Garage Foundation

AlexHouse · 03-23-2012, 11:40 PM

My understanding of foundations is that it is essential that the footings be built on undisturbed soil. If this understanding is correct, then I'm puzzled about the process involved in building garage foundations which abut a basement, kind of like in this picture.

The soil under the garage which is near the basement foundation has been disturbed in order to allow the basement to be built and then the hole was backfilled.

Can someone take a moment and explain how garage foundations like this are built so that they avoid settling that occurs from having footings poured onto disturbed soil.

Thanks.

gregzoll · 03-23-2012, 11:45 PM

Open Phone Book. Look up phone number for your city hall. Find number for permit department. Make call to speak to engineer that handles building permits, and they will be able to tell you their stand on building garages next to new builds, for your local.

jomama45 · 03-24-2012, 04:55 AM

Quote:

Originally Posted by gregzoll

Open Phone Book. Look up phone number for your city hall. Find number for permit department. Make call to speak to engineer that handles building permits, and they will be able to tell you their stand on building garages next to new builds, for your local.

Maybe we should just shut the forum down on make it one big phone directory?

Alex, the answer to you're question, which is a good question IMO, is that the footing stops short of the basement wall and is only poured to the end of the excavation. Typically, there is a span somewhere between 2-4' left between the garage footing & the basement wall.

On a poured wall foundation, the wall in this area ties into the basement foundation (they're typically poured at the same time monolythically) and is re-enforced with rebar. Think of a 4' garage wall as a 4' high, 8" wide beam.

With a block foundation, a precast concrete lintel is the most common approach.

AlexHouse · 03-24-2012, 05:35 AM

Quote:

Originally Posted by jomama45

Maybe we should just shut the forum down on make it one big phone directory?

Alex, the answer to you're question, which is a good question IMO, is that the footing stops short of the basement wall and is only poured to the end of the excavation. Typically, there is a span somewhere between 2-4' left between the garage footing & the basement wall.

On a poured wall foundation, the wall in this area ties into the basement foundation (they're typically poured at the same time monolythically) and is re-enforced with rebar. Think of a 4' garage wall as a 4' high, 8" wide beam.

With a block foundation, a precast concrete lintel is the most common approach.

Thanks, I appreciate you taking the time to answer that question. I now understand but I also have a few follow-on questions.

I take it that settling does occur and that if it does then the "beam" has reduced support underneath it and thus the loads bearing down on the "beam" get transferred to the small points on either side of the "beam" - so do contractors see increased levels of damage from this concentration of loads?

My understanding is that monolithic pours are done principally to reduce water infiltration. Seeing how the garage is outside the residential building envelope, other than the convenience of doing the pour all at once, is there a reason to prefer a monolithic pour? Delaying the garage pour would allow for the backfill to settle and that could be helped along by compacting. Is there a real benefit to this or am I making a mountain out of a molehill?

Daniel Holzman · 03-24-2012, 07:00 AM

Foundations can be placed on undisturbed mineral soil (sand and gravel type soil). However, it is common to place foundations on structural fill, which is select structural fill material (typically mostly sand and gravel) that is carefully placed and compacted as it is placed, typically in six inch lifts. Structural fill is just as good, probably better, than undisturbed soil, and does not settle.

The undisturbed "rule" is typically a way to construct without the need for an engineer to write a specification for structural fill, and a way to allow the building inspector to pass a site without actually witnessing the placement of fill material. However, there are engineers whose entire careers are devoted to proper placement of fill for foundation and other support applications, so it is absolutely possible to place a garage, house or most anything else on properly placed structural fill. The fill is typically placed above naturally occurring soil to raise grade to an appropriate level.

jomama45 · 03-24-2012, 11:36 AM

Quote:

Originally Posted by AlexHouse

Thanks, I appreciate you taking the time to answer that question. I now understand but I also have a few follow-on questions.

You're welcome.....

I take it that settling does occur and that if it does then the "beam" has reduced support underneath it and thus the loads bearing down on the "beam" get transferred to the small points on either side of the "beam" - so do contractors see increased levels of damage from this concentration of loads?

Seeing as it's intended to act as a re-enforced "beam", it doesn't require any support under it. I realize that it seems like the ends are carrying a large amount of weight, but the reality is that most commonly in a lightweight wood construction (like that of a garage wall) the concrete foundation wall itself weighs more than anything it's supporting. If the forms can be removed the day following the placement of the concrete and span under it's own weight while the concrete is only at about 15-30% cured strength, imagine the load it can carry after it cures to it's designed strength.

My understanding is that monolithic pours are done principally to reduce water infiltration. Seeing how the garage is outside the residential building envelope, other than the convenience of doing the pour all at once, is there a reason to prefer a monolithic pour?

This is undoubtedly the main reason to pour it monolithically. I'll give you an idea how a new poured foundation goes in here:

- Footing crew shows up on day 1 and typically has the footings set-up, inspected and poured by noon

- Wall crew crew comes in on day 2 and set's forms on footings, generally pouring walls by 2 pm. The rest of the day usually consists of going to yesterday's job & stripping and loading the forms from the previous basement.

- On either day 3 or 4, another small crew comes in to damproof and insulate the exterior walls, set temporary wall braces before backfill, supply anchor bolt nut's & washers, etc...

As you can probably see, pouring the foundation in separate pours would be extremely innefficient for a production-based crew.

Delaying the garage pour would allow for the backfill to settle and that could be helped along by compacting. Is there a real benefit to this or am I making a mountain out of a molehill?

It would undoubtedly require compacting, as Dan states above. I would think it would also require soil analysis as he mentioned, as it is indeed disturbed soil, compacted or not. The other thing to keep in mind is that typically, there is draintile running around the exterior of the house foundation with clear stone covering it. A barrier would need to be in place with this method to ensure the placed & compacted fill didn't infiltrate into the open stone, and eventually settle. For all of these reasons, it's just far simpler in almost every case to pour the foundations together......