Just tossing an idea around.
My house is on a slab, 4″ thick with 6″ mesh.
I saw the online article at JLC about dropping an existing basement another 15″. I was thinking about dropping my existing basement from 0″ to 9′.
Obviously the kicker is the slab. the JLC article showed how to dig around the foundation and pour sections to the new height then fill the gap with non-shrink grout.
But what about the slab? I’d like to not have to demolish the house to dig a basement.
Any ideas?
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Replies
Cut the slab around the perimeter (if necessary; you might be able to just lift the house off it if you can get to the J-bolts), and jack up the house high enough to be able to dig under it. Set the house on I-beams sitting on caissons outside the square of the house.
Dig, form, and pour. Drop the house back on the new foundation. There ya go....
I don't see how you can save the slab and it's probably not worth it in the long run.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
I remember seeing on some show, probably This Old House, where in Florida? they were jacking up whole houses with the slab attached. They'd slide beams through the house, and drill holes through the slab and attach it all somehow, then jack the whole thing up and pour a foundation underneath to get them above hurricane surges. Neat process, but probably pretty expensive.
The point is to not have to move out! This is a small home - 1400sqft. It also has a 2nd story hanging off an edge - not over the main house.My thought was to dig under the garage and the 2nd story overhang, then use that as a basis for cutting in under the house.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
I am assuming this 2nd storey hanging off one edge cannot easily be detatched and reattached. If that's the case, I suggest you come up with a different idea.
However: If that assumption is wrong, the answer is, do what I said in Post #2; you'll only have to move to a motel for about 10 days or so.
It only takes an afternoon to jack the place up; a day max to dig; a day to form and pour the footings; two days to dry; another morning to form the walls; and the pour can be done in the pm.
Give the walls time to dry then drop the house on 'em and move back in. Then bolt the 2nd storey back in place.
Oh, yeah: arrange with the powers that be for no earthquakes to occur while it's jacked up there....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
What about going up with a second floor?
"Preach the Gospel at all times; if necessary, use words." - St. Francis of Assisi
No, I didn't vote for him; but he IS my president. I pray for the his safety, and the safety of his family every day. And I pray that he makes wise decisions.
I actually want the natural coolness of the basement, a below ground garage is attractive too.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Interesting set of parameters!
Not move out, slab is only 4 in thick, etc.
#1, you obviously dont want to even try to get a permit, that will only slow you down.
#2, how to support the slab? Even with mesh, which is probably 6" grid, you can only count on about 300 psi in tension for the slab, assuming it is not already cracked.
live plus dead load is 40+50 psf, or about 8 #/in.
section modulus of the slab, say it is only 3-1/2" in most places, is 12*3.5^2*(1/6) = 24.5. Thus it can take a moment M of 300*24.5 = 7350 in-#
Solving M= w*L^2/8 ; solving leaves "questionable' span 'L' capability of a whopping 7 feet with no safety margin, say 5 ft with some little margin.
#3, how to support the slab every 4 or 5 ft - suggestion - buy some nice jr. I beams, too lazy to do the calcs on those for you today. Anyway, dig a 5 ft wide trench under the house (NOT under any bearing wall) and install an I beam at each side, dig out from there, installing beams as you go.
Off top of head guess for I-beams at 30 ft length and a post in the middle would be 7I15 with the post, 12I40 or for no post
Good luck, you may need it.
EDIT: A LOT depends on if the slab was poured over a relatively smooth surface, such a a 6 mil layer of poly over sand. If it were poured over raw rough ground so there are low stres riser points, you may have ZERO capability of the slab to support any type of span.
Edited 10/8/2009 3:38 pm ET by junkhound
That was one of the things I was wondering - what was the span of that slab between supports? Thanks for roughing that in for me. Sounds like a beam every 4 feet is about right to start with.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
surely you don't mean do this while living in it?
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I've known 2 guys who dug basements under their houses while living in them. Both houses were framed and had were built on piers in red clay. I thought they were both nuts but neither had any real problems.
My Dad had us excavating a portion of the crawl into a deep basement bacck in the sixties - Cuban missile cisis and bomb shelter thing back then. it's more labor than dollars to do if it is a DIY thing
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Yeah. My uncle was one of the two. Had a small house, probably about 1200 total in Concord, NC. Had one of those small homeowner type wheelbarrows that hold about 10 shovelfuls, a pick mattock and a shovel. He's come home from work every day and do 10 loads before dinner. Slow and steady, took him a little over year as I recall.I also worked here in Ft Myers for a guy 35 years ago whose parents lived out at the beach on the end of a canal. They were both in their mid-seventies but were jacking the old house up 8 feet. They had collected the biggest load of pieces of one bys, 2 bys, and four bys you've ever seen from trash piles at construction sites. You never saw such cribbing! They had two $20.00 3 ton hydraulic jacks and would jack up each point one inch a day. Still lived in the house too.
