Foundation near a Leach Line 3

[UofAGrad]

Hi Guys,

I had a customer request that I design an independently supported deck for their front yard, the issue is that there is a leach field that parallels the house 10’ away. This is where the outside of the footings will be. The county has setback requirements saying “all buildings or manufactured homes must be at least 10’ away from Septic Tanks and Leach Fields.” But the building official gave them my card and asked them to see if I was ok with footings there.

That being said, this seems like a bad idea for several reasons. But I was hoping to find some guidance for this in the IRC or IBC, seems like crushed stone below the footing and a lower allowable soil bearing pressure would be OK provisions. Does anyone have any thoughts, experience or literature they can reference to educate me? I did tell the client there’s a chance that he won’t be able to do this, but let me do some research first.

TIA!

 

[r13]

I'll keep my deck away from a septic tank and leach line as far as I can, or abandon the idea altogether, for sanitary concerns. But it is doable, if you can be sure the bearing will consistently be there at all time.

 

[SlideRuleEra]

UoAGrad - What is the presumptive load-bearing value for the soil? (That is for soil not close to the leach line.)

http://www.SlideRuleEra.net

 

[emmgjld]

In a similar problem, I put the footings as far out as 'comfortable' and cantilevered the deck about 4 feet to get the desired size. It required hold-downs on the main foundation wall. My project included a 'hot tub', partially on the cantilevered portion of course.

 

[UofAGrad]

SlideRule,

There is no soils report in this area so I use 1,500 psf in this area.

Emmgjld,

A cantilevered support is a great idea, minus the hot tub lol, this would provide a few feet between the footings and the leach line. Still, I would feel alot better about it if there is some design standard saying it is OK if these provisions are taken.

 

[jayrod12]

I would also use the provisions for manure retaining facilities when it comes to specifying and designing the concrete. Here where I practice this would be classified as A-1 exposure requiring significantly more cover, etc.

 

[r13]

No design code/standard/guide will tell you it is OK, or Not OK, to place a deck near a septic tank system, since the potential leachate is a matter of concern of personal hygiene rather than structural.

The cantilever is an excellent idea, as it pushing the footing farther away from the leach line. I think the leachate should be chemically (PH) neutral, as it is only water carries decomposed human waste. But, it can affect the surface soil, as it might always in a moist state, that is highly susceptible to frost heave action. However, the problem is not difficult to overcome. I suggest to follow ACI recommendation to provide adequate cover for a watertight structure, with the note that the leach line may disperse phosphorus (P) in nearby soil.

 

[SlideRuleEra]

UofAGrad - Rather than get (and pay for) a geotech report for a deck (which code appears to require for soil with <1500 psi presumptive load bearing value), I would combine several measures:

Use emmgjld's excellent cantilever concept to move the footing(s) away from the the leach line.

Per your comments, construct oversize footings (on crushed stone backfill) to cut soil bearing pressure to half or even one third of the presumptive value. Note: To my knowledge, "codes" do not "ban" a design that is (voluntarily) more conservative than required.

Address jayrod12's comment about potential rebar corrosion by designing footings for (then using) unreinforced concrete.

Finally, I'll suggest designing all footing the same... to be both conservative and to guarantee that oversized footing(s) are constructed where they should be (near the leach line). Note: Surely, oversized footings for a deck are not going to "break the budget".

http://www.SlideRuleEra.net