Fill an old cistern with concrete?

earlieq

Hi everyone,

I have an old, unused cistern in my basement. It is a 6' x 6' square hole, about 7' deep, and it is built right into the stone foundation walls of our 1880's-era house. The top of it is level with the basement floor, so it's basically an open square shaft 7 feet deep. It is completely under the house, and completely open to the basement, no part of it is obscured or covered.

There seems to be a foot or two of loose soil, old bricks and general crap on the bottom. I've dug down through it and found what I think may have been a clay layer, but it is not watertight now. (Hurricane Irene occurred the week after we moved in -- the water that got into the basement all went into the cistern, and then it quickly disappeared.) (I've fixed gutters and grades since then, and no more water gets in.)

I'm going to clean it out, take pictures of it for the house's historical record, and sift through the dirt at the bottom of it to see if any interesting bits of history fell into it over the years.

Then I would like to fill it up with concrete and reclaim that floor space. I could also fill it with sand or crusher run, but concrete seems like much less labor. I can't picture how to get sand or gravel in, except in wheelbarrow-loads.

Questions:

Structurally, is there any reason this is a bad idea? The four walls of the cistern are stone foundation walls, presumably as thick and strong as the rest of the house's stone foundation walls. The cistern walls are parged with some kind of mortar that has some damaged spots but is surprisingly tight and solid.

Is there any reason not to pour one giant 8-yard cube all at once? Any kind of prep to do in advance other than clean out the bottom and tamp it down?

With regard to access, I think I can pop an old former basement window out on the driveway side of my house, right over the cistern, so that the delivery driver can get the truck quite close to the house and put the chute through the former window opening, and let the concrete drop right down into the hole.

How high should I pour the top of the concrete relative to the rest of the basement floor? The floor is bricks, basically set in dirt. Was thinking I would pour concrete to within 8" or so of the existing brick floor level, then put a layer of crusher run or sand on top of that, and brick it over to match the level of the rest of the floor.

Any advice and/or better ideas welcomed!

Thanks,
Earl

PS - multiple friends and neighbors have already suggested making it into a bomb shelter, wine cellar, root cellar, priest-hole, gun safe, panic room, etc. There is no possible renovation that will get my wife to set foot into it, since she saw the spiders living down there, and it is second-quality space at best, no matter what you do to it. I'd rather have the floorspace back for my workshop.

Bondo

Ayuh,... That's a 'ell of a Waste of Money,....

Stone is $6. a yard, concretes nearly $100. a yard...
yer talkin' almost 9 yards...

Stone can be sluiced, just like concrete, just add water...

earlieq

I didn't know that -- do you mean I can ask a concrete transport company to just deliver stone, sluiced from the truck down a chute?

Bondo

I donno if Yer concrete company offers that service, or not....

I'm just sayin', that stone is easily sluiced...

I'd guess they offer "Flowable Fill",...
Which is sluiced stone, with a tiny bit of concrete added...
looks like concrete, but is "Dig-able"...
It's used where mechanical compaction ain't an option...

Fix'n it

if it were me, and i would have to see it first. i would put joists and sheathing on it. with a trap door, to make it storage.

post some pics.

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oh'mike

Sand can be delivered in a cement truck---and will flow down the chute nicely---A local not-for -profit club raised money each spring delivering sand box fill that way---

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Canarywood1

Ask if the redi-mix provider has a mud coat,similar to what Bondo suggested,it's sand with 2 bags of cement added per cubic yard,lot cheaper than regular concrete.

itsreallyconc

we used it for pipe crossings under the wearing course of blacktop - your plant will know it as ' flowable fill ' - can't imagine any plant NOT having it by now - faster, stronger, fewer callbacks due to settling, etc -

http://www.flowablefill.org

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if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help
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Tscarborough

Sand fill with a concrete cap.

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Maintenance 6

Around here you can order 2B stone delivered in a mixer. I'd chute that in to within 4" of the top and then cap it with concrete. You said the water drains away from inside it, so I might even consider putting a floor drain in the cap unless radon is a concern.

earlieq

Thanks everyone, for the very useful ideas and suggestions. I learn something new every day from diychatroom -- it never occurred to me that ready-mix trucks could also deliver stone or sand, and I had never heard of flowable fill or mud coat. Live and learn!

Fix'nIt, I have considered flooring the space and using it for some kind of storage, but it seems like a lot of effort for a poor-quality storage space that would need maintenance attention (moisture, bugs, etc). Still on the fence about it.

I'll try to post some pictures this weekend.
Earl

Fix'n it

yeah, it would kinda be a pita. and that is a small space.

oh, and be carefull with having a cement truck on your property. they can destroy your driveway.

i found out that my asphalt driveway cost the PO's $6100 in 2009

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jomama45

This would be my suggestion............./\ /\ /\

Clear stone (sometimes called "washed stone" or #1 stone) would be the best option IMO, as it would allow water to perc through it yet and can be delivered via concrete truck & chute, just like concrete. We use this method quite often and just to give you a rough idea, stone delivered via concrete truck is about $38 a yard vs. ~$90 a yard for concrete.

I would suggest installing a sump crock in the corner of the pour as well, just in case you need is down the road........

Mort

My first thought before I read the responses was CDF (controlled density fill, aka flowable fill, lean mix, etc.). It's basically sand, water, and a little cement so you don't have to compact it. It has between 50-300 psi of strength once it sets, so you can dig it out later if you want.

Also, don't try to walk on it right after you fill the hole. Ask me how I know.

AllanJ

If I were to make provision for a sump pump I would set up a pit (unfilled space) witin the cistern at least 3 feet deep and 30 gallons of capacity. Much larger than the typical crock you buy from Home Depot.

With all of that open space it is not necessary to set up such a small space that under many conditions a sump pump would cycle on and off frequently.

A super large pit makes it unnecessary to have a check valve in the outlet pipe, an additional item that can malfunction or make noise. The much longer pump run time means that the amount of water that falls back into the pit with no check valve is a very small fraction of the pit contents. Thus the pump is not continually pumping out the same water over and over again.

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