Adding a basement next to existing house.

[POC]

So, I have a 24x36 Cape Cod style house on a 4 block crawl space. I want to add an addition at one end of the house with a basement. Has anyone done anything like this? How would I go about doing it? I've read that you can't dig right next to the existing foundation due to the compression "plume"(?) being wider than the actual foundation.

So, what say you TBN? Can it be done?

 

[s219]

So, I have a 24x36 Cape Cod style house on a 4 block crawl space. I want to add an addition at one end of the house with a basement. Has anyone done anything like this? How would I go about doing it? I've read that you can't dig right next to the existing foundation due to the compression "plume"(?) being wider than the actual foundation.

So, what say you TBN? Can it be done?

Well, the current footing needs to have plenty of undisturbed soil underneath to bear down on and spread the load, so you certainly can't dig down below that level adjacent to the footing. Normally, footing loads widen as you go down into the soil, with about a 60-90 degree cone. Depends on the soil type and other factors. At best, you'd need to leave a tapered slope there, so you couldn't dig down vertically right next to the wall. An engineer would know for sure. Some good pictures here:

Bearing capacity of soil

I suppose there could be a way to replace the footing with columns down to new footings or bearings below, but that will take a bit of fuss to get done with all sorts of temporary cribbing/bracing/etc.

 

[Billrog]

A pic. would be very helpful and soil type I've done a few commercial and residential buildings where we were able to dig down adjacent the fting. The load spread out over the width & length of the fting. doesn't necessarily equate to much of a load per sq, ft. You may also have the option to run a beam 3 or 4" ft. from the exterior wall under the existing building.

 

[POC]

What I've figured I'd do, since everything I've seen has said I can't go right next to the current footing, was to leave 4'(?) of undisturbed soil before starting my basement dig. Then cantilever the new part of the first floor over this area of undisturbed soil.

Thought I might get some other ideas here.

 

[BHD]

there could be a number of ways to do this, some depends on what one want to spend,

one do dig leaving a few feet and put in a stem wall up by the current craw space wall,
ending up with some thing like this http://www.markswiss.net/Images/518Elmwood/BasementStorage3.jpg

the second way one can dig down similar to above, then go in and support the existing house and remove the craw space wall, get a skid steer, and dig out the section that is left, and some hand digging and then pour a new footer and rebuild the wall at basement depth. something like this http://www.gfmexcavating.com/wp-content/gallery/the-church-of-saint-mary/church-of-saint-mary.JPG
or
http://www.finehomebuilding.com/assets/uploads/posts/28649/Needle_beam.png

one may be able to go into the craw space and build a supporting wall for the job and shim the joists up to take pressure off the stem wall,
http://foundationrepairshop.com/wp-content/uploads/2010/04/Stabilizer_w-inset.jpg
or
http://www.finehomebuilding.com/assets/uploads/posts/28649/Non-needle_beam.png

 

[Raspy]

You can dig a "well" right under the existing footing in several places. Then pour a concrete base in each "well" and temporarily support the footing on posts. Then dig out around the posts and build the rest of a supporting/basement wall. You might even have to put your temporary posts inside the existing footing, meaning under the house and next to it, with a temporary beam under the floor joists to support the house. This if your block footing is not tied together with concrete and bar. Once that is all done the whole foundation will be stronger than ever.

This is the method they used on a large house I was working on where they added a full depth basement under the whole area of an existing large 2 story house. The first wells in that case were about 13 feet deep, 4' in diameter and dug in a conventional crawlspace under the house. All dirt was moved out by laborers crawling around down there. Amazing. But yours sounds much easier and much safer.

Another similar method I used on one of my small buildings that was sitting on Redwood beams, right on the ground, was to dig down and slip in sono tubes under them. 12" diameter tubes about 3' deep if I remember correctly. I then backfilled around the tubes and attached brackets to the beams. Then poured the tubes full of concrete which encased the brackets and came up to the bottom of the Redwood. Six of them did the trick.

 

[Sodo]

If you can rent some large steel I-beams to support the end of the house, you can jack it up and remove the end posts. When you pour the basement wall it becomes footing that the house is sitting on after the I-Beam is removed.

Try calling a house moving company, maybe they rent the beams or even come out and set the house on it. Just moving 24+ foot beams is a chore and going down the hiway with them takes some eqpt. Could be a few thousand dollars for that 4 feet. Certainly a foundation company can do this but I gather you are DIY.

24+ foot beams is a tall order. It seems reasonable theres a way to do one corner at a time with shorter beams. How many posts are on that end?

