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Making Basement Deeper?

sawdustom's picture

In another post I detailed the woes of my 100 year old basement floor. If I'm going to bust it out and pour a new floor I have a question.

Can I make the basement deeper? If so, how do you handle the foundation blocks? Can I put new blocks underneath the old blocks? What are the alternatives for handling foundation support?

(post #64356, reply #1 of 9)

I assume this is a do-it-yourself project. in that case I'd take out a section of the floor and build a poured wall under the block    carefull to fill the space under the block completely  with cement  (don't be afraid to include rebar, although rebar under a bloack wall is kinda overkill. .. then go to another section and dig that out and pour that etc.. if you really are ambitious you might try doing a section under each wall.  pour that,..  do another section,  pour that etc..when you have the foundation as deep as you want it and  a new poured wall under  everything then you dig out the basement and go as deep as is pratical.. lots of cavots here but it can be done..

(post #64356, reply #2 of 9)

Don't mess with the footings or walls. Without professional investigation you have no way of knowing what structural condition those walls are in.
A better way of going is to come inside of your existing basement walls a certain distance and then dig down for your new floor. A rule of thumb is one to one. If you want to go down one foot, then go inside the FOOTING one foot and start your excavation. Build a nonstrucural stem wall and "bench" over the existing footing and what's left of the floor. You could probably do this in one pour. Then dig out the middle and pour the new floor. Now's the time to also put in drain tile and sump if needed.
You're using this area as a workshop, so I'd consider coming in two feet and making that step-up the base for shelving, cabinetry, or work benches.
Floor drains and interior column footings - you're on your own.
A few years ago I hand-dug a basement under an existing house so I have a bit of experience. You could build your perimeter stem walls, replace the interior columns and footings, bust up the existing slab into manageable pieces, then rent a conveyor for a couple days and get all that stuff out. Just make sure you've got a place to convey it to and have somebody lined up with a truck to haul it away. Speaking from experience, don't even consider hauling it upstairs in 5 gal. buckets- rent the conveyor!
Given the age of your house, and that you probably don't want it falling in on you, a couple hundred dollars of engineering advice is money well spent.
Have fun!

(post #64356, reply #3 of 9)

Digging out your basement is a pretty big job.  I was asked to put a foundation under a house that didn't have one and learned alot that I really didn't want to know.  However, we had I beams delivered and supported the house with those.  My guess is that you could use beams of some sort and lally columns bearing on your existing basement floor.  The beams go perpendicular to the floor joists and I would use some thing to fasten them temporarily to the the joists--don't want them to flip out when jacked up.

With a beam near the existing foundation wall (3' to 4') I would break out the floor.  Use ear muffs.  An electrical jackhammer might be enough.  I have used them to do most of the earth digging too--especially if it is clay.  I don't know what you have for a footer.  It could have been poured and the block layed on top.  If that is the case it should have rebar in it.  You can dig out laterally about 5' under it.  Leave "pedestals" to support the weight of the wall.  It may be possible and even advisable to also support the wall weight with some steel props that will never be removed.  The new concrete footer will surround the props when you pour it.  IF you can break out the old floor, dig under the footer, add props, and rebar with the beams and lally columns taking the house load for one entire wall that would be best.  Form up just the inside of the new footer and pour it.  The earth on the outside of the existing wall will act as the form for the other side of the footer, dig neat and square.

Go around the perimeter until you have the house supported on new footers deeper than the original footers, moving the beam and columns as necessary.  Now you can break out the old floor and dig the rest of the floor to level (+/-).  If the first stage seemed lengthy you'll love this.  I came up with a method to move all of this material that may or may not work for you.  UniStrut fastened to the floor joists can give you a track to a window.  Ball bearing wheeled carriers roll in the UniStrut.  A chain hanging from that carries a 5 gallon bucket that you fill with all of the material.  The dirt will appear to expand about 4-5 times from when you dig it out.  The bucket goes out (and maybe up) the window and sooner or later you will be walking around on a dirt floored basement as deep as you want to dig. 

Use a tape to measure down from the joists to figure your depth or a water level.  Put in remesh, call the trucks and pour your new floor.  Have the concrete finishers ready.  Don't forget the expander all around between the new footers and the new floor.  Now all you have to do is cut off that chunk of old footer sticking out  about 18" above your new floor! (Oh, you remembered that before pouring the new floor!)  I didn't include work on any perimeter drain, existing floor drains near the HW heater or partitions, etc. 

One time I saw a guy use a little conveyer belt about 18" wide.  It allowed him to dig and toss the dirt on the moving belt--he started with only a crawl space.  Now if you could get one of those little stand-behind BobCat's down there to do the digging & lifting.........Thor

Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden.... Roman Poet Phaedrus 15BC–50AD

(post #64356, reply #4 of 9)

Tom, how much deeper do you want to go?I'm assuming you will pour a 4" slab. Have an architect or engineer confirm my next suggestion.After you dig one side, leave it a trench for a form. Or form up to bottom of slab with plywood. Stay back from existing foundation line 8".Pour this stem wall. After stripping forms and adding expansion joint to old walls ,your ready for concrete.I would not pour this yourself, get a concrete contractor or at least a cement finisher and laborers.You may fet surprised when digging, many 100 year old houses have no footing. They sit on flat stones,or the walls are rubble foundation.In any case , check with a engineer first.


(post #64356, reply #5 of 9)

That book by Nash "Renovating Old Houses" shows how to do it. Never had tried it, but now redoing a house in Baltimore that had its basement filled in in the early 1900's. When you try it, keep us posted, and maybe you'll convince me to get off my bum and get diggin.


(post #64356, reply #6 of 9)

All, Thanks for the detailed advice. I especially appreciate "Tyr's" advice - you made it all sound sooo easy ;) A few years ago I replaced my driveway. I dug out the exterior foundation wall adjacent to the driveway and waterproofed / installed drains before the concrete was poured. There is no "footer under the blocks that make up the basement walls. The area of OH I live in is all clay and our house is actually about 150 ft above lake level (Lake Erie). We are basically land locked - the basement is the only place to get more usable space. From all you advice here's what I plan to do: Consult a structural engineer; Consider the "in two feet, down two feet option"; Get a conveyor and hauler to handle the debris; Get lots of licensed / insured help to do the work; Get a qualified cement contractor to pour the new floor. Thanks again - you have all helped me out quite a bit.

(post #64356, reply #7 of 9)

Best way to do it is to hire a house moving outfit to come in, jack up the house, and rebuild the entire foundation.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #64356, reply #8 of 9)

I know of a remodel where the same thing is being done.

They are digging down 18". The existing foundation on 3 sides is not being disturbed. The house is getting an addition on the fourth side, so that side will be temporarily supported and dug out.

The floor is being dug up next to the existing footings and a 12" thick new footing is being poured into the space.

It is a small house, about 800 square feet per floor originally, and I was told the cost is around $30k for the basement part.

I wonder if raising the house to install a deeper basement would have been much more money.

(post #64356, reply #9 of 9)

you could jack the house and then dig all you want.