concrete footings for house jacks
marlboroman
| Posted in Construction Techniques on
I’m putting in some footings in my basement to set some house jacks on. I was planning to put five 2ftx2ft footings under the length of the beam overhead the problem is after cutting the first hole and digging down about a foot theres water in the hole and is just a wet clay mess. I can push a piece of rebar down another 18 inches easily so what do I need to do to make this suitable to pour a footing on?
Replies
spread footing
The Cliff Notes Version: The soil can suppoort a certain amount of weight per square foot. You will need to determine the bearing capacity, and the load you are going to put on it. Then you size them to cover enough ground to hold the load.
The other alternative is to drive pilings until you get enough resistance to support the load. But since you are in the basement I doubt that will be a practical solution.
You really should spring for the few hundred dollars it will take to get an Engineer involved. Look for a Geotechnical Engineer in the yellow pages.
And, fill out your profile with at least enough information that it shows where you are.
I gather this is just jack posts for a sagging beam? If so, the footing doesn't need to be perfect -- you can give the jack screws a quarter twist now and again, and if a footing disappears into the muck then you can crib up the beam and install a larger footing.
It would help to guestimate how much load you'll put on the pad. Your standard telescoping jackpost is rated roughly 10K to 20K pounds, and generally by the time you approach such a load the screw becomes inoperable, so you're unlikely to exceed 10-15K in most cases. (Non-telescoping posts are rated higher -- up tp about 40K.)
Typical home construction weighs in at around 120#/sf of footprint for a two-story strucure, maybe a little less for single-story. You can guestimate from there how much weight is on your beam. Figure it's not quite divided evenly -- the center pad out of 5 posts (and 2 endpoints, for a total of 7 bearing points) would carry maybe a quarter of the total load.
If you have a simple frame house that's 24x48 with the beam running down the middle, the beam would carry no more than a third of the load (probably less than a quarter). 24x48 is 1152 sq ft, and at 120#/sf that's 138240 pounds. A third of that is 46080, and a quarter of that (the heaviest-loaded post) is 11520.
If you fill your holes with compacted crushed rock you should achieve a minimum of 20 PSI soil capacity, meaning a 2x2 foot pad would carry (ta dah!!) 11520 pounds. Increasing the pad to 2.5 feet square gets you 18000. Increasing it to 3 feet gets you 25920.
Junkhound can probably even give you a technique to measure the actual soil capacity, using a few kitchen utensils and parts from a cast-off rocket engine.