How do I pour a concrete "lid" for a cistern or other large hollow?

Question

I have a 9' diameter by 8' deep cistern. The concrete top was poured in the 1920's and is failing; it has developed 2 holes through the 6-8" thick concrete and is getting worse.

I can mix concrete with the best of them and have poured a bunch of walkways and footers and whatnot. But how do I pour over such a large area and end up with a 'cap'? Usually the ground or tamped gravel holds up the concrete while it cures. How do I 'hold it up' so I can make a cap?

Idea A -

Unless there's a better way, I'm just going to use 2x4's and make some 16" OC stud walls to fit underneath and cover it with plywood cut into 18" widths laid perpendicular to the walls (cut to 18" widths so I can disassemble it afterward and pull it all back through the access hole). I'll lay rebar and mesh, then pour the concrete on top and let it cure. When it has set for a week, then I'll remove the walls and plywood from below.

Idea B-

Just cover it with plywood, pour the concrete over the rebar and mesh and leave the plywood in place.

I assume idea A is probably 'the right way' but I'd like someone who has done this sort of thing to weigh in. =)

Chris

Answer 1

An 8" thick reinforced concrete slab weighs about 110 lb/sq. ft. That's actually not all that much, so I think you're on the right track with method A. Here's what I'd recommend:

First, demo the old lid so you can work unobstructed from the top.

Use double top and bottom plates on your stud walls. Place the walls parallel to each other, only far enough apart that you can get between them to work. You'll have to build each one a different length, or two of each length, to accommodate the curve. Build each one long enough so it's right up against the cistern walls at the ends. Be sure to build the 2 on the outsides as close to the cistern walls as possible - this means they'll be really short - like only 2 studs each. Brace all of them top and bottom with horizontal 2Xs nailed across.

For the top, don't just use plywood. Lay 4X4 joists across the tops of the stud walls, pretty close together, like 6" (again, cutting each one to go all the way from cistern wall to cistern wall). Then lay your plywood strips perpendicular to the joists.

Edit: Is the bottom also concrete, or just dirt? All this presumes concrete. If it's dirt, I'd put closely-spaced joists down first, then the stud walls.

Answer 2

Your "lid" dry weight will be over 3600 lbs not including steel.

I'm no engineer but I think pouring it over a form on a solid surface with chains in it would be the way to go.  Then rent a backhoe or trackloader to move it into place.  This means especially hard concrete so it won't break while moving it.

With this weight even steel plating with reinforcement might not hold.

Answer 3

I had a friend several years ago who built a house in a similar way.  He had an outbuilding that was pretty large and set up a form on a trailer.  The form was built of corrugated steel panels supported by steel I beams on the bottom, with wooden sides.  He would put in his wire mesh, rebar, electrical conduit, etc., then pour the concrete in.  After it cured he just rolled the trailer over to a corner where he had a chain hoist and lifted the wall into an upright position for storage until he got them all built.  The walls were not totally concrete, as he put in closed cell foam like styrofoam about 2 inches thick for insulation.  When he poured in the concrete, the foam had about 3/4 inch of concrete on the inside and outside.

Answer 4

i would go with plan A...that way you can get about 5 other friends to help you pick it up and place it on the cistern after it cures then break away the frame...good luck

Answer 5

Hi, I have installed a lid like you are describing but called a precast company to form up the lid, deliver, and set it. It was in the end a safe, relatively easy job that was engineered to hold the weight and last.

Answer 6

Pour it in a frame, built and sitting on solid ground with rebar through chains and then use a back hoe to lift it into place.  Concrete weighs a bunch and the substructure to hold an 8' by 9' lid, say 4 or more inches thick would be prohibitive and very dangerous.

An engineer has said 8 inches thick is 110 lbs per sq foot isn't very much weight.  How much is 110 lbs times the nine or ten foot diameter going to weigh?  Plus rebar.  Wouldn't that make your top about three tons give or take a half a ton?

What a mess that would make if your form fails, and could be very dangerous too.  I stand by my original idea, pour it on solid ground and pay a backhoe operator to place it.  Don't forget to make a port for access in your top.

Answer 7

I think that if you are only using this for reserve water, I would find a way to put on a metal cone roof like on a grain bin.  With a concrete lid, how would YOU get to it if needed?  Do you really need this cistern, can't you drill another well?

Answer 8

mmmmm

I presume your going to demo the existing cover.If so,then option A is more correct.I would use 4x4's as support bracing,included a link for shoring supports.I would use 3/4" plywood cut in say 16" squares stacked 3 high beneath each 4x4.The stability of the ground on bottom is what really concerns me.

Make sure your plywood is oiled so you can get it off.I believe I would opt for 2 mats of #5 rebar.Probably vibrating a good idea,the more water in the mix the weaker the mix.

I have been on 3 decks that have failed in the last 25 years,the only good thing in no injuries.These were commercial jobs and the carpenters did the forming for a living.I can only say don't pour to fast and let the forms absorb the weight slowly.If you feel/hear anything it's probably a warning of what's to come.

Disclaimer: I have to do this.It would be best to have a contractor do this.My opinions are not engineered advice.I am and have been a wall/flatwork contractor and have seen much go wrong.

Good Luck

Answer 9

Since the cap is failing, perfect time to bust the rest of the cap in, fill it with dirt/gravel.  A lot cheaper than building a cap for it.

I have a cistern tank too, although my isn't failing, I do know the dangers of having one.  Looking into getting the thing taken out.  I would rather spend the money than to have someone fall into it.