Septic Tank Minimum Design Loads 3

[gte447f]

I am assessing a situation for a residential homeowner who has discovered that a wood post supporting part of an exterior deck is located on top of a septic tank. The tank is buried approximately 2 feet below grade. The post is an 8x8 timber post approximately 15-20 feet tall with a tributary area of approximately 40 square feet of deck floor and 40 square feet of roof. My initial inclination is to recommend that the tank should be abandoned and replaced, since the tank was not designed to support the loads from the deck structure. A new tank would be installed a short distance away and most likely connected to the existing leach field. The existing tank would be abandoned and backfilled with concrete. Does this course of action sound reasonable? Does backfilling the existing tank with concrete create any new design conditions that need to be considered that I am not thinking about? The only issue I have considered is the possibility of future settlement of the tank due to the increased weight of the tank when filled with concrete. If that happens, my view is that as long as the amount of future settlement is relatively small (ballpark 1" or less), then the wood deck construction should be flexible enough that minor settlement of the tank below the deck post should not create any serious problems.

Alternatively, I am interested in how the current loads on the tank (i.e. soil overburden and concentrated load from deck post) compare to the typical design loads for a residential septic tank. I have a copy of ASTM C1227-20 "Standard Specification for Precast Concrete Septic Tanks", but I am having trouble interpreting the minimum design loads stated in Section 6.1 of the document (excerpt attached). Section 6.1.5 mentions a minimum live load of 100 psf or a concentrated load of 2250 lbs, but then Section 6.1.6 mentions a minimum live load of 300 psf. Why the difference? What am I missing?

Regardless, I am convinced that the existing loading could exceed the minimum loads stated in ASTM C1227, and furthermore, I have no way of knowing whether the existing tank even meets the ASTM standard. Hence, why I am leaning toward recommending replacing the tank and backfilling the abandoned tank.

Thoughts?

 

[dhengr]

Oh, sh:t!

 

[JedClampett]

It's poorly worded, but for septic tanks in lawns, you can use 100 psf. If you're not sure, use 300 psf.
You're always safe moving the septic tank away from the load, but if you want alternatives:
[ul]
[li]Put in two posts, one on either side of the septic tank.[/li]
[li]Span a buried member from wall to wall of the septic tank and put the post on it.[/li]
[/ul]

 

[gte447f]

dhengr, yes, exactly!

JedClampett, thanks for your interpretation of the minimum live loads. That's sort of what I thought as well (lower live load permitted for residential lawns), but I wasn't sure, because it does seem poorly worded to me. I discussed moving the column with the owner, but it's a corner column, so it would mean moving the post beyond the corner of the deck and adding some sort of flying beam to reach the new column. I also thought about adding two sonotube piers (one on either side of the septic tank) and spanning a beam between them over the septic tank at the ground line to support the existing column above the septic tank (sort of like your second alternative, but not using the walls of the tank as supports). I'm still leaning toward replace the tank. Whenever I encounter boneheaded screwups, I'm always inclined towards "do it again, the right way this time". Do you see any problem with backfilling the abandoned tank with concrete?

 

[dik]

Any chance for something heavy like a garbage truck or a fire engine?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[gte447f]

dik, no, its in a residential back yard. It just has a deck post sitting on top of it.

 

[dik]

post not good... how far down... and tank manufacturers often have stipulated loadings if a precast one...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DBronson]

gte447f:

The Health Departments in every jurisdiction we've worked in have regulations that prohibit having a septic tank within 10', 15', or 20' of a building. I think they'd consider the deck a building. I wouldn't put 40 psf on any of the tanks we usually see, unless they are specified with hs-20 loading. You're right to propose a new tank. I'd then have the installer of the new tank remove the top 1/2 or the lid of the existing tank and punch the bottom with the excavator bucket to break up the concrete so it won't trap ground water. Then pour a, say, 30" square footing on the tank bottom to set your post on. Or install a piece of Sonotube from the footing to grade if you don't want to bury the post that deeply. The tank can then be back filled with spoils from the new tank excavation. Filling the tank with concrete is not a great idea.

Regards,

DB

 

[dhengr]

Gte447f:
If the tank is new enough, and a manufactured tank, you might be able to just pump it dry, excavate around it and move it to the new location, and reconnect it. They are brought in on a truck and lifted into their hole, so maybe a (the original) supplier could move it. Check with a few local septic tank suppliers and see what they have to say, did they supply it? This might not cost more than trying to do a proper job of filling that kind of a mess to carry a significant load. Alternatively, could you install two new posts under the two beefed up, existing rim joists, and allowing each rim joist to canti. 4’ or 5’ to a modified connection at the corner of the deck?

 

[SWComposites]

So how long has this tank and post been in place?

 

[gte447f]

SWComposites:
The home is 28 years old, but multiple additions have been added over the years, including the deck in question, maybe 5+ years ago. Home recently sold to new owner. New owner discovered the deck post on the septic tank when he called a septic contractor to inspect the septic tank to see if it needed to be pumped.

dhengr:
There are multiple options, like relocating the existing tank and modifying the deck foundations/framing layout, as you point out, but I am inclined toward locating a new tank elsewhere and abandoning the existing tank as the most straight forward and expedient remedy, especially if the existing tank can be backfilled with concrete; then essentially no modifications will be required for the deck or the deck foundation.

