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Old 12-25-2011, 12:35 PM   #1
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Digging basement in a crawl space


Does anybody know if there is a required amount of space that should be left next to an existing footer when digging a basement in a crawl space so as to not undermine the existing footing? I live in Greenville, SC and am a framer by trade so I am familiar with construction but not dont have any knowledge it this peticular area.

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Old 12-25-2011, 12:49 PM   #2
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Digging basement in a crawl space


I would get a soil engineer in to see if it will work, and then work with an architect to help draw up the plans. The worst thing that you would want, is the home coming down on you, when the foundation & footers give out. Plus you would have to put in Oak cribbing to support the structure along with lifting it if needed.

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Old 12-25-2011, 01:57 PM   #3
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Digging basement in a crawl space


The general rule of thumb is to keep the bottom of the excavation 12"away from the footing for every 12" below the bottom of the footing.

Think of it as the load from the footings spreads out at a 45 degree line from the edge of the bottom of the footings. You do not want to dig the the area on the footing side of the 45 degree line.

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Old 12-25-2011, 02:30 PM   #4
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Digging basement in a crawl space


You scare me

I HOPE that noone comes on here and ends up with a house on top of them, or burnt because of something you told them.
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Old 12-25-2011, 02:56 PM   #5
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Digging basement in a crawl space


I just asked a ? didnt mean to start a fight chill out. Digging at a 45 degree angle wuldnt help me cuz be the time I got down 10 ft from the bottom of the floor joists Id have a skinny basement which would defit the purpose. I ll have to find an engineer to talk to, I know of one at a local lumber company where we get lumber for our jobs. Thanks for the advice!
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Old 12-25-2011, 03:07 PM   #6
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Digging basement in a crawl space


What an amusing argument. I am a civil engineering by degree, with a masters degree in geotechnical engineering. I am registered in six states currently as a PE. None of the states I am currently registered in has a specific designation for a geotechnical engineer, therefore I am registered as a civil engineer in all states. There are specialty designations for structural engineers, electrical engineers, mechanical engineers, and chemical engineers, although not all categories apply in all states. There may be states where you can register as a geotechnical engineer. All that said, the issue is whether the engineer actually understands the principals of geotechnical (soil) engineering, not what particular discipline they are registered in.

Now on to the question. The 1:1 rule of thumb is valid for most soils, not all. The "rule" is based on some simplifying assumptions, and application of the Rankine theory of soil strength. Like most rules of thumb, it has limits, which are well known to engineers trained in soil mechanics, but probably unknown to the average engineer or contractor not experienced in earth support.

In your case, since you need to dig apparently in a near vertical orientation, you are going to need bracing and support (underpinning) for your foundation. Design of bracing and underpinning is not simple, and failure to get it correct can lead to foundation collapse, with potential for severe injury or death. This is NOT a DIY job in my opinion, and requires careful evaluation by a professional engineer trained in soil mechanics. Opinions offered by individuals who lack the appropriate training and experience should be viewed skeptically. Opinions offered by individuals who have not seen the site and are unfamiliar with critical factors such as soil type, groundwater level, size and type of foundation, loading, and unusual geometric conditions, are worthless in my opinion. You need a professional who makes an on site visit to prepare plans and details.
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Old 12-25-2011, 04:02 PM   #7
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Digging basement in a crawl space


Dan - I posted it a "rule of thumb" for general guidance. All of us registered engineers use these "crutches" for a quick and dirty" estimate factor. In most cases it is realistic for the small amounts. There was a recommendation for professional assistance made and I did not fell it necessary to repeat it.

jjjohnson - A lumber yard engineer is usually not qualified to do this. If he is registered and recognizes he will have sign the drawings and reports he created, he may tell you where to get proper advice, rather than risk his registration and assume liability for his employer.

