Spoke with engineer and some others....consensus is to pour a slab over the area, use lots of rebar and good cement. I already priced a 24inch manhole cover. I want to have it part of the slab so all of it's weight is in slab and not on a riser to the tank. Now, I need to find a good concrete guy who can actually figure out weight of concrete and how to form it up from below. I would like to have the slab poured on top of some wood or something that is supported from the outsides of the tank so I don't have the weight of the wet concrete on the tank. I may have to expose the entire top of the tank to figure out just what to do.
Septic Tank Driveway
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Eagleview
Veteran Member
The way it is done , is to pump out the tank and cut holes in the lid with a concrete core drill . Then they stand PVC pipe up in the holes which reach the bottom of the tank and are cut flush with the lid . The PVC pipe/s are then filled with concrete which form solid columbs like super market car parks have .
JB4310
Super Member
I don't think you have to build it suspended/supported like a bridge, or like you said have it all empty under the formed slab.
If you went 6' x 8' x 6" thick that's 48 sq ft and 24 cu ft. @ 150 lbs/cu ft = 3600 lbs. 200 lbs of steel = 3800lbs. that's a fairly static load of, (3800 divided by 48) = 79 lbs per sq ft. (0.55 psi) so pouring it on the ground would not be an issue IMO.
Once cured, the dynamics of driving a small car over the slab would have little significance in terms of additional compression on any one small footprint on the top of the tank as the load would be disbursed throughout the entire area of the slab. The car would obviously add to the gross weight.
Wow, I'm starting to sound like an engineer? I might be completely off my rocker with my ciphering and reasoning, if so, I don't mind if someone says so
If your worried about the gross weight on the tank and/or want to be sure any possible future heavier traffic could be supported, then you would have to have 2 foundation walls, supporting the slab, one on each end, built outside the parameter of the tank.
The cast in the slab man hole/frame is the way to go, was gonna say you don't really need the riser cause you could just dig it out like you have always done, but there might be an issue of back filling under the slab, so it would be better to have a riser of the same diameter as the cover.
Be sure to "crown" the cover, making it the highest point in the slab, grading the slab all around away from the cover so storm water doesn't flood the tank, having the open riser may provide a free unabated path for storm run off.
Good Luck, JB.
If you went 6' x 8' x 6" thick that's 48 sq ft and 24 cu ft. @ 150 lbs/cu ft = 3600 lbs. 200 lbs of steel = 3800lbs. that's a fairly static load of, (3800 divided by 48) = 79 lbs per sq ft. (0.55 psi) so pouring it on the ground would not be an issue IMO.
Once cured, the dynamics of driving a small car over the slab would have little significance in terms of additional compression on any one small footprint on the top of the tank as the load would be disbursed throughout the entire area of the slab. The car would obviously add to the gross weight.
Wow, I'm starting to sound like an engineer? I might be completely off my rocker with my ciphering and reasoning, if so, I don't mind if someone says so
If your worried about the gross weight on the tank and/or want to be sure any possible future heavier traffic could be supported, then you would have to have 2 foundation walls, supporting the slab, one on each end, built outside the parameter of the tank.
The cast in the slab man hole/frame is the way to go, was gonna say you don't really need the riser cause you could just dig it out like you have always done, but there might be an issue of back filling under the slab, so it would be better to have a riser of the same diameter as the cover.
Be sure to "crown" the cover, making it the highest point in the slab, grading the slab all around away from the cover so storm water doesn't flood the tank, having the open riser may provide a free unabated path for storm run off.
Good Luck, JB.
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JB4310
Super Member
Interesting and fairly simple, you would have to flare out the top of the columns so they made contact with and supported the underside of the top of the tank. or use steel plate some how at the top of the column, that would be the trickiest detail but still such a simple concept
Maybe not the ultimate protection, since one could argue that any significant weight on the columns could punch through the bottom of the tank. probably the weakest part of the tank.
JB.
Eagleview
Veteran Member
Photos are hard to find , this is similar to the columns i was thinking of . These ones support the roof , the ones i had seen support the slab , which in it's own construction is inherently strong .
CurlyDave
Elite Member
How big is this tank? You probably know gallons, but I am more thinking in terms of feet and inches.
There are two failure modes I would be concerned about, the first is collapse of the tank lid, which is probably a thin flat slab.
The second is buckling of the tank walls if too much load is transferred to them. No matter how thin the walls, the compressive strength of concrete is high enough that they will theoretically hold any reasonable load. The place where the theory breaks down is buckling. There are engineering criteria, but they are hard to apply. You also do not know the amount of rebar in the tank, but I would suspect the answer is "not very much".
I would strongly consider building what is essentially a bridge over the tank, not just a slab. You need supports for the slab buried to a depth approximately equal to the depth of the tank. Despite the attraction of putting them in the tank, you do not know the condition of the earth under the tank, and finding out is a major undertaking. If you put an internal support over a void in the earth beneath the tank, the tank floor will fail.
It is far safer to put the supports outboard of the tank. You can look into the hole and determine the condition of the earth, and if you pour concrete supports, you know there is no void.
I see this as a two stage process. Wait until the ground is firm and dry, then build footers for the slab, pour and allow time for strength to develop. Then pour your slab.
And alternative to a poured in place slab, would be to pour long narrow concrete rectangles, capable of bridging between the footers, and place them next to each other on the footers with your tractor. This eliminates the need for a fairly complex form under the slab, which is going to have to be removed after the slab is poured.
A second alternative would be to pour the slab on the ground next to the tank, and then drag it onto the top of the footers after it has cured. To me, this seems fraught with danger, since the slab is going to be heavy and your tractor might not be up to the job. If you do it in small pieces, you know what each one will weigh and you know your lift capability.
You guys rock. Lots of great info. I'll skip the pvc columns in the tank. I really don't want anything touching the tank. This all could have been avoided if I could reach the previous owner and ask him tank origin info. Oh well. Never thought about elevating tank lid too for rain runoff (crown).
J_J
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Pirate,
You said you installed a 1/2 steel plate as the top cover. A simple solution I believe would be to pull the plate off and weld some heavy angle or I beam to the underside of the cover. If done correctly, It should support whatever, and never slide. Weld some lifting hooks on the top. A large tractor or forklift should be able to lift and reset the lid. Done deal.
You said you installed a 1/2 steel plate as the top cover. A simple solution I believe would be to pull the plate off and weld some heavy angle or I beam to the underside of the cover. If done correctly, It should support whatever, and never slide. Weld some lifting hooks on the top. A large tractor or forklift should be able to lift and reset the lid. Done deal.
atgreene
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Do you know if it is a heavy duty tank or a standard duty?
I install septics on a regular bassis. There generally are several different types of tanks.
Low-boy, for shallow soils where you cannot dig deep enough to burry a regular tank, generally 10' +-.
A regular tank, generally 6'x8' in size.
And a heavy duty, designed to be driven over. It is cast much heavier and thicker usually with more wire/rebar.
If you have a heavy duty tank, this entire operation is moot. Especially if the tank is deeply buried. Covers are prone to breakage, not unusual even for heavy duty ones.
FWIW, most septic systems nowadays can be made safe to drive over by placing 2' of gravel over them, a layer of filter fabric/geo-tex and paving them.
I think you may be over-engineering this problem.
I install septics on a regular bassis. There generally are several different types of tanks.
Low-boy, for shallow soils where you cannot dig deep enough to burry a regular tank, generally 10' +-.
A regular tank, generally 6'x8' in size.
And a heavy duty, designed to be driven over. It is cast much heavier and thicker usually with more wire/rebar.
If you have a heavy duty tank, this entire operation is moot. Especially if the tank is deeply buried. Covers are prone to breakage, not unusual even for heavy duty ones.
FWIW, most septic systems nowadays can be made safe to drive over by placing 2' of gravel over them, a layer of filter fabric/geo-tex and paving them.
I think you may be over-engineering this problem.
I don't know if it is heavy duty or not. I don't know how to find out. Any ideas? My cover (round 2 foot) already broke, that is why I just replaced it with a 1/2 thick steel plate. It is buried 18" deep right outside the garage.
Over engineering has crossed my mind more than once. The deeper I dig into, the more I wonder if I'm just paranoid or worry too much. I'm really only worried about the entire tank top collapsing.
