Source
r/AskEngineers•
Asilus

For the civil engineers: Modern applications of platform mounds?

Hello Redditors,

I am a non-engineer with a question for the civil engineers here. I realize my question may have a lot of variable due to state laws, local ordinances, etc., and that consulting with a local earth works firm may be my best bet, BUT I wanted to take my question to the Redditors first.

I am going to build an off-the-grid house in Florida. I would like to utilize a platform mound like the indigenous people of this area used centuries ago for their homes; this way I will be protected from flooding. But instead of a primitive house I will have a home with modern amenities. The total amount of land on top of the mound that I need would be equivalent to one normal, suburban sized lot. Or maybe one like this, or this one

Will a platform mound support a foundation, and house, etc? Is this thing sound enough to safely and permanently support a foundation and house? Or am I barking up the wrong tree?

Thanks!

Posted May 14, 2014, 10:50 PM UTC

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tewew•

It will probably work.

It does depend on what you build it out of. If you want to use local materials, the steepness of your side slopes will depend on the strength of the material. Buildings much larger than a residential house are built on fills all the time. Also, depending on your proximity to the river and the expected flood velocities, it may be smart to look at some kind of armament such as rip-rap on the slopes to prevent them from washing away in the flood.

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AsilusOP•

I was just thinking to use local dirt, rocks, etc. If reinforcement via rebar. etc., is necessary, than that too. I presume this is what you mean when you say rip-rap?

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TOLstrykCivil - Construction/Railroad•

Yes, you are correct in your assumption of rip-rap.

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tewew•

Not really, although you could pour some concrete armoring if you were really worried.

Rip-rap is just large rocks placed on the slope to help prevent water from eroding the soil. Like this: Riprap wiki

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AsilusOP•

Is concrete armoring just what it sounds like: concrete poured over and to ensconce the mound?

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SirLeepsALotCivil/Transportation Infrastructure•

Ya people reinforce slopes nowadays with shot crete, which is just a sprayed concrete. But it really depends on how often and intense you plan on getting flooded. I'm sure there's records for the area telling how high the area has flooded in the past. Simple native grasses could be enough to prevent erosion.

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UrungusAmongUs•

Yes, a compacted fill will have no trouble supporting a residential home. Talk to a geotechnical engineer before having a contractor build it. (Or ask on here about compaction specs.) I look forward to visiting your island when western Antarctica melts.

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AsilusOP•

Yes, please, tell me more about a compacted fill/compaction specs? And yea, it'll be party time when the deluge happens!

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UrungusAmongUs•

Oftentimes good contractors are familiar with their state DOT's compaction practices. In Florida, my guess is you have mostly sand so it shouldn't be too big of an issue. You want density and uniformity. The main thing is to build the fill in thin lifts (no more than a foot), compacting each lift. And you'll want some tests to make sure the roller is adequate and the moisture of the soil is right. A quick google led me to this primer on compaction, which actually doesn't look too bad.

Whatever you do, don't just give a contractor free reign to build a pile. You'll create a high likelihood of uneven settlement of your home.

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AsilusOP•

Thanks for the link! Right, I get it: get the geo-engineers to build the fill/mound (btw, is fill just a synonym for mound?) and have the contractor build the actual house.

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UrungusAmongUs•

Ha! No, I'm a geotechnical engineer and I have no interest in building your mound. I just tell contractors how to do it. Fill is just what it sounds like: soil to fill something in (or build something up, in this case).

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LazySprocket•

The problem if you have mostly sand, is that it doesn’t make a very good mound does it? Unless you have a lot of land to make very gently sloping sides.

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DrBridgeCivil - Structural•

Depends on where in FL. The inland areas and particularly northern Florida have plenty of clay and silt. Coastal, yeah it's pretty much all sand (or silty sand) on a layer of limestone.

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UrungusAmongUs•

If you compact it and vegetate the slopes it should be stable up to about 30 degrees. That's steeper than I'd think you want for practical purposes anyway (mowing, walking, etc).

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LazySprocket•

I’d think that cost wise you’d be much better off just building on concrete stilts. Fill and compaction can get expensive, depending on size required. And even if the geotech guys tell the contractor how to do it, there are times it doesn’t go well and settling occurs.

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AsilusOP•

Expensive? Any idea what a geotech firm will charge for consulting, and do the earthworks companies charge per hour or per square foot of earth moved?

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SirLeepsALotCivil/Transportation Infrastructure•

Cubic yards is the unit for fill where I'm from. You could structure a contract a couple different ways, lump sum for them to get it done or unit cost where is based off quantities.

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