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Old 12-25-2011, 04:14 PM   #8
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Digging basement in a crawl space


In all honesty itd probably be better just to go buy a house that already had a basement instead trying to dig one under my current home. As mentioned all the bracing and underpinning to support the house while I dug and poured new footers is not an easy task and wuld require much money which I do not have. Its only 1300 sq ft home on half an acre. Id like a bigger house and more acreage some day. Thanks for the info.
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Old 12-25-2011, 07:55 PM   #9
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Digging basement in a crawl space


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Originally Posted by Daniel Holzman View Post
What an amusing argument. I am a civil engineering by degree, with a masters degree in geotechnical engineering. I am registered in six states currently as a PE. None of the states I am currently registered in has a specific designation for a geotechnical engineer, therefore I am registered as a civil engineer in all states. There are specialty designations for structural engineers, electrical engineers, mechanical engineers, and chemical engineers, although not all categories apply in all states. There may be states where you can register as a geotechnical engineer. All that said, the issue is whether the engineer actually understands the principals of geotechnical (soil) engineering, not what particular discipline they are registered in.

Now on to the question. The 1:1 rule of thumb is valid for most soils, not all. The "rule" is based on some simplifying assumptions, and application of the Rankine theory of soil strength. Like most rules of thumb, it has limits, which are well known to engineers trained in soil mechanics, but probably unknown to the average engineer or contractor not experienced in earth support.

In your case, since you need to dig apparently in a near vertical orientation, you are going to need bracing and support (underpinning) for your foundation. Design of bracing and underpinning is not simple, and failure to get it correct can lead to foundation collapse, with potential for severe injury or death. This is NOT a DIY job in my opinion, and requires careful evaluation by a professional engineer trained in soil mechanics. Opinions offered by individuals who lack the appropriate training and experience should be viewed skeptically. Opinions offered by individuals who have not seen the site and are unfamiliar with critical factors such as soil type, groundwater level, size and type of foundation, loading, and unusual geometric conditions, are worthless in my opinion. You need a professional who makes an on site visit to prepare plans and details.
So basically, he should call up a Civil Engineering Firm, and have them do the leg work for him, correct?
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Old 12-25-2011, 07:59 PM   #10
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Digging basement in a crawl space


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Originally Posted by jjjohnson81 View Post
In all honesty itd probably be better just to go buy a house that already had a basement instead trying to dig one under my current home. As mentioned all the bracing and underpinning to support the house while I dug and poured new footers is not an easy task and wuld require much money which I do not have. Its only 1300 sq ft home on half an acre. Id like a bigger house and more acreage some day. Thanks for the info.
You could prob talk to an Engineering Firm, and they can do all the work from there. From figuring out a materials, amounts, etc. that would be for the specific purpose you have going.

Make sure they are Civil Engineers, and they can walk you through the process. Including information on permits, cost estimations, contractors, etc. I don't know of any Engineering Firms (friends of mine who're graduates from UW/WSU Civil Engineering Programs who're working for Engineering firms and Construction Company's, their company's all have contractors in many disciplines on speed dial, with contact info etc. Because the contractors also use them and vice-versa)
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Old 12-25-2011, 11:40 PM   #11
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Digging basement in a crawl space


Sometimes - better off to jack up house, move house, dig basement, put in basement, put house on.
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Old 12-25-2011, 11:40 PM   #12
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Digging basement in a crawl space


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Sometimes - better off to jack up house, move house, dig basement, put in basement, put house on.


lol!!!!!!!!!!!!!!!
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Old 12-25-2011, 11:44 PM   #13
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Digging basement in a crawl space


No Joke, it's been done many times. Around here about 5 grand to move house 10 grand to dig and put in new basement. (Wood basement shell only) of course these are very approximate and assume a rectangular 1200sf house with good access and reasonable digging.) Your price may vary greatly
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Old 12-25-2011, 11:56 PM   #14
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Digging basement in a crawl space


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No Joke, it's been done many times. Around here about 5 grand to move house 10 grand to dig and put in new basement. (Wood basement shell only) of course these are very approximate and assume a rectangular 1200sf house with good access and reasonable digging.) Your price may vary greatly
Have you seen house movers on TLC? When I read what you posted, I immediately got a flashback of a house falling over its side because a support wasn't hooked up correctly.

I know you're serious, which is why it made me laugh, because no one here ever thought to mention it lol!

But that would be a solution...
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Old 12-26-2011, 12:24 AM   #15
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Digging basement in a crawl space


Why must the basement go *under* the entire house?

It's not just a matter of digging out an opening that's adequate without weakening the home's support and foundation - but adding in drainage and other such measures. . . wouldn't it be easier to set it off the side and not underneath?

But I wouldn't even attempt it - anything subterranean is where I don't tread as a DIYer.

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