Septic Tank Depth
Our septic tank didn’t get properly buried when our final grading was done. One of the two lids is just a couple of inches below the surface, and in fact because the soil is so sandy it’s blown that portion away. The (concrete) tank itself is about 3-4 inches below the top of the lid, if memory serves me correctly.
What is the minimum depth of soil I need to add? We’re on the warm edge of zone 6, with typical winter lows being -15C, occasionally down to -20C.
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Trevor, it varies by state. Here in Maine (zone 6) there is no minimum coverage required that I know of, but the access hatch needs to be within 6" of the surface, or a riser needs to be included to reach the 6" dimension. I know of several tanks that are only a couple of inches below the surface.
My only concern is the risk of freezing and damage to the tank, which it sounds like is not really much of a concern. We've already had all our inspections, so whatever code may be in effect isn't likely to be enforced.
At my previous house I had to dig down a full two feet to get to the lid for emptying, and it wasn't even marked so I had to guess its location and explore until I found it.
You shouldn't have an issue with freezing for a septic tank. When the tank is new, it's empty and won't have anything that can freeze. As the tank fills up with waste, the decomposition activity will warm things up just like a compost pile.
When I moved into my new house some years back, I didn't know exactly where the septic tank was located. I waited for winter, and when we go some early snowfall I went out and marked out the clear, snow-free rectangle where the tank was located. 2 inches or so of snow everywhere except over the now very clearly defined location of the tank. Later, as things get colder and more snow falls, the tank DOES get covered, but it will still be producing heat inside and won't freeze.
Remember that you don't want the tank buried too deeply since you'll need to expose those covers for periodic cleaning and pumping. Check with your local city for whatever your local codes require for cover, but it probably won't be very much. If you have issues with sandy soil on top blowing away, you can get a big planter as a movable cover and use that on top. Just be sure that whatever you do doesn't prevent maintenance access to the tank.
I am not sure how much heat is generated by the biological activity in a septic tank, but I suspect that an active tank will generate more than one in "vacation" mode. If you are a two person household like mine, then the total amount of warm water and "food" put into the tank may not be very much in general. Leaving home for a long vacation might result in stratification of the temperatures in the tank. Depending on soil types and moisture levels, your actual frost line depth might approach the set depth of your tank. If the frost line penetration matches the tank height, then the overall temperature in the tank might finally get to freezing.
I set my 56" tall tank with the bottom 6' below finished grade to ensure that ground warmth would always be above freezing point. The ground temp 6' down may only be 35-40 in the dead of winter, but that should be enough to keep an inactive tank from freezing. It should also help keep the little bugs more active when in use by limiting heat losses. The heated added by a warm shower is not terribly great when flowing into a 1500 gallon tank.
Taking an extended winter vacation when local temperatures are dropping down to -5F sounds a bit risky to me. Ice expands in all directions, which could exert pressure on the tank walls. I do not know whether this could crack the tank. The tank I have is quite thin in comparison to the ones I remember from decades past, so maybe someone out there can provide anecdotal observation about how strong the new thin tanks really are.
The inlet and outlet pipes may be of more concern as they too are quite close to the surface. My neighbor had to rescue a friend one very cold winter when the frost went deep enough into the ground to freeze all the water flowing from the house. The run was long and crossed a driveway, which together dropped the soil temperatures well below freezing. Oddly, I don't recall my neighbor saying if the outlet pipe froze up as well. Around here the word is don't put any pipes to or from your tank under a driveway. The reason proposed is that vehicles drive frost into the ground. I am not quite sure if that is valid, but I made sure that all lines in and out of the tank are well away from traffic. I think a bigger issue would be driving a vehicle across the tank top. Too little cover will not redistribute the foot print of a tire safely.
Snow has some insulation value, but it sounds like windy conditions might keep the snow thin like your sand has done. Maybe Michael can add real life observations about the inlet and outlet risk for your area. I am supposedly in a 6B zone, but we do get down to -20F at times.
The other side of the equation is that concrete tank tops have a weight limit and how much soil they can support. Most concrete tanks have a max rating of no more than 24" of soil on top of the tank. Anything more, can cause the top of the tank to fail/collapse. Definitely, never drive on top of it.
I'd be more worried about other parts of the septic system freezing. Especially during periods of no use.
If there is a concern anywhere (like a driveway), 1" of rigid foam does a lot more than soil.
I hope I didn't sound like I was promoting driving over septic tanks as a habit. I trusted all the details to others responsible for putting in the tank and field system. I was continually surprised by how unconcerned many were about getting near my tank with assorted equipment. The two compartment tank is fortunately supposed to be cast with extra high strength concrete. The tank is a bit special in that it has a lower profile than most of that capacity.
I did a bit of poking about and it seems that max depth of burial may be 3' with an additional limit of 150lbs a square foot. I will definitely look into getting more precise info. I also found that many tanks are as much as 6 feet tall, so I guess the risk of losing the ground warmth is less likely if not putting in a "shorty" tank.
To clarify the line freezing incident, the tank was safely inside the driveway circle. Just the lines going in and out were being crossed over. They were sleeved for crush protection, but apparently the act of driving across the line made the ground extra icy. Still no idea why the exit pipe did not also choke up. Maybe the lack of lumps. Guess it shows why good design and installation is so important.
To Jon R's point, I often wondered if my distribution field would be a block of ice as the depth is only 24". I peeked in the distribution boxes one winter to find that at least the output water was flowing freely. Can't tell what happened after that point, but all seems well with the system. It may just be that the local 48" frost depth in the code is just a CYA value for the building department.
I would contact the septic tank company and see what the specs are for cover on top of the tank. My septic tank company specs out 24" max soil cover. It's also worth pointing out that there is usually a 20% margin of error built into those specs, so they are conservative estimates.
I wouldn't bury the D-Box. Get a landscape box and enclose the D-Box under the landscape box. You can then put rigid foam inside of it to protect it during winter. Access to the D-Box is vital to see what is going on and to also check on things during winter to see IF the lines are freezing. I think it's bad practice to bury D-Boxes. I have access to my D-Box and I can also close off zones (4 chamber field) to let zones rest and recover.
Not to worry, I haven't buried the D boxes. The ones I had put in are much like deep landscape boxes with built in flow adjusters. They are set a few inches below grade, but not covered with soil. I used the infiltrator chamber style leach field so the 24" depth I was referring to is the surface to leach level difference. Maybe someday I will get one of those fiber optic inspection cameras and see what's happening out in the chambers. The tank company has closed, so for now I will just guard the tank zone. My over topping is only about 20" so I think it will be okay.
When we bought our 200+ year old farmhouse almost thirty years ago, it was a wreck. We had to hunt for the septic tank. It turned out to be a 4' wide metal cylindrical tank, with no leach field. The top had evidently rusted away, because some previous owner had covered the top with nothing but a piece of 2" thick Styrofoam. We'd been walking over it for months, fortunately without falling through.
When we had a new tank installed, we carefully measured the location with reference to a couple of house corners and recorded the info. The next owner was glad to get that information.