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Question DetailsAsked on 2/18/2015

What's the easiest & cheapest way to excavate (under a garage that has a floating slab) then pour concrete walls there?

I'm adding a walkout room under my garage. The garage has a cinder block foundation and the ground level is flush with the slab in front and on the left side. The right side and back a cinder block foundation is about 6 feet. The ground falls from the back down a slope and I would like to lower the ground level out back and cut into the cinder block and dig out all the dirt. The dirt sits about a foot below the concrete garage floor (like I previously stated, it is a floating slab).

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OK - I am confused - a "floating slab" sits on compacted structural fill without rigid connection to the foundation walls, so the "dirt" should be right up under the slab. If your dirt is 1 foot below the slab you have some slab cantilevering out over thin air (due to erosion or earth slumping), or the support soil has settled and the slab is hanging up on the perimeter foundation wall - either way, drive back there and you might have yourself a Youtube moment just before calling your insurance agent and a towtruck.

If your slab were a true elevated one-way slab - structurally spanning the garage from side to side (which is extremely rare, especially with concrete) but NOT being supported on the back side or by the soil under it, then you could excavate the dirt out from underneath and build your room walls - just being carefulnot to hit the slab with bobcat or whatever was doing the excavation. However, an engineer would have to check the foundation design to see if it could be excavated to safely without first excavating the fill aroundthe outside, to avoid collapsing the wall. With some moment-connection designs, even that could not be done - foundation needs the fill on both sides to prevent it from buckling.

If you are trying to put a room under a conventional slap on grade, then while it is possible if necessary for technical or access reasons to incrementally dig out small sections and then build a supporting wall along the edge for a new room out in back of the garage, going in under it is out of the question of cost feasibility. A normal slab on grade is designed to sit on grade - not span unsupported across a room, so you should just take it out (at least over the room area) and build the room, then cast new appropriately designed structural slab back over the top - with construction joints at the outside edge of the underlying supporting walls, because it will crack there. That new slab would usually become the rough ceiling of the new daylight room.

Be sure to check with your local building department or designing architect/engineer also - generally there are special requirements for waterproofing and airtightness of the slab to prevent any gasoline or carbon monoxide leak from getting into the underlying living space, if they were to happen.

Either way, you are going to need plans from an architect or structural engineer for this to get a building permit, and for contractors to bid on and build to, so talk to one first about what you can reasonably do. My guess - probably most economic solution will be, assuming you have the space in the yard, is to put the room entirely in back of the garage wall, with no structural connection - as a physically disconnected structure, with the only connections being flexible connections in siding and interior finishes for watertightness and appearance, and of course utilities running through the interface with flexible design to accomodate any differential movement relative to the much older garage, which the added room if certainly likely to have.

If you mean this entire new daylight basement will be totally under the garage, then you have a major reconstruction on your hands, especially if anywherenear the width of the garage so it is getting near the goundation walls - which might mean substantial changes to the garage foundation as well, depending on whether heated or not and frost penetration in your area. Again, talk to engineer or architect before you get committe to a particular design.

Let me reiterate the key point in case I did not make it clear - the garage slab is NOT designed to serve as an unsupported slab - at a minimum you would have to put a structural slab under it to carry the load to the new room walls, but generally just jackhammering out the part over the new room and replacing with new thicker structural reinforced slab is likely to be much quicker and cheaper - taking out a slab is pretty cheap, and then excavation with an open floor with also be much cheaper than digging in under a slab.

Answered 5 years ago by LCD

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