Building up the ground level to put a basement in a high water table area
We are looking to build a house in an area with a high water table (like 6″ below the surface, basically if you dig a hole you have a pond). The land is quite flat so there’s not really a “high point” to build the house on. This is on a rural property and we would have access to all the fill dirt we would want, by digging a new pond and using that dirt.
I’m trying to figure out if it would work to construct the basement so that the basement floor would basically be at current ground level, and then adding fill dirt around the basement so in effect we would be creating a hill that the basement would be under. I realize this would require a lot of fill dirt, but are there any other downsides that I’m not thinking of? If we did this, we would be above the water table and we could drain the footings to daylight. Without elevating it like this, there would be no way to drain to daylight and I envision the sump would never turn off as there seems to be basically unlimited water below the ground.
Is there a term for what I’m looking at doing?
Thanks for the advice.
Replies
Yeah, there's a term...
It's referred to as "foolish". Why someone would want to build a basement on a flat lot with a water table 6" below grade is beyond my comprehension. How are you going to drain footings to daylight on a flat lot no matter what you bring in? Footings on fill that's tamped over ground water? If anything you should be designing as if you live in a swamp and be thinking about a practical elevated structure that keeps your conditioned space seperated from moisture, not living perpetually within in it.
Why do you want a basement?
Why do you want a basement at all? Why not just build above grade? You'd still have issues with the high water table, but it will still be cheaper and easier to build without a basement. Where are you located and what is your frost line? If all that water freezes every winter you are going to have some serious issues.
You're basically describing an "earth sheltered" structure.
http://en.wikipedia.org/wiki/Earth_sheltering
In your proposed case it would be only partially earth sheltered. Apparently this is called "earth bunding", but I'm no expert. (Hopefully the couple of
energy nuts, uh, environmentally sensitive folks who hang around here will chime in with more info.)Perhaps I'm not explaining this sufficiently, but in the area where I live this is quite common as the water table is high everywhere around, always higher than a basement floor, but nearly every house has a basement. The land isn't a swamp, it just has clay soil and a high water table (at least this time of year), and I'm looking to avoid water problems in the basement. The frost depth is over 4 feet. It apparently is quite common to do this kind of building up, but in most cases they may build up the house maybe 4 feet if they have a water table at 3 feet, but in my case the water table is a little higher than some so I'm looking at building up more. The land doesn't drop off enough to drain to daylight if one were to just build into the ground without building it up (the elevation doesn't change the 8+ feet that would be required) but if you build up a few feet, then there would be enough elevation change across the land to drain to daylight. I'm looking to find more information because it seems like builders sometimes do things that may not always make sense, and perhaps this is one of those things, so I'm trying to determine that. A basement isn't strictly necessary, but we would like one if possible due to the fact that 99+% of the houses around here have one and they are nice for all the reasons a basement is ever nice, and the fact that you need to dig down 4+ feet already to get below the frost line, even to do a slab. But we want to avoid water problems in the basement, and it seems that by elevating it above the natural grade one could avoid problems due to the high water table. Thanks.
Oh, so you need to dig down 4' in order to "do a slab", but you intend to drain house footings (to light) by elevating them above "natural" grade over fill.Then you harbor the idea that you're going to have a finished basement above the same natural grade by elevating the surrounding grade artificially to avoid "water problems". Everybody's doing it. Yeah; okay.
You definitely need to consult an engineer, to determine, among other things, how to construct this in a way that is safe from frost heaving and associated damage. Berming up the soil around the structure will not (quite) be equivalent to excavating that deep.
This is the right answer. You will need to build your bottom floor above grade to guarantee you are not going to wick water up there and you will need compaction analysis before you pour your footings.
I suspect that if you pile enough dirt around your walls you can alleviate the frost heaving by insulating the ground under the house but I would want to see that over an engineer's seal too. Engineers are cheap in the long run.
And, of course, the foundation/walls will need to be rigorously waterproofed, just as if it were below grade.
I did something similar to this on my house in Maryland. The original plans were for a basement/lower level that was ~4 feet below grade but my experience with wet basements had me changing the plan so the back door was at grade so it was "walk out" and they backfilled the rest up to the normal "split foyer" depth. It worked out fine and I never had any moisture problems.
The only real issue was that they needed to dig the footers deeper and come up with a stem wall on that side.
Thanks for the advice. Good stuff to think about.
I don't see why it wouldn't work, but - Wouldn't it be easier to build a 2 story house? Then you wouldn't have to move all the dirt.
I wonder how deep you'd have to go with your footings in order to make it work. Depends on the soils in your area.
If you really do have plenty of land and it is wet, do you need a septic too. That would be my bigger concern. The basement is not a problem if your soils will accept the pressure. You don't need to go 4' deep due to frost. The soil you mound the house with will take care of frost. If you need a mound system for septic, your pond is going to be pretty big too. That alone may help the water table. Another concern is the garage. Attached? On the basement level or the "first" level. Compacting for the floor will take a lot of care. You want it solid, but you don't want to push in the basement wall adjoining it. If the soil is clay that won't drain, it may not be the best to backfill with either. If nothing else, I hope I have shown you that you need a special kind of professional to help you through this.