Frost penetrating drainage gravel too deeply?
I just had a contractor warn me against installing drainage gravel the full height of the foundation wall (below grade).
He told me that here in the mountains of Virginia (Climate zone 4A, mixed humid), he had a client that once had their water line freeze solid next to the foundation, at a 5 foot depth – when our maximum frost depth is normally only 2 feet.
He figured out that the frost had penetrated extremely deep next to the foundation because someone had put drainage gravel the full depth of the backfill. And the open gravel allowed the cold to penetrate much deeper than usual.
Has anyone here heard of that happening before?
I was planning on installing drainage gravel most of the height below grade, with the exception of an underground gutter draining water away from the foundation below a 2″ thick layer of finely crushed stone.
However, the top of my footings are only 28″ below grade, and my drainage gravel extends to the bottom of the footings on the one side with the foundation drain. We also have dimple membrane as a water barrier on the poured concrete foundation wall for our crawl space, and a 2 foot wide eave overhang above.
He recommended backfilling with soil for the top 12″, to prevent frost from penetrating too deeply. I’m a stickler for good moisture control, which is why I had wanted gravel backfill for nearly the full depth. But now I’m concerned about the cold.
Any thoughts or suggestions about this? Thanks.
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Replies
I don't have an answer to your question concerning increased frost penetration, but a couple thoughts:
1) If you're worried about a water pipe, you could over-insulate with rigid foam (EPS/XPS).
2) My understanding (i'm not a foundation / soils expert mind you) is that a soil cap is usually desirable, regardless of the frost penetration concern, because it sheds surface water farther away from the foundation wall, actually reducing over-all water presence near the foundation (assuming it is properly graded). The drainage system (stone) below will still ensure water that moves in otherwise (laterally) has a path to exit (if you have drain tile).
3) It obviously helps if you have roof gutters with either system. You may also be familiar with underground roofs, which could potentially incorporate insulation and/or a secondary drain tile to bear the load of the roof flow.
https://www.greenbuildingadvisor.com/article/an-underground-roof
4) If you do do a soil cap, it should be separated from the drain stone by a filtering fabric to keep the fines from migrating into and clogging the stone.
Burying insulation over a pipe is a good idea. Since GBA hasn't yet implemented a bot to do this, I'm chiming in to say that XPS has a disproportionate climate impact and we shouldn't be recommending it.
Charlie,
I agree. If the only potential damage is freezing the incoming water supply, a $10 piece of EPS would solve it.
Hi Debra -
had never heard of this technique until literally just a couple of months ago when doing a building assessment in Manchester