's the bouyancy force we are wrestling with.  The first design was to anchor down the interior slab with helical anchors to counteract the bouyancy....  this got real expensive.

Now the Geotechnical Engineer comes up with the theory of densifying the soil around and below the building such that hydrostatic pressure needed to cause bouyancy under the building would take so long that the flood would be gone before it could develop.

Obviously we aren't quite comfortable with this approach or I wouldn't be posting here.  I'm wondering if anyone else has used this approach....

Thanks for your time.

### RE: Floodproof by compaction

get the geotechnical engineering recommendations clearly written in a sealed document.  Pay for a third party review - somebody also in the geotechnical engineering business within the region.  Expect a \$200.00/hr senior professional and several hours of time.  See if there is agreement between the two geotechnical engineers, both practicing in the area.

I'd concur that you need to account for buoyancy for slab support.

f-d

¡papá gordo ain't no madre flaca!

### RE: Floodproof by compaction

if  this is a slab on grade, then the bouyancy is caused by displacement of the water above the ground. compaction of the soil  below grade will have almost zero effect. you could make your foundation slab thicker and anchor the building down to it. It would be very expensive, but might work. a better option would be to build a flood wall or levee around your structure.

### RE: Floodproof by compaction

It might be cheaper to build a 10 ft high parking structure on grade under the building. And start the actual bldg. well above the flood level. Because when the flood comes, no one will be at work.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

### RE: Floodproof by compaction

Your Geotech consultant is dreaming. That is not practicable.

### RE: Floodproof by compaction

This seems like a terrible idea.  It only takes a small gap between the slab and the subgrade for hydrostatic pressure to develop almost instantly.  It would start at the edges of the building and work its way to the middle, and the building comes off the foundation.  What did you look at for tiedowns?  Helical anchors may be a cheaper option if you were only looking at drilled tiedowns.

### RE: Floodproof by compaction

Whatever way you go should include a permanant dewatering system to maintain nuetral pressure. We did this on a house in the mudflats of the Sac. river. Kikcked on a few hours twice a day. Foundation stayed high and dry for the most part on a bed of three inch rock. The water is pumped out as fast as it seeps in. worked real well but I wasn't paying the pg&e bill. But it couldn't have been done more cheaply any other way. It takes a hell of a lot to hold down a ship but not so much to keep her dry

### RE: Floodproof by compaction

Ya know, thinking back on it we were right at 8 feet of water at high tide. water poured in pumps kicked on and problem solved. We had a basement garage, rather large with ten foot ceilings and I think it was about eight foot of water at high tide

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