Drive equipment over septic drainfield?

[Richard]

I believe that Richard痴 field is done with what we call 兎asy lay here. It has perforated pipe that is surrounded with what looks like the Styrofoam pellets you use to pack fragile items and it is all held together with a wrap of soil fabric. The total package ends up about 10 round. This type of system has the advantage of not requiring gravel backfill and it does not have to be run in a perfectly straight line.

That description would blend nicely with the image in my memory (flexible ductwork that might be found in your home, other than being black)

I'm resizing pictures now, hopefully they'll help a bit?

 

[Richard]

Here's a side view. Builder said they couln't bring a top heavy concrete truck here with this angle (truck would be facing picture and would be leaning sideways)

Septic tank is on other side of tree and field is below tree, extending into the yard on far side

 

[Richard]

This is looking up

The red "X" is the approximate location of the current tank that they're going to fill in. The new tank will go to the left and will actually be a bit off the picture. Under (downhill) of the tanks is the drainfield.

Since the new tank will be to the left, you can see how the drainfield blocks the access. Also... the yard itself slopes down to a dropoff that is about 3' high so it's not easy bringing a truck up from the bottom. I don't think any of these pictures show the row of trees at the bottom of the yard

 

[Richard]

Looking down the hill, the house blocks some access, this dogwood blocks some access and then the drainfield blocks some access.

He told me yesterday he wants to take the dogwood down as that might give them better access.

We did everything we could to SAVE that tree so I'm not too keen on doing that, however, I did tell the wife last night that if he wants to replace the tree (his expense) then I'm ok with that

I might also offer to him that if he wants to replace the drainfield (with proper inspections when done) that he can rip it up all he wants. Again, this would be at HIS expense.

I was very clear about this situation prior to him quoting anything so if he's got some challanges now, he had ample warning and didn't do enough homework.

My feeling is, if I knew enough about this situation to 'warn' him upfront about these obsticles, (that he should be ahead of me on) then I don't really care what his challange is to get over them, nor his expense.

 

[Richard]

You can see here the dirt path where the elctrical has been routed around the drainfield. The electric is above the grass going to the left, almost out of the picture and then returns to the very near left. My transformer is right next to the tree who's leaves you see in the picture. Both (among others) are yet more items in the way of easily moving equipment around on my yard

 

[CurlyDave]

Richard:

MarkV very nicely describes what a grade beam is.

One reason I suggest it is that I have a hard time understanding how filling a septic tank with gravel and then pouring concrete on top of it is going to help with potential settling. If the gravel settles, the concrete on top of it will settle also. The concrete will not be reinforced, will be impossible to inspect during or after the pour, and will not be particularly strong. I think the concrete would crack into pieces which would settle with the gravel.

A grade beam is a poured-in-place reinforced concrete beam. The steel reinforcement can be easily inspected before the pour, it is supported by earth on both sides of the tank. A common construction technique with the advantages of being inspected and reliable.

In order to properly abandon the old tank, it must be filled to prevent anyone from falling into it, but there is no reason at all to use it as a support for your foundation.

Another possibility here is to think about mapping out a route for heavy equipment that goes over the end of your leach field furthest from the tank. If there is any damage that would produce the least damage.

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Just a piece of advice. You need to be careful about your relationship with your contractor. If he decides he can't ever make any money on this job, he could just walk away. You could lose whatever you have paid him so far. Especially if the contract doesn't spell out some things, like the dogwood tree. I know you told him about this stuff, far in advance, but if his interpretation was "it is OK to drive on the septic field" and yours is "it isn't", unless your contract says it isn't OK, it is not a clear cut win for you if there is a dispute.

 

[Richard]

I fear I've been a bit vague...?

The foundation will not be OVER my old septic tank at all.... the fear is the weight of the foundation might cause lateral pressure on the side of the (hollow) tank and allow the tank to cave inwards if it wasn't filled first. Then allow the foundation to settle.

That's why they want to fill it rather than remove it... they tell me if they remove it (other than I suppose it being gross) they will then not have packed soil in that location. If they simply fill the tank with gravel and "entomb" the gravel with a 4 or 6 inch layer of concrete on the top then that mass should (would?) be strong enough to prevent the sidewalls from ever caving in.

The tank is currently maybe 20' from the edge of the house, after we add the room, the edge of the house will probably be within 3/5 feet of the current (and soon to be old) tank but NOT on top of it.

Does that change any logic?

 

[horse7]

The equipment for the foundation (digging, concrete, backfill) is only the beginning. If those vehicles can't get in, what about the trucks for the rest of the building? They may have a higher ground pressure than a TLB (narrower tires for instance).

The existing structure looks multistory, so probably the addition is, usually (if you are sane) you want a boom truck or crane to get stuff onto the 2nd floor and roof but those need access... if you had access, you would not have a problem now. So it looks like whatever solution chosen ought to also apply to everything else that will be needed.
.

 

[Red55]

I fear I've been a bit vague...?

The tank is currently maybe 20' from the edge of the house, after we add the room, the edge of the house will probably be within 3/5 feet of the current (and soon to be old) tank but NOT on top of it.

Does that change any logic?

I'm NOT an engineer but the way your contractor is handling is O. K. the problem would be if you disturbed the soil or didn't fill the tank. Just a word of caution Curly Dave is right about your contractor. He's in the business and he will find ways to recover monery losses on your addition. It is much better to work with him that trying to put the screws to him for his own mistakes It's usually cheaper too! I always run into people who are looking to do it the cheapest way possible and I tell them that it is ALWAYS cheaper to do it right that to do it over. Had a customer that wanted me to use cheaper roofing materials to match a lower estimate ($350) and I asked her when the roof leaks will she blame me or the cheaper materials? She laughed and I ended up giving the job to my BIL -Ed

 

[CurlyDave]

If it were my job, I would use the grade beam, but there is a good chance that gravel fill plus concrete cap will be OK if the addition stays 3-5' away from the tank. Of course, if it were my job, I wouldn't be hiring a contractor, I would do it myself. My philosophy has always been that I would rather spend $500 on extra concrete and steel than spend $500 on engineering to prove I didn't need the concrete and steel.

There is a another area of non-compacted earth you should be aware of, and that is the backfill around the existing tank.

When the tank was installed, they dug a hole which was bigger than the tank, probably ~12" clearance on all sides, put the tank in, and then backfilled. It is almost impossible to compact that backfill, and in most situations it doesn't really matter. Over the years, it settles some, but it is never, ever, going to reach full compaction. Put a force on that soil in a new way, and it will settle some more.

The problem with building on unstable soils, and this soil is unstable, even if the rest of the property is stable, is that settlement issues frequently don't show up for years. In most states, a contractor is liable for his work for one year. By the time settlement shows up, he is no longer liable, if you can even find him. In fact, a lot of contractors build something, see that it is OK for a few months, and then go away thinking they have built something which was just fine.