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Digging out a crawl space

I have a crawl space that is roughly 13'x40' that is completely open that I would like to excavate to expand my existing basement. What is the best avenue to take on this project? Underpinning the existing crawl space wall or erecting a knee wall around the inside perimeter of the existing crawl space wall. the digging and labor doesn't scare me because I'm not on a time frame and I have alot of favors to call in.

Re: Digging out a crawl space


It's easiest, less costly, & less use of materials if you install a bench footing (knee wall) all along the exterior foundation, using steel-reinforced concrete.

This would avoid digging directly under the present foundation (called underpinning), which is more complicated & usually unnecessary.

The Nemmar site below has illustrations of bench footings & underpinnings---click onto "structural problems-page 1 at the site, then onto diagram 0239, 0231, 0232 to view the technique used.

If the full Nemmar diagrams don't load on your system, Google "Nemmar real estate house illustrations" and load the "cached" version of the site.

In theory, this is how this project is done---in reality there's a lot of work & other issues involved & you'll have to decide how you're going to excavate all that dirt ---(soil expands ~30% when it's excavated).

You'll have to calculate how many cubic yards are in there & what do you do with it when you get it out.

If you attempt to excavate the entire 13' X 40' by 6' you would get 3120 cu.ft./27 = 116 cu.yds + 30% = 150 cu.yds.---a tremendous amount of soil.

If you hire a 15 yd dumpster to haul the dirt that's 10 dumpster trips @ $300/trip = $3000 just for the dumpster.

If you rent a small skid loader (the best way to remove the dirt if you can fit it in there) to remove the dirt it would cost $300 to $600/day.

You would also have to decide how to shore up the center portion of the building (with jacks, etc.) & also put a supporting wall on that side of the diggings.

The bench footing in the Nemmar diagram looks a little wider than necessary (unreinforced concrete)--the bench footing need be only 9" to 12" if steel reinforced with 3/4" rebar.

The town & engineer may require that instead of a bench footing, a full-length "concrete steel-reinforced sister wall" is needed, due to the poor condition of the present foundation wall---this would be an added expense & problem in getting the fresh concrete in there into the forms if there is no access from the top of the work.

Since this involves the structure of the building, you would have to get a permit from the town building inspector---they often demand that you hire a civil engineer ($500) to draw up drawings and plans as to how it will be done before the city/town will issue you a permit.

Some town inspectors won't issue a permit for this type of work unless you are a licensed foundation contractor---you can always appeal a negative decision to the town council, who may or may not agree to let you do it.

Injuries: this is dangerous work---if anyone is injured during the course of the excavation, you will be directly liable if they decide to sue--check your homeowner's insurance, or perhaps ask your agent to estimate how much an amendment to your policy will cost to cover this.

A wiser course is to consider excavating only A PORTION of the entire crawlspace---enough to address your storage needs, & whatever else you intend as usage for the new space---13' X 20' may be all you need---let common sense prevail.


Re: Digging out a crawl space

Jackthe Shack is pretty much spot on.

To add to that ... underpinning needs to be done properly and is best left to someone that is experienced. It requirers to be done in sections as well requirers an engineer to sign off as to the proceedure , depth , thickness , strength of concrete.

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