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Old 11-10-2011, 10:38 PM   #1
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Partial basement excavation?

Hey all,

I am interested in possibly excavating to create some additional head room in my existing basement. I have read several related posts on the general topic, but I am looking for some additional thoughts on my specific situation.

I live in an approx. 90 y.o. bungalow. The basement is approximately 800 sq ft, and I am looking to excavate an approx 100-130 sq ft area in one corner. I want to excavate about 12-18".

What I am wondering is how much I need to worry about disturbing existing foundation. I figured if I left about 6-8" of existing floor around the walls, and given that the majority of the original floor and foundation/walls would still be in place, that the risk of structural issues would be fairly low.

Am I crazy and/or naive?



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Old 11-11-2011, 06:03 AM   #2
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Way to little information----How deep are your footings?
Where are you---

What is your soil?

What is the foundation made of?

Seriously,You need an engineer on site to help you---


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Old 11-11-2011, 07:03 AM   #3
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You must not disturb the soil below a imagined 1:3 slope down from the bottom surfaces of the foundation footings. For example one foot away from the sides of the footings you can be at most four inches below the footing bottom surfaces. Before you can be a foot lower you have to be 3 feet in from the footings or approximately 3' 4" in from the walls.

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Last edited by AllanJ; 11-11-2011 at 07:06 AM.
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Old 11-11-2011, 10:30 AM   #4
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A little more info...

Thanks for the input. I am gathering that this project is likely beyond my capabilities, at least without professional help.

My foundation is cinder block, not sure what the footings look like.

I really only need to go down less than a foot. Right now, the joists/rafters of the first floor are exactly 7 ft above the concrete basement floor. 7 ft meets code in Minnesota, but by the time you put in drywall on the ceiling and a subfloor over the concrete, it will be something less than 7 ft. Not usre how strictly code is interpreted, but I would feel much better if I had at least few inches to spare.

Is there a good way to determined what the footings look like below foundation, other than digging a section out?

Also, the soils in my area are a sandy loam, generally pretty good drainage.

Thanks for all the input!
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Old 11-11-2011, 10:53 AM   #5
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There is no way to determine what your footings look like (or even if you have footings) without digging out a section. Ground penetrating radar has been used for commercial scale projects, but the cost is prohibitive. As previously noted, it can be very dangerous to excavate near a foundation, especially an unreinforced block wall (based on your description, this is probably what you have). The 3:1 "rule" is only a rule of thumb, it is useful for preliminary design, but CANNOT BE RELIED UPON for final design. The actual zone of "no dig" depends on the foundation geometry, type of soil, water table, and loading of the foundation. Careful people hire a structural engineer to tell them how much it is safe to dig. DIY'ers sometimes wing it, occasionally with catastrophic results.
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Old 11-11-2011, 08:40 PM   #6
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I may be the biggest idiot ever born, but it seams like it might be easier to have the house lifted and slip in a course or 2 since we are talking a small footprint house.

ps. I don't know what was involved, but my house was at least partially on crawl space with real shallow footings, and then they dug under them to make a basement with vertical walls and poured floor. They might have accomplished most of it by digging under the non-load-bearing sections (not the walls with the floor joist on top of them)

Last edited by forresth; 11-11-2011 at 08:44 PM.
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Old 11-11-2011, 10:54 PM   #7
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Thanks to Daniel H. and Forresth.

Not sure what I am going to do. My wife and I are expecting twins. "Due" date is April 3, but we have already been told that they will recommend inducing/c-section at 38 weeks (mid-March). We currently have a 2 bedroom home, and a six year old son. So, we need to expand. The house is a 1 1/2 story, but the second story is definitely only a half story--it is about 6 ft and 3-4 inches to the rafters at the highest point. So we know that finishing that could provide livable space for a 6 year old, but there is no way we can make it a code-approved bedroom/living space w/o raising the entire roof, at a likely cost of AT LEAST 100k. We are about to buy a new car (a minivan, to accommodate the expanded family), and just spent 25k on IVF (in-vitro fertilization). Also, we lost pretty much whatever increase in value plus mortgage paydown we had; we aren't underwater, but we don't have much equity to borrow against. This is on top of student loans, mortgage, etc.

So, to make a long story short, we need to expand, but aren't in a position to direct a lot of funds toward the project. So I was hoping to find a low-cost way to make an home improvement that solve our immediate problem of need for additional living space, but in a way that also provides long-term value by creating space that would meet building code requirements for bedrooms/livable space.

Given our short timeline, my conclusion is that my goal is not realistic. Instead, we will attempt to finish space that is usable and safe, and hope that at least some of it (the basement space) meets code and adds longterm value.

Thanks again to everyone for your advice.


Last edited by jawol2000; 11-11-2011 at 11:00 PM.
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basement , excavation , foundation

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