Buy a For Sale sign and then buy what you want. You'll spend way more than if you just bought it.
true, but Paul is one of those guys with more **** than ****!I do recall reading a tory somebody linked here once, where a HO had cut out a trap door in a closet down thruy his slab and had created a fairly large room down there.Don't reccall why
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
"I do recall reading a story somebody linked here once, where a HO had cut out a trap door in a closet down thru his slab and had created a fairly large room down there. Don't recall why."
Did he really need a reason? He had an impulse to dig a room under his closet and he did. Surely this illustrates the American Dream more profoundly than anything Donald Trump has ever thought up.
Of course, could have been just one of the many who went overboard during the 60s building bomb shelters.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
I don't kn ow what it has to do with the 'American Dream' I just figured somebody would ask me why, so I was typing a pre-emptive answer. Your query shows me I can pre-empt every possible rabbit chase.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
The idea that a man would wake up one morning and decide to dig a hole big enough for a room under his closet caught my imagination. Perhaps it is a stretch, but surely his motivation must have more to do with self-expression than any practical purpose.
Like a number of schemes put forward here on the board, say for example digging out a full basement under a slab, I suspect the rational given is window dressing for some else. Being in general practical types, we simply give advice on how best to proceed, but trying to understand the motivation behind some of the things proposed here intrigues me.
I recently saw it done where they did jack the house up for the operation. They used those Styrofoam units that you pump cement in for the walls. Apparently it worked real well. I'm not sure you could do it without jacking the house up, that is unless the square footage of the basement was a lot less than the footprint of the house.
I see no remotely cost effective way to do this kind of work while keeping the slab intact. Unless your slab is 8" thick and reinforced with a grid of rebar, you cannot practically jack it up to do anything under it.
If it would cost $100k to add a second floor, it will cost double or triple that to do it with the slab intact.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Dang, all the naysayers.
If I were in FL, would come over and take a look.
I'd bet, not counting DIY labor, the whole structural job could be done for less than $20K. Without moving the slab per previous post. Say $10K for recycled steel beams, rod, and lally columns; plus 50 yards of concrete brings it to about $16K, leaves $4K to buy (and keep) a used bobcat !!!!.
The last basement I dug out was in '94. Did need to find a way to remove 4 ea 4x4 3 ft high posts supporting a few transverse 4x12s. Welded together some scrap sections (with doublers for margin since I'm not a certified structural welder <G>) of 4" by 12" 1/4" wall box beams, good to go. Whole structural job material cost was 99% just the concrete. May have had to buy a few bolt anchors for the beam ends.
If the bottom of the existing slab is NOT smooth and free of cracks, another $5 K or so of steel may be needed for support between the main 5 ft on center support beams.
Dig yourself a hole under one side of the house to see what the slab was poured onto. That will tell you how much support you need to install.
check with a structural engineer, they'll probably tell you your crazy. a 4" unsupported slab doesn't have a lot of strength.
I recall seeing an article about a slab house that was moved or raised without destroying the slab. Basically, they installed strongbacks through the house and drove anchors into the slab every couple of feet to carry the weight of the slab, then lifted the whole thing.
You could presumably do this, then build (probably steel) framework under the slab to support it permanently. That could then be left on cribbing until the basement was excavated.
But keep in mind that the slab bottom would not be smooth, and the framework would consume some headroom (though maybe not much more than a conventional wood-framed floor). Probably some areas of the slab would break and require repair. Might be that the best approach would be to build the framework with a "floor" and then inject grout between it and the slab.
You'd of course have to deal with plumbing that was below the slab, and would have to re-jigger the HVAC for the basement.
No way you could do any of this without emptying the house. The strongbacks would require cutting holes in the walls.
The strong backs above the slab are an interesting idea I hand't considered - thanks for that.What I am trying to imagine is some way of supporting the slab underneath every 4 feet, then making that into a finished permanent part of the final product. The underside of the slab will be rough, so it would have a lot of tooth to grab onto whatever gets poured up underneath.But can you pour concrete UP to a surface? I know it needs to be vibrated to get out air pockets, and these air pockets would then be trapped against the upper surface. Hmmm. Is it possible to injection pour from the side?
