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Old 10-23-2012, 12:03 PM   #1
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House foundation questions


Hey guys, first post here so go easy : )

were in the middle of having some engineering done in order to satisfy the BS stipulations by our conservation authority for building on our lake front lot (just a cool $7000)
anyways enough crying.
once this is out of the way, our designers will be able to draw up a plan to add on to our tiny 800 sq foot home.
with permission from the above mentioned we should be able to add up to half the existing, so 400 sq...we currently live in a converted cottage that me and my wife have gutted and made into a lovely home (but now with baby # 2 the walls are closing in fast)
the house is over a crawlspace where i ran ducting a furnace, air exchanger and even dug in a sump and layed big O around the parimiter (only 1 foot down due to digging in a 17" high crawlspace, ouch)
when we do our adding on, the new build will be over a basement, the existing house is story and a half, so the half will be raised a bit and the upstairs will be continued over to the addition, so it will be completely 2 story....lots of extra space.\
my question for you guys is that i know they need to under pin the corners of the house that the addition will be on, and we have talked about lifting the house and putting a basement under, but since weve done a lot of work, and dont want our slate flooring/drywall/beautiful stone shower to all get cracked and busted i think were going to not go this route....
now being a very avid diyer, even though my wife laughs at me about this idea...i'd like to eventually (even after the addition is done) dig out the crawlspace and pour cement and basically have a basement under the old part of the house.
now i dont even know if this is possible since the current house has a big old wood beam running the lenght of the house, had 1 footing with some block under the beam in the middle, i added 2 more footings under the main (just for my own peace of mind due to the heavy slate flooring) just 2 , 2 foot by 2 foot by around 4 inches thick cement, and a home made screw jack....i also poured one for under the shower and i also made and installed steel stairs so figured i'd support that too.
so if i dug out the basement i guess i would either have to put in 8' jack stands or put in a few steel beams.....however i dont even know how the old block foundations work...i know they leak haha
if i dug out all the dirt would the existing foundation be as strong as with the dirt, or is it holding the block walls in position?
and if i was to just replace the jack stands with the longer ones...how long can you wait to do that, or does it need to be supported completely the entire time of the dig?

sorry about the long winded post, but its hard to type this all out : )
any advise would be great
thanks

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Old 10-23-2012, 12:24 PM   #2
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House foundation questions


I am no expert by no means, but if you were going to dig out the basement of the existing structure, the structure would have to be completely lifted from its existing foundation and temporarily supported. Then the hole can be dug, new foundation poured or built and then the structure lowered back down onto the new foundation.

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Old 10-23-2012, 12:32 PM   #3
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the soil on both sides of the existing foundation wall keeps the wall from moving. the soil under the foundation wall (connected to the soil inside and out) support the vertical loads of the foundation. if you remove the soil on the interior/exterior side below the depth of the existing foundation wall where do you think the soil under the foundation wall will go? if the soil falls from under the foundation wall into your new hole what keeps the foundation supported? if nothing keeps the foundation supported what will keep your floors, walls and roof supported?

replacing a foundation IS NOT a typical DIY project. it can be done, but you should seek the services of a professional engineer to evaluate your existing conditions and determine a course of action. typically you can underpin (replace small sections of foundation at a time). these procedures would be determined by your engineer. doing the right thing at the wrong time can create structural issues, and of course doing the wrong thing at any time can lead to serious problems.

an online forum, even as good as this one is SHOULD NOT BE USED FOR STRUCTURAL EVALUATION AND DESIGN. you do not want to risk you and your family's safety and financial investment to structural procedures given to you from an online forum as you do not know the posters level of knowledge and experience. They have not seen all of your existing conditions therefore cannot properly evaluate your home.

Good luck!
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Old 10-23-2012, 02:22 PM   #4
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ok maybe I need to re ask my question as i must have sounded quite dumb fro m the sounds of your answers ; )
my bad.

ok jackofall thanks for the reply, but i didn't mean dig out with a backhoe as per usual, there is a company that comes into my work for soil removal with hi press water and basically a vacuum to daylight leaks, i was going to use them, just shove the hose in the access hole to the crawlspace and suck away, i beleive there 500$/hr and the size of my house im sure they would be done in much less..

gbrackins thanks for the reply also, that was what i was looking for in an answer, but im kinda looking to see if there is a way people do this from the inside out by bracing during the dig...
obviously the wall would fall or sink if you dug under it haha
would there be a normal set back, that i would leave say 5 feet of soil from the wall all the way around the perimeter, put down some temporary footings (large steel plate perhaps) then support the house floor with 4 temporary beams until the rest of the dirt was removed and the floor was poured as well as new cement under certain points of the existing foundation?
of coarse ill be talking to our arch/eng/designer but i thought i would get some ideas on here first to see if anyones done anything like this before
thanks again
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Old 10-23-2012, 02:35 PM   #5
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yes, the house can be support on beams and excavation can be performed under the home and a new foundation installed. but to determine the number and size of beams, as well as the support for those beams would require a professional engineer.

