Septic Tank Driveway

[LBrown59]

I'm worried about the tank itself. I already replaced the "cover" with 1/2 steel plate. I believe you are talking about the small access cover usually only a couple of feet across, right?

Go through the tank and it won't matter how strong the cover.

 

[tommu56]

I'm worried about the tank itself. I already replaced the "cover" with 1/2 steel plate. I believe you are talking about the small access cover usually only a couple of feet across, right?

Think the cover you are talking about is the access to pump it.
I was thinking about the whole thing say 6' by 8" small patio sized concrete top of the tank that would only load the sides of tank not the unsupported middle of the top.


tom

 

[Pirate]

Think the cover you are talking about is the access to pump it.
I was thinking about the whole thing say 6' by 8" small patio sized concrete top of the tank that would only load the sides of tank not the unsupported middle of the top.


tom

Dang it, not me, the other guy. I'm talking about the entire tank too. Some people get confused when you talk about tank cover and what I would call manhole cover on the tank for pumping (except I wouldn't want to be the man in the hole, yech!). I'm going to get an engineer to come out (if he will, they already balked when I said "septic") and then do some cost analysis of slab vs. moving tank. I'm sure slab will be much cheaper and easier.

 

[JB4310]

I've been a contractor for 26 years, I'll tell you nothing makes me more uncomfortable than when someone gets an engineer involved in a relatively simple problem.
No offence to any engineers here, but I just think they should be left alone to tackle the real tough jobs.

Call the tank manufacturer, since it's only 16 years old they have to have the specs on it, ask the guys that install them and work on them.

My mother called me up one day to say there was a cave in in her front yard, it was an old dry well or gray water tank, was pretty deep, maybe 4 feet down, but as far as I could tell the entire thing was built out of wood, for sure the entire top was thick wood planks, about 3ft x 5ft. The point of this story is, it was 90 years old! and I use to park my full size pick ups right on top of it, we never knew it was there.

Good luck with whatever route you take,

EDIT: well I just found this, doesn't bolster my opinion that it's not a big deal, but it may be helpful to you. hope it doesn't make you more nervous
http://www.inspect-ny.com/septic/Dont_Drive_On_Septics.htm

JB.

 

[Pirate]

Hey JB, I read that link the other day. Nice stuff. I think they should have dumped that load before trying to push it out of that tank though.

I don't mind the engineer bit. I look at it as a bit of insurance. He says slab should be x amount thick to allow for car under 5k to park here. Car falls through, I go after engineer.

 

[CurlyDave]

Well, I am an engineer, chemical, not mechanical, although I minored in mechanical, and do all of my own calculations. If something needs to be stamped by a professional engineer I pay a registered engineer to sign off and stamp my plans.

A lot of "relatively simple" things are very easy to underdesign, and underbuild.

Over the course of about 2 years I watched my neighbor's retaining wall collapse three times. Fortunately, no one got hurt, and when it went it didn't undermine his house, just his driveway. It was made of stacked 2'x2'x4' concrete pieces. He is a good friend, and the first time it fell down, I explained how to re-stack the blocks so they wouldn't collapse, but it was not intuitive and he just didn't believe me. So he had them re-stacked and the blocks fell over a second time. The third time he got the same contractor out as the first two times and told him he had to set each course of blocks back, the way I wanted to, but he didn't set them back far enough and the fool thing fell over again.

For the fourth stacking, he took my advice and told the contractor how far to set each course back. Of course the contractor didn't believe him and didn't set the blocks back as far as I wanted, but he did set them back enough to work--just very little safety factor.

Then his son started to build a water feature at one end of the wall -- a pond at the base and a waterfall flowing into it. The first winter all the rocks fell into the pool. Somehow the neighbor got smarter, and asked me to design the water feature before his son re-built it. Well, nothing has fallen into the pond again.

Bottom line here is that a little bit of engineering can save you a lot of grief...

 

[scott_vt]

I know a lady that was launched off the toilet when her husband drove a loaded dump truck over their steel tank. The bending cover worked like a giant plunger, in reverse. She made him clean the bathroom. MikeD74T

Good Mornin Mike,
That was a classic !!!

 

[JB4310]

I don't mind the engineer bit. I look at it as a bit of insurance. He says slab should be x amount thick to allow for car under 5k to park here. Car falls through, I go after engineer.

Just to clarify, I don't want to sound like a braggart, when I say I've been a contractor for so many years and engineers make me uncomfortable, I don't mean because I know it all and don't need them.

It's more that I'm am intimidated by them cause I don't understand their language. I'm a patch up/fix up guy, that's what an old timer called me, not very flattering but fairly accurate. We do some specialty restoration work for institutional type customers but are often working with just a trowel and a pail of cement or roof cement. I've never worked on new construction in my life.

When a customer gets an engineer involved with trying to figure out why the exterior of the building needs maintenance after the New England winters. It turns into a circus, turning a relatively simple problem into a Government project.

If your gonna go all out, make it capable of holding up to any possible load that will fit down that driveway, like a fire truck, oil truck, tree truck or loaded dump truck like in the pictures.
For just the 5K car, that's only 1250 lbs at any time on top center, above the ground over the buried tank, a pc of plywood wood would be sufficient, even if the tank was made by gophers digging out a burrow 2-1/2 ft down

JB.

Good Mornin Mike,
That was a classic !!!

I thought it would be funny to direct the air flow from a back pack blower down the roof vent stack on my shop while a buddy was in the bathroom, he wasn't on the throne but came out looking like he just got out of the shower, whole room was wet, ceiling and walls were dripping. I didn't realize it would have such a powerful effect, this building is on a septic system so I guess by blowing down the only vent, there was no where else for the pressure to release. DUH,
I didn't premeditate this foolish act, it just came to me while I was up on the roof blowing debris off and saw him going inside.

.

 

[scott_vt]

I thought it would be funny to direct the air flow from a back pack blower down the roof vent stack on my shop while a buddy was in the bathroom, he wasn't on the throne but came out looking like he just got out of the shower, whole room was wet, ceiling walls. I didn't realize it would have such a powerful effect, this building is on a septic system so I guess by blowing down the only vent, there was no where else for the pressure to release. DUH,
I didn't premeditate this foolish act, it just came to me while I was up on the roof blowing debris off and saw him going inside.

.

Good Mornin John,
I gotta laugh when I hear these stories, but of course I have never been on the receiving end..... when something like that happens !

BTW, heavy rain today John... good for business !

 

[J_J]

My tank had steel plate on it and I noticed that it was bent from something rolling over it. I measures the top of the tank, and built my own cover. On a flat surface, I screwed 2 x 6 together for half the tank size because the center of the tank had a rib built at the top which supported the two halves. I then put wire and re-bar in the form with the wire and re-bar at 2 in from bottom, about center. I also welded a chain to the re-bar so I could lift it whenever. I then pored cement in the form and leveled it at 4 in.

 

[Pirate]

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.

 

[Eagleview]

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]

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.

 

[JB4310]

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 .

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]

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]

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.

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.

 

[Pirate]

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]

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.

 

[atgreene]

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.

 

[Pirate]

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 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.