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Dirt Basement Stabilization

Posted by mebits (My Page) on
Sat, May 10, 08 at 8:41

OK, here's the deal. We're rehabbing an 1860's brick victorian. We've got a 2/3 basement and a 1/3 dug out dirt crawlspace. By dug out, I mean that they literally dug a large space out of the dirt of the crawl space as though they were going to extend the basement. But they didn't.

So, it's dirt and I'm certain that the foundation back there goes all the way down. In fact, I'm pretty sure it doesn't.

I'd like to stabilize the dirt that's there (in most places it's 2'-3' out from the foundation, but not everywhere). The house took on a lot of water due to roof leakage and some bad drainage, so that the dirt back there has eroded in some spots too. Basically it's a mess.

Now, no one has told me that I HAVE to do anything, and I have been told to cover it in plastic and then foam the walls above grade. Fine.

But I've got the floor up already and I can get a truck right up to the window above. The entire space is maybe 15x15, with only part of it dug out and maybe 3' up the wall of dirt that needs to be supported and another 3' or so that I'd like covered with something.

So, I'm thinking, preliminarily, that I could build a form leaning toward the foundation, pour that with a few inches of concrete, leave the form in place and pour a "floor" while I'm at it. I don't need to use the space for anything other than access to pipes and maybe electrical and I don't see anything structural that the concrete would do save keep the dirt from being disturbed and then undermining the foundation. So, I'm thinking that I could stabilize the dirt and also get a vapor barrier at the same time that could be safely walked upon.

I'm guessing this is about 3.5 yards of concrete to complete. Anyone have any thoughts or experience with this?

M


Follow-Up Postings:

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RE: Dirt Basement Stabilization

I think you REALLY need professional advice on this one - what might seem totally obvious to you may be totally wrong for various reasons you can't guess at. BTW... "So, it's dirt and I'm certain that the foundation back there goes all the way down. In fact, I'm pretty sure it doesn't." What??


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RE: Dirt Basement Stabilization

Sorry! That's supposed to be, "So it's dirt and I'm NOT certain that the foundation back there goes all the way down. In fact, I'm pretty sure it doesn't.

Now, while I intend to get professional advice, my experience on this project has been that competence in professional advice is hit or miss. There's nothing like hitting folks who've been there for their accumulated knowledge.

FWIW, my professional advice thus far has been, "Just clean it out, cover it with plastic and leave it."

I'm looking for more info as I have an opportunity here, one time only. Things are about to start happening fast on this project, so if I want to exploit the opportunity of having the floor up, it has to be like in the next 10-days.

Mark

Here is a link that might be useful: Our Project, RehabOrDie.com


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RE: Dirt Basement Stabilization

You need to be very careful about removing dirt in partial basements, especially on older houses.
There may not be actual footers under the wall, and digging even close to the wall can cause shifting of the earth under the wall load.
A geotechnical engineer can look over the building and the soil and make recommendations.
A lot depends on the soil type under the foundation.


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RE: Dirt Basement Stabilization

Mark,
I don't know where your house is, but in central VA most of the brick houses of that era have English basements, but no footings. There is one across the street from 1862 that has a partial basement but I don't know the particulars - possibly built cheaper because of the Civil War. I'd be very careful about digging around/under the walls. I've known several instances where walls collapsed because of digging around the base.


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RE: Dirt Basement Stabilization

You need a professional engineer who knows (and appreciates) old houses to advise you about digging in partial basements - especially basments with recently unsually wet soil. Call you local historical society or your State Historic Preservation Office (SHPO - every state has one, I believe, and they can start you in the right direction) to get names of people who fit my description. It's quite important to get someone with old house experience and affection. Houses without full foundations (i.e.with footings) are completely dependant on undisturbed soil for stability. Even a ground hog burrow can create problems. Bad things can happen under those circumstances, so get someone with an engineering license in there to help you decide what to do. I would think this is particularly important in brick houses (vs interconnected wooden hulks like my timber-frame structure.)

And, yes, you should put down some vapor barrier while you're at it. It is easier to do with the floor open, but it can be done later, even if you have to practically push the material forward with your nose as you slither along in the crawl space. BTW, there are materials that are better for vapor blocking than contractor's poly. I can make some suggestions if you need them.

HTH,

Molly


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RE: Dirt Basement Stabilization

Molly,

I'm up for suggestions on the vapor barrier, which I have to put down regardless.

Just to clarify--the "hole is dug" so to speak. I don't want to dig anything at all. I just want to make sure that things are stable so I don't have to worry about it. So, if it's cheaper to stabilize that foundation now (and no matter what, it would be) and I SHOULD do it, then I want to. Cement is cheap, right?

At this point, all of the foundation is supported and drainage is decent (up from bad) and getting better. I'm just worried about a few areas (a couple feet here and a couple feet there) where I don't have a full 2' of loam up against it.

If anyone is curious, and perhaps has dealt with this in a similar context, this is urban Ky, there's cement on 3 sides and brick on one at grade. The soil is clay and loam, mostly the latter. Not much sand.

M

Here is a link that might be useful: RehabOrDie--our blog


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RE: Dirt Basement Stabilization

Even placing a vapor barrier can change the weight bearing characteristics of the soil by keeping the soil itself damper.

In a number of cases I have worked on the geotechnical engineering recommendation was to insulate the underside of the floor and place the vapor barrier there, keeping the dirt completely uncovered and the space well ventilated.


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RE: Dirt Basement Stabilization

I'm not at all surprised that a vapor barrier could change the weight bearing characteristics.

This takes us back to an idea that my wife had, which is at odds with what the HVAC guy recommended. She wanted to spray the bottom of the floor with closed cell foam, he wanted to spray the the walls and where the joist meets the foundation and use a vapor barrier and condition the entire basement.

During the summer time, humidity is a big issue, and we know radon to be one around here too.

Mark


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