had a project one time we where adding onto the rear of a home build in 1840 that had a crawl space. Engineer specified the number and size of beams to support the existing home. we dug the foundation of the new addition (full basement) and then lowered a small backhoe into the hole with an excavator (used to dig the addition) and used the smaller backhoe to dig under the existing home. installed a cast-in-place foundation and then lowered the existing home down on the new foundation.

It can be done, but needs a professional engineer.
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Old 10-23-2012, 02:53 PM   #6
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ya that would certainly work im just trying to avoid lifting the existing home due to all the new construction on the inside would definitely crack. were also leaving the existing foundation where is on the addition since theres nothing wrong with it.
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Old 10-23-2012, 03:12 PM   #7
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For the new addition you will want to dig the hole for your full height basement and stay 4 to 6 ft away from the exisiting structure. In the 4 to 6 ft gap you will dig down to the bottom of the existing walls/footing. Footings will then be poured for the full height walls as well as for the short run of new crawl space that will be created by not digging down the full height next to the existing house. A footing will also be poured running parralell to the existing wall you are connecting to. You will then pour your full height walls, your connecting walls, and the wall that will retain the crawl space from the new basement. Then a quick dustcover for the new crawl space and a floor for the new basement. As far as creating a new basement in your existing crawl space it can be done with one sided forming equipment that works off of a-frame connectors. You will be able to dig within 3 to 4 ft of the outside perimeter if you have good soil conditions.
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Old 10-23-2012, 06:41 PM   #8
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there is always a certain amount of damage/repairs necessary about this type of work
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Old 10-23-2012, 10:21 PM   #9
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ok now were getting somewhere, thats great info. my crawlspace is mostly clay so i beleive it would be strong enough to do as you say by keeping 4 feet away, but what about if it rained hard? would'n't that wash the walls away, or do you just have to have all your ducks in a row and get everything done before it has a chance to rain?
Could you explain how the one side forming works exactly? also what would the norm be to do for my extra screw jack supports? would you just dig around them and leave a 4' circumference around the jacks/cement pads? im not so confident in how strong that would be, clay or not. Would you just replace the existing beam with a metal support beam or 2 and just do away with them all together, they were just added extra (except for the one in the very middle of the house which was just soft wood before i replaced it with the metal jack.
thanks again for the great info
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Old 10-24-2012, 10:45 AM   #10
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any suggestions concrete man? just wondering what you normally would do if you saw a set up in a small house like mine with 5 or so support points in the crawlspace?
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Old 10-24-2012, 07:45 PM   #11
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if there are existing supports that need to be kept and cannot be easily replaced with stronger beams then I would temporarily remove one support and dig down to the bottom elevation of where the new wall footing will be. Build a crib with 3'6" 6x6 a few ft tall and add a temp screw jack from there up to the frame. Complete this process for all the support columns. At this point one would then pour concrete inside the cribbing which has handily formed a 30" wide pad. Once the pad has cured remove the cribbing and place your permanent supports onto the new concrete pad. These pads will also offer excellent anchoring points for the one sided forming equipment. After the foundation walls have been poured a floor would be poured over the pads. As for the new addition rain is generally not a problem unless your talking about a severe storm in which case one would protect the banking with a tarp.

Last edited by concreteman; 10-24-2012 at 07:48 PM.
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Old 10-24-2012, 10:08 PM   #12
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ok thanks for the response, i guess if i decided to do it without putting in new stronger steel supports (at least at first) then that would be a great way to go...and realitevly in expensive braceing just using 6/6's...somethign to really think about.
what is your prefered way to dig out a basement? do you have a slope dug with a skid steer so you can drive right under, or another way?
not sure if you read earlier but theres a hi pressure water/vacuum excavation company that would make real quick work of a small space like mine, but after reading and looking into this further, it might not be the best route since it might impeed on the soil next to the foundation, and they would only be the cost effective choice if they were to just come in and suck it all out, would take little time and they charge like 500/hr, dont think i would want to have them suck down each spot where a brace is, then do the temp supports while they wait...well i guess if all the bracing 6/6's were cut and ready and the screw jacks pre made it might not be that long of a wait...then they could just suck the rest out maybe leaving an even safer say 6 feet from the foundation just to be safe....huh food for thought.

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