DBronson:
I agree with you most of your points. In my state there is a regulation that mandates that if use of a septic tank is discontinued, then the tank either has to be pumped out and backfilled "to prevent entrapment", or it has to be removed. The regulation does not mention puncturing the bottom to prevent the water from accumulating in the backfilled tank, but I have seen that guidance recommended elsewhere. I like your suggestion of casting a footing on the bottom slab of the tank and using a sonotube pier on the footing to support the existing wood column, but that is more involved than simply backfilling the tank with concrete. Also, the wood column in this case is not located over the center of the tank, but near one side of the tank, so there probably isn't clearance for casting a footing on the base slab of the tank and a sonotube pier without encountering interference from the wall of the tank. That being said, why do think that filling the tank with concrete is not a great idea? The tank cannot fill with water if it is filled with concrete. Also, due to the relatively large footprint of the tank (at least 4 ft x 8 ft), the solid block of concrete plus the structural loads from the deck will only result in a soil bearing pressure of approximately 800 psf at the base of the tank; seems like settlement potential should be minimal. Is there something else I am not thinking about?

 

[DBronson]

gte447f:

Cost is my only concern about using so much concrete on a residential deck project. Filling the tank here in MN would run $600-$700. The excavator is already on site, so if an existing tank wall is in the way just knock that wall out and pour the pad where you need it. The soil excavated to install the new tank has to be either used on site or moved - it's cheaper to use it for back filling the old tank than to bring in concrete.

My thinking here is to try to utilize the resources available in the context of a small residential project. Also, bear in mind that $600-$700 would go a long way towards buying the new septic tank.

Regards,

DB

 

[phamENG]

As long as the health department is ok with the proximity and there's no damage to the tank, I'd find a way to span over the existing tank and leave it in place. Even at that elevation, putting in a larger beam and beefing up a couple of deck footings (or adding two new posts and footings) is going to be waaaaaayyy cheaper than pumping, filling, excavating, and installing a new tank. Remember, it's not just the tank. If this is a gravity system, you have to consider a plumber is going to have to redo the existing drain pipe or the tank will have to be installed deeper than it is now to accommodate the existing slope. The drain field may also need to be move. If it's an active treatment system, you may be moving additional tanks. You could probably rip the entire deck down and build a new one for less than what you're suggesting once you factor in all the downstream costs.

 

[dik]

Have to be able to access the hatch...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

The post is an 8x8 timber post approximately 15-20 feet tall

I've met some big septic workers, but I think they'd all be able to work with that kind of head room.

 

[Canuck65]

In my area it is specifically prohibited from building any form of structure, including a deck, within about 12 ft to 15 ft of a tank or septic bed. You are also not allowed to build anything spanning overtop the septic tank.

The precast septic tanks I have dealt with in the past are very marginally designed. I can't recall the top loads, would have to look that up and I don't have the standard in front of me. However, they tend to design very skinny and for full certification they are required to pressure test model tanks and recertify every 2 years. They rely on the testing to pass, more than the calcs. So in my opinion, I would avoid point loading a septic tank.

Why fill the old tank with concrete, buy a new septic tank, dig a hole, and then have to find somewhere to dispose of the fill? I would find another place for the 'new' tank, dig a hole, take the existing tank and crane it over to the new hole (you would need a crane anyways for the delivery of a new tank), take the fill and put it in the old hole and be done with it. You could extend a bigfoot type form to the bottom of the excavation at the original location and build up from there to grade (to make sure you have bearing on undisturbed fill), to support the deck post.

 

[gte447f]

Canuck65:
Thanks for your feedback. I agree with everything you have said about not loading the tank with a concentrated load from a structure built over top of the tank. And your idea to relocate the existing tank makes sense as an option. However, to me, the appeal of abandoning the existing tank, leaving it in place, and backfilling it with concrete, is that it is the simplest approach, in that it requires absolutely zero construction activities on the existing deck structure (no shoring, no modifications to the existing framing, no excavation under/around the existing footings, no construction of new footings/piers, etc.) Therefore, it requires only one trade, a septic contractor, no general contractor/builder, no framer, no concrete sub. You dont't want a septic tank guy messing with structures or foundations. Have him install a new tank. Have him fill the old tank with concrete. Done. Simple. He can handle that. Don't let him mess with the structure/foundation, and no need to hire another contractor.

However, I will probably present multiple alternatives to the owner and let him decide.

Another point, anything other than abandon and fill with concrete is likely to trigger a requirement by the county for stamped plans from an engineer. That's more cost for the owner. So far, the engineering stops with an assessment letter.

 

[XR250]

I'm with JedClampett. Span an 8x8 timber or a cast-in-place grade beam across it an be done. Seems like the cheapest from a design and installation standpoint.