Frost action on septic tank 2

[n1cq]

I have found a contractor with an interesting theory about frost heaving.

The scenario concerns a "monolithic" 2000 gallon concrete "tight tank" which is actually a precast "box" with an 8" thick h-20 rated lid which fits into the typical set of grooves one finds on standard "split" tanks. The only difference is that the groove is near the top of the tank.

This fellow claims that this past winter frost action had lifted the tank cap allowing water to leak into the tank.

He claims a 2"+/- lip (casting defect) overhanging the remainder of the tank body was enough of an area for the lens to form and push upward.

Next week I am going view the unearthing of the top portion of the tank and take a few photos for documentation purposes and my report of findings.

My major reservations of the contractors theory are:
1. Thermal heat produced by enclosed wastwaters would always maintain the outwall wall of the tank slightly above 32f.
2. The soils below the suspected ice lens having enough bearing capacity to react against the expanding ice.
3. The fact that this theory is an easy way for the contractor to skip out of a bad waterproofing job.

What do you think?

n1cq

 

[CarlB]

I agree with your point number 1, especially if the septic tank was in use and was provided with the usual amount of earth cover. I don't agree with your point number 2 only if an ice lense magically formed simultaneously under a few feet length of this lip. But ice lenses form perpendicular to the heat flow, which would be more radial (or at least hot horizontal) right next to a heat source like a tank. If a lense could be caused to form under the lip, it would first start in a small area. Pushing up on the lip would engage the weight of the entire lid and a frozen slab of earth over the top. I see the lip or slab cracking before the lid lifts.

Also shouldn't the lid have lowered upon thawing, preventing leakage?

So yes I agree with your point 3, he's trying to divert attention from the real cause, whatever that is.

 

[LHA]

I think it is a feasable scenario.

The frost heave would occur evenly around the entire lip, which would have a similar force-distribution as strapping it and lifting it with a crane. It is concievable that the lid could support this, even with a foot or so of cover...but I can see CarlB's point, it also could break.

After thaw, the tongue and groove may or may not reseat perfectly; odds are that it would not, due to the shifting of aggregate in the gravel. Even if it did reseat, the Mastic/seal would almost certainly be damaged. This alone would allow leakage, although it would be so minor as to have no effect on tank capacity.

The quality of the initial waterproofing application may be hard to determine...make sure they hand dig near the tank.

Decreased tank capacity is probably what lead to the investigation and discovery of this problem. Which means the lid has to be significantly (and visibly) unseated. From this perspective, your job should be easy...once agian, make sure they hand dig near the tank. If a bucket gets anywhere near that lid, you will never know for certain how it looked prior to excavation, and no definite/binding conclusion will be possible.

 

[n1cq]

Thanks for the input CarlB.

Actually this is a rather interesting heat transfer problem.

The boundary conditions are as follows:

Ground surface - say 0 f
Tank wall perpendicular to grade - 32f+
infinite depth - 50f

The tank has a 12" anti-buoyancy slab cast atop it and 12" earth over the slab.

The difficult things to model is the amount of heat (BTUs/day) generated within the tank.

I recall from pavement design years ago a method to estimate frost penetration as a function of time (involves soil type, heating degree days, ect.)

I would think a key question would be how long into the winter season did the leaking become apparent. Insight into this matter would allow one to distiguish between frost action and a lousy sealing job to begin with.

n1cq

 

[n1cq]

Thank IHA,

Good observation.

Yes, the presence of sand/gravel in the groove will be a key indicator of at least one a "lift/drop" sequence.

The weight of the tank top including the anti-buoyancy slab is 18,000 lbs - quite a bit. I don't think a plate rupture would neccessily occur and the effective unit load is only about 325 psf and the cover is rated for AASHTO H-20 traffic.

Given the fact the the installer repeatedly had to apply epoxy and hydrophillic sealants to maintain the seal leads me to believe a bad initial seal may be the culprit.

Bear in mind that the precast tank trade group defines watertightness as <1% tank capacity per day! That's 20 gpd in this case.

n1cq

 

[DwattedWabbit]

A possible new factor in detrimental freezing issues with septic tanks has begun to occur in the past couple of years. This past winter, there were several local instances of septic tanks freezing here in Massachusetts. Granted, it was a cold winter, but it wasn't the worst in recent memory by any stretch, contrary to the weathermen's doomsday hysteria. (Only rarely did temperatures drop below zero, this rarity being pretty unusual for a New England winter in my experience. One to three weeks of steady sub-zero temperatures, at least once during the winter, is more common - referred to as a cold snap.)

I've heard a hypothesis which argues that these instances of freezing septic tanks (which should be generating enough heat to keep well above freezing) are victims of the recent popularity of antibacterial soaps and detergents. Used excessively (which fad-prone Americans are wont to do), these cleaners not only kill bacteria on our hands and clothing and dishes when we use them, but are also swept down the drain into the septic tank, where they continue to kill the beneficial bacteria that breaks down septic wastes and which generate heat in a septic system. If the bacteria are dead, the wastes are not broken down, the system freezes in the winter, it fails to operate correctly, and so forth.

I didn't invent the hypothesis, but it sure makes sense. Perhaps engineers who are involved in septic design need to be explaining this problem to homeowners for whom they are designing new systems, to encourage them to worry less about bacteria. (How did the human race ever survive to get as far as inventing antibacterial soaps and detergents?)

 

[CarlB]

I doubt that excessive use of antibacterial soap, by itself, would kill off a septic tanks's bacteria. Maybe a bottle a day. Here's a web site that discusses it, and does say it can contribute to improper functioning. Site also has some discussion of freezing of septic systems.

http://septic.coafes.umn.edu/Homeowner/index.html