 

[EddieWalker]

When my brother had a house in Salt Lake City, he had a basement dug under his house. They dug a ramp under the exterior wall with a skid steer, braced up the concrete footing, then just kept driving that skid steer farther and farther under his house until they got all the dirt out of there they needed to. The rest of the house was still undisturbed and when they where done digging, they braced everything up and poured concrete from the ground to the bottom of that exterior foundation wall. It all seemed pretty simple and straight forward to me, but I wasn't there, I just saw pics and talked to my brother while it was going on.

I'm planning on putting a storm shelter under the slab of an addition that I'm also planning on building. My plan is to start the stairs 2 feet from my existing exterior footing and go down from there. I'm not planning on doing anything special to support that wall while digging. I think that's far enough away that it wont affect anything.

Eddie

 

[dave1949]

What is a Michigan Basement? | Prugar Consulting, Inc.

My grandparent's house had a basement like this. There was a concrete shelf about 2-3 feet wide that went from the top of the actual basement block wall to the bottom of the original crawlspace foundation. The basement wall is set inside the original house perimeter.

Perhaps you could do something like that on the wall where the addition meets the existing house, and a regular poured or laid block wall for the other three addition basement walls.

Tying the two parts of the house together would depend on which direction the new floor joists run, parallel or perpendicular to the existing house floor. Parallel sounds easier. I assume your goal is same height floors in both parts.

 

[JPRambo]

Many year ago my Dad used to move homes. Occasionally he would get a job to do nothing but raise the home off the footing (usually a small home or bungalow on pilings). Another company would then dig out the basement and pour slab and walls. Dad would then come back and set the house down. I remember him using 18" X 12" I beams and large jacks to crib the home.

 

[POC]

What is a Michigan Basement? | Prugar Consulting, Inc.

My grandparent's house had a basement like this. There was a concrete shelf about 2-3 feet wide that went from the top of the actual basement block wall to the bottom of the original crawlspace foundation. The basement wall is set inside the original house perimeter.

Perhaps you could do something like that on the wall where the addition meets the existing house, and a regular poured or laid block wall for the other three addition basement walls.

Tying the two parts of the house together would depend on which direction the new floor joists run, parallel or perpendicular to the existing house floor. Parallel sounds easier. I assume your goal is same height floors in both parts.

This is the way I was planning on doing it.

 

[dave1949]

This is the way I was planning on doing it.

That way would be the least messing with the original crawlspace foundation or disturbing the house framing. The trick is to do it well--as usual.

If you put a footer and wall for the addition out about 3' from the existing crawlspace wall, make the new wall even with the top of the existing crawlspace footer, back-fill and compact the 3' gap, then cap that 3' gap with 3" of concrete that rests on the top of the new wall over to the top of the existing footer maybe.

If needed for regular joist support, you could put short adjustable floor jack posts on top of the new wall before pouring the concrete cap. Being height adjustable it will help with getting the new and old floors at the same height.

If you have decent drainage now and put good drainage around the new basement, I don't think hydrostatic pressure on the "Michigan" wall would be an issue since it will be under the center of the combined old and new sections and not exposed to much exterior water runoff. A Tee buttress/deadman in the middle of the "Michigan" wall would add some insurance against that wall being pushed by soil pressure.

If you don't have good drainage or a wet site in general it may not be the best method. My grandparent's basement never had moisture or support issues and looked a lot nicer than the pic in the link I posted. But they were on a dry site in a city with storm/sewer drains also.

 

[Cord]

So, I have a 24x36 Cape Cod style house on a 4 block crawl space. I want to add an addition at one end of the house with a basement. Has anyone done anything like this? How would I go about doing it? I've read that you can't dig right next to the existing foundation due to the compression "plume"(?) being wider than the actual foundation.

So, what say you TBN? Can it be done?

Couple of ways to accomplish this. If you don't mind losing some basement space, the easier, cheaper method is to build a knee wall in front of the existing footing so you won't have to disturb the existing foundation. If you want to maximize the space, then you will need to underpin the existing footing. Two options: you can either pour the wall in segments or temporarily support the house and then demo the existing wall and footing and pour it back.

 

[SPIKER]

one other option may be to dig a basement that will fit the new addition AND the existing home then move the existing home onto the new build and new basement.

otherwise I would jack up existing house support system with some beams so you can make it attache directly to new build/addition. I have dug out basements under existing houses not easy or quick by hand or by small equipment.

Mark