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
It would be more grout than concrete -- not structural, at least not the top couple of inches.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Hmm - that would be interesting...Excavate a tunnel 5 feet wide, with steel beams near the edges at 4'O.C.Between the beams and under the slab, frame up a mold to pour a concrete reinforced horizontal panel, with linking rebar protruding out the sides.Pour it and let it cure, then grout the top and lift it into its final position. From here you can continue expanding each side of that initial support.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Put it on some beams jack it up and dig it out
Don't you have extra seismic concerns in CA? I don't see how your AHJ will approve attempting to support a 4" slab without some form of internal tensioning, no matter what you do for support from below.
You might be better off cutting loose the framing from the slab, as well as the waste and water supply, lifting the entire structure enough to platform frame beneath, demo the slab and footings and while the place is up on beams under the platform, get under there with your excavation, footings and new basement walls. Then drop the platform to the foundation and strap it down. Reconnect the waste and water.
As for living in it... treat yourself and your wife to running water and indoor plumbing offsite.
I've seen this done, but it's not a job for just any contractor.
What is usually done is that a Bobcat is used to first excavate a 'ramp' into the yard, down to the sidw of the house. Then a tunnei is made by the Bobcat across the narrow dimension of the house. At this point, jacks and beams are placed to support the house on either side of this tunnel.
Once this is done, it becomes a ballet of tunnel- support - tunnel, until the area is excavated.
When all the dirt is removed, then a proper support structure is made under the house, and the temporary posts removed. Footings are poured, walls are built, and the finished floor is made. The last step is closing the hole the backhoe has been using, and filling in the ramp.
I think you can see how critical the 'ballet' is. Not enough support, and you break the house.
Yep, that'd work; just like digging a mine, really, except there's a house over you.
But I wouldn't want to see the premiums for the insurance rider....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Yes, this is kind of similar to what I thought might be involved. As Dino said - Mining under your house!
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Sounds like a job for Larry, Darryl, and Darryl.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
I have a slab that is 12" thick and no house on it. Was wanting to add two rooms under before building on top. Is this possible in the same manner considering I have no weight of house on it or safer at least??
Since your house is on a slab. you don't have any floor joists, subfloor, etc. Your big problem will be supporting the house while the basement is excavated, a new foundation and basement walls are poured (or built from CMU's), floor joists are put in, and subfloor laid.
I doubt if you will be able to stay in the house while all this is going on because the house will probably have to be raised off of the slab so it can be removed before the digging starts.
It's do-able, but you'll probably need a well stuffed checkbook. - lol
Dig 6' x 6' sections and pour interior bearing walls. When you are done, you'll have a 30 room mansion.
Basement Added Under a Slab
Just wondered if you ever did this. I regret not building a partial basement under my home. I considered sawing out the approx. 4 inch thick floor in the den (largest room with no load bearing areas), then excavating and building the basement, then pouring the floor back. Ta Da! Instant (well, not quite, but you understand...) basement room, great for utilities, storage, HVAC, storm shelter, et al. it would be better to have a way out from below, which would be doable.
Still a back of my mind project. If it happens, it may just happen to my garage and patio so I can create underground parking. But there I can pull out the slabs first.
Junkhound
We are in a similiar situation and are wanting to get in contact with someone who is knowledgabke in this field. We are in Cali, concrete foundation. Want to dig. We need a refferal to a structural engineer or someone who has experience in this area. Thanks Kindly
Finding an engineer is aguably the hardest part of the project, as I spend about a month looking for mine. Good luck!
Re:General Discussion
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more stink'n poorly done spam links removed.
Missed it - the Khan guy is Chinese, the other one is legit I bet.
Lowering an existing basement without demolishing the house or removing the existing slab can be a challenging project. It's important to note that this is a complex construction endeavor that may require the expertise of a structural engineer and professional contractors. Here are a few ideas to consider, but please consult with professionals before proceeding:
Underpinning: Underpinning involves excavating beneath the existing foundation and reinforcing it to support the lowered basement. This technique typically requires installing temporary supports and sequentially pouring new reinforced concrete sections beneath the existing foundation to achieve the desired depth. Underpinning should be carried out by experienced professionals who can assess the structural integrity of the existing foundation.
Bench Footings: Another approach is to construct bench footings. This involves excavating sections along the perimeter of the existing foundation and pouring new footings at the desired depth. These footings act as supports for the lowered basement walls. The space between the old and new footings can be filled with non-shrink grout or other suitable materials.
Mini-Piles: Mini-piles, also known as micropiles or pin piles, are small-diameter deep foundation elements that can be used to transfer loads to stable soil layers. They can be installed around the perimeter of the existing foundation to provide additional support. The basement can then be excavated within the new supported area.
It's crucial to consult with professionals who specialize in foundation work and have experience with basement lowering projects. They will be able to evaluate the feasibility of your specific situation and provide guidance on the best approach, taking into account your house's structural integrity and local building codes and regulations.