building a basement under garage (Austin: houses, theater, construction)

[Austin97]

Looking to build a basement under my garage. Does anyone know of any contractors that do this? House is built on limestone.

[centralaustinite]

Is your budget infinite? Is the garage attached to the rest of the house? What is the foundation of the house and garage, slab?

Whatever your goals are for creating a basement under your garage -- storage, man-cave, workshop -- they would most likely be better addressed by some alternative to a basement (storage unit, on or off-site, accessory building, remodel of the main house).

Here is a discussion that address putting a basement under a garage:
http://www.garagejournal.com/forum/s...ad.php?t=65448

But I see no mention of doing it in limestone, I can't imagine blasting out a basement with an existing house attached.

Here is a discussion of working with limestone:
http://www.homesteadingtoday.com/cou...limestone.html

[Austin97]

What gave me the idea was the cost to dig out our pool. The backhoe with jackhammer cost around 500/day and it took 8 days for them to dig out our pool. Everyone talks about how expensive it is and it wasnt expensive at all. The pool builder charged us 10k for the dig (their margin over the subs).

The garage is attached. We want to expand the area above the garage for additional living space. So I was thinking we would take the roof off, then remove the garage floor, then dig the basement. Then use span crete (load bearing) + I beams for support. Then rebuild the garage. I feel like it should cost less than 100K which is a lot less than moving. We would add about 1800 sq ft

We are out of impervious cover so we cant expand the house sideways. Another alternative is to build the basement under the driveway so we only have to tear out the driveway, we still would like to build out the attic above the garage..

[Komeht]

Can you? Probably.

Should you? Probably not.

Sounds like you'd be better off scraping the garage and rebuilding. Going down through limestone with the structure in place would be a logistical nightmare.

[centralaustinite]

Span-crete and I beams is the way to go, I would worry that the excavation could damage the foundation of the house in some way -- but you have the right ballpark for a budget (I was afraid you were thinking this could be done for $3K or something). If you can get a structural engineer and the City to sign off on it, go for it, I guess.

Lots of ways it could go sideways but if you have the stomach (and the wallet) for it . . .

[Austin97]

Originally Posted by centralaustinite

Span-crete and I beams is the way to go, I would worry that the excavation could damage the foundation of the house in some way -- but you have the right ballpark for a budget (I was afraid you were thinking this could be done for $3K or something). If you can get a structural engineer and the City to sign off on it, go for it, I guess.

Lots of ways it could go sideways but if you have the stomach (and the wallet) for it . . .

our plan would be to redo the attic above the garage anyway so we dont mind pulling it out. One nice thing is that the inside of the house is blocked off from the garage so there shouldnt be a ton of dust in the house.

Our values are at 180/sq ft so if we can get 1800 sq ft for 150K I feel like we are doing good. Especially since we will be one of the rare houses that has a basement/theater/wine cellar/tornado shelter etc

[BriansZ]

This is the reason that I've most commonly heard for why we can't have basements in Limestone...
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With a pool, you have it filled with water which exerts pressure against the walls from the inside. With a basement, you don't have that, so water on the other side which is flowing through the limestone presses in on the basement wall and could collapse the wall.

Another thing to consider is your foundation. You probably have a post-tension slab, and I assume your garage is part of that. You couldn't just dig up part of the slab without cutting those cables. To do it right you'd probably have to demo the house and start from scratch.

[centralaustinite]

Ooh! Lots of great info in the link Brian provided!

The layered limestone that makes up the ground here in Austin has water running through it as all depths, including just under the surface - especially after a heavy rain. Add to that the fact that the water in inconsistent, running high after a rain and going away altogether 5 days later, and you are in a terrible situation for putting anything under the ground.

They would have to build a completely water tight enclosure to build the basement in and that would have to never settle, shift or move or it will crack and leak. It is possible to build a basement, but no contractor would guarantee it's water-tight status and the cost would still be extremely high. As such, they just stay away from them completely.

[TXEX06]

Good luck with that!

[car957]

If you proceed down this path, you need to confirm this is not a post-tensioned foundation. If it is post-tension construction and the garage foundation is part of the house foundation, the foundation will be destroyed if the reinforcing cables are cut.

A simple x-ray examination of the foundation can confirm if post tension construction was employed.