Cover needed for line to leaching field
Porsche998
| Posted in Construction Techniques on
I am in the process of building a house in NH. My lot is similar to much of the state – quite a bit of ledge. To obtain an adequate site for the septic leaching field. the field was located more than 200 feet from where the house is being built. At least 90% of the 200 foot plus run has ledge less than a foot from the surface. Everyone has told me that it is cheaper to construct the line on the ledge and then fill in around it to get the 4 feet of coverage required to get below the frost line. My question is – are there any ways that I can have a properly functioning line and use less than 4 feet of cover? For instance with foam insulation or something?
Thanks
Replies
Are you sure the waste line has to be 4 ft down?
Unless this is under a driveway this is news to me. In any case you should ask your septic designer.
The designer just stated that "adequate" cover is required. I was thinking that you would want the line below the frost line but perhaps someone has other thoughts. Thanks for your info.
Ron
There are h20 requirements if you go under a driveway, but otherwise I have seen wastelines close to the surface. If you look at the specs for your field you will see that it is only a foot down.
What I would worry about is ledge doesn't run straight when you want it to, so good luck with the trench. It would be wise to use schedule 40, make sure there are no dips that collect water, and backfill with clean material.
And no, I am far from being a septic designer, so run any of this by your designer or installer. Good luck with this, ledge lots are always fun.
Ron--
The answer to your question depends on your specific design and NH's environmental regs. For instance, in Massachusetts pressurized system lines must be either below the frost line, or insulated; tops of all system components (e.g.: septic tanks) must be within 36" of the surface; minimum of schedule 20 PVC required, with schedule 40 mandated where automobile or heavy equipment traffic might occur; minimum pipe slope of 1/8" with 1/4"/foot preferred--the local boards of health do have the ability to grant variances. In non-pressurized systems, 12-18" of cover usually adequate...
In Massachusetts, all new on-site systems must be designed by a registered professional engineer or registered sanitarian. Obviously do not know NH regs or credentials of your "designer". However, with a 200 foot run, and operating in ledge, would suggest requiring him to specify the cover. Down these parts have never seen a plan where this was not done. Stating the obvious: the elevations for the entire system need to be integrated from the furthest outlet in the leaching field, through the distribution box, to and through the spetic tank, back to where the building sewer exits the building; and these need to be integrated with your final grading...Have seen where this exercise has been botched...
Hope this helps a little, if not, a bit more detail about your system might assist,
Regards,
Rework
My present home is in MA and as a PE here and in NH, I am aware of the general requirements but do not deal with sanitary systems. The NH regs require individuals designing the system to be aware of the regulations and pass the state test - but that is about it. The design provides all elevation information - that the distribution box will require baffling and show the lot topo but was told that their work was not to determine the amount of cover except where required under the regulations. The depth of the field, the setting of the tank is all detailed, but not the depth of the line. That is why I initially asked the question. The designer has told me that it is my choice- I can balst to put it below grade, insulate, heat trace or whatever works but that is not provided as part of the design as that would be past their expertise.
Thanks for your info.
Ron
Ron--
Hope I didn't put my foot in my mouth mentioning things you already knew...I was actually incorrect about even the pressurized systems, under Mass regs they do not have to be either below grade or insulated; they can also be "self-draining". Minimum specified cover over septic tank is 9"; over leach lines is 9" (excluding topsoil). Your common sense concern over effluent freezing is not directly addressed; (similar issue: what is the true effective area of mounded systems during the winter months?). Certainly not a PE; my father was and I installed many systems under his guidance in my younger years; including ones such as yours (i.e. ledge). General method was 1/4"/foot; and 18" of cover; never insulated.
Believe "biological" zero is around 41 degrees, and hopefully its warmer than that in the tank. Now wondering what distance of pipe actually allows enough heat transfer to achieve freezing. Leaching lines, at a lower slope, are generally limited to 50'; which may be taking this factor into account...Would be interested in hearing how you decide to handle.
Regards,
Rework
My tendency when there is a lack of information and I can correct the situation is trial and error. I will likely put about 24" of cover on the line on see what happens over a period of time. It has been suffested to throw some heat tracing on the line in case it starts to freeze and then when warm weather returns, put more cover on. I will probably put some instrumentation in place to monitor.
In Maine, there have been studies re footings, foundations and piers protected by foam board (Dow Blue2" R7.2?) Assuming the same four foot depth of frost requirement.
If you have a pier poured 18" deep on more or less gravelly soil (Forget how they specified it) and run foam horizontally out from the pier four feet all the way around and about eight inches down, You will be OK.That is because as frost works down, it also works it's way horizontally.
If we assumed that the same or similar figures would work in your case, you would need to place foam 8'wide for two hundred feet and cover it with a foot of soil. That seems expensive to me.
Alternatively, we might think of spraying foam around the drain line, especially on top of it, to help.
That would probably help keep the pipe and its transient tenants from freezing but there is another concern, in my mind. Qtrmeg is most likely right that the contents won't freeze solid in a waste line but you can have damage in another way.
If this ledge pans water under the drain pipe and that water freezes, it will heave and lift the pipe via the pressure created under it, even if the pipes contents don't freeze. that lifting pressure could break pipe or separate joints. This is not theoretical. I have seen the results on a couple jobs. It's less than fun to clean up after. Better to prevent when feasable.
Personally, my choice would be to
1.) make sure the trench doesn't hold and puddle water that can freeze. Move over and or drill/break rock to let it drain if it doesn't do so voluntarily.
2.) run the line in a bed of sand.
3.) rip some of that Astrofoil about 18" wide to cover it, on the theory that it will not only grant minimal insulation value and flexibility to stay with the pipe, but also will reflect radiant heat from the bowels of the earth to the underside of the pipe where it beds. This should help some without nearly as much cost as the massive amt of foam board and without as much agravation.
4.) cover it with as much soil as you can afford, looking for four feet, but willing to settle for about two.
I'm sure there's plenty to argue about in the above suggestion, but it offers a compromise between reality, cost, frost, and probably, landscaping.
.
Excellence is its own reward!
Ron,
I wouldn't put foam board over your leaching field. Here in Michigan they have put fields in clay and they work fine because the liquid evaporates upward as much as it leaches downward. I just put two field's in, in Northern Michigan, they're only 12" to 18" below the surface and that is considered the standard up here due to the high water table.
Definitely!
I was only speaking of foam over the line leading to the field. Evaporation is part of the design. That's why structures ar e not allowed on top of it..
Excellence is its own reward!