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Question DetailsAsked on 2/8/2018

I bought a house that was built in the 1850s. The flag stone basement walls are collapsing almost everywhere.

I want to fill it in with concrete. Is this a safe way to fix the problem. I can not dig on the outside due to the fact the naibors house is only 18 inches apart and can’t get to the hill in the back to put in drainage. Is a big house where I can put the utilities in a room on the main floor and I got a couple quotes on fixing it that where around $50,000 but it got a price of $15,000 to fill it in. I know it will bring the value down but I also think paying 50 grand is not cost effective and will equal out the lose. So I would rather fill it in just want to know if it is safe to do

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The other response is one way to do it. Depending on the foundation condition, just putting in compacted structural fill rather than concete might be a viable option.


I presume the "fixing it" estimate fo $50K was to rebuild the foundation section by section, probably with concrete block ?


Depending on the condition and construction method of the house, whether the house is wood frame (good) or stone/brick (bad for this case), the type and condition of wall, and whether you have a "wet" basement or not, common options (usually coming in at more like half that $50K number) include:


1) jacking the house up and rebuilding a new foundation under it - usually cast concrete in that case. Good with most wood frame houses with exterior foundation only, not so easy with ones with lots of intermediate support posts under it.


2) locally jacking it to just prevent collapse while working, and rebuild the wall - in stone if desired, but usually cheaper by a fair bit to do it with concrete block, though with the stone already there (assuming the stone itself is not deteriorating, just the mortar or it is dry-stacked) might be about a break-even - working progressively from one end to the other, with only maybe 3-5 feet opened up at any one time. Not hard to do as long as the soil supporting the adjacent house is stable and water table is at or below the basement floor level.


3) put in drilled-in tieback anchors into the ground through the wall and extending inside of it, then tying those into a cast concrete interior wall (and poured concrete basement floor if desired) to provide a structural waterproof wall inside the existing one,supporting it (the exterior soil will support the outside with all but very poor soils). With minor wood frame additions underneath to transfer the house load from the existing wall to the new one just inside it. This will typically take away 16-24" in each clear dimension in the basement (due to 8-12" thick wall all around). But in your case, with only 18" to next house, that is probably out because the anchorages would have to be well (like 15-20') in under the house next door - a hassle to arrange and invokes risk of any future leaks or cracks in THEIR basement or house settlement cracks being blamed on your anchors.


4) backfill with compacted structural fill - usually about 2-3 foot wide (at top, just below top of wall) highly compacted zone around the inside perimeter of the basement, made of structural fill sloping down inside to maybe 5-7 feet wide at the floor level, then backfill the rest with sand. Typically, unless very dry foundations, drain pipe and sump pump is put in to prevent this fill from becoming a "swamp".


5) backfill with concrete - totally like you said, but more commonly just a triangle against the inside of the wall dug down a foot or so into the floor (or anchored in it if floor is already concrete) - maybe 6-8" wide at the top of the triangle, and 4-6' wide at the base - as a buttress for the stone wall. A lot less concrete than totally filling the basement, and might be designed to leave the utilities largely untouched in the basement. With drain pipes and sump pump again as applicable to prevent water rising up to near house wood level.


6) if the house has a good massive frame in good condition, support it on pin piles or minipiles or such, though that is tuypically a higher-end solution and the 18" clearance to next house makes that a tight job, which may require "importing" a contractor with special different equipment designed for use in confined spaces like commercial building basements.

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Considering the $ involved and my gut feeling it should be more like $15-25K range WITH being able to keep at least some of the basement space, I would strongly suggest you see a Civil Engineering (no Angies List category for that) or Architect/Engineer firm with good foundations repair experience to assess your situation and come up with recommendation and rough repair cost for probably about $500-1000, and maybe that range again for the final design needed to get building permit and for contractors to bid on and build to.


To get a building permit you are almost certainly going to need remediation plans from a Structural or Geotechnical (soils and foundations) engineer anyway, so bring him on now for assessment of alternatives. This can be very important, because as I did bove he will look at a number of alternative remediation methods, whereas most Foundation Repair contractors tend to push their favorite method regardless of whether it is the best for the situation - plus some of the methods being pushed these days are pretty hokey, more like a bandaid than a true structural solution.

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One thing on the $15K number to fill it - you said you were thinking concrete fill - I don't think you could fill a large house basement with concrete for that. Say basement is probably at least 30x50 feet roughly, and even if only 7 foot high basement, that would be (leaving some space under the joists as a crawlspace for future access to underside of framing and any pipes running in it) 30x50x5' say = a bit less than 300CY of concrete - around $25-30K in pretty much all areas of the country even for low-strength fill concrete, and that does not allow for pumping cost which would be the easiest way to place this. I suspect the $15K number was for structural fill (crushed rock or gravel) - so might check on that.


Your favorite realtor (assuming you think you may sell this house someday) can give you a ballpark valuation on what the basement is "worth" come resale time, and how much having it backfilled may detract from the attractiveness to buyers - that is a "cost" or "missed opportunity cost" to be factored into the decision too.


One other thing to consider - a "fill it in" solution does not remove the weight from the stone foundation, so you will still get some at least nominal settlement over the years, and possibly a lot of the stone itself is what is deteriorating rather than just the mortar - or drystak stone moving around. Putting a new structural support (wall ofr new foundation pilings or such) should pretty well eliminate that future settlement issue.

Answered 9 months ago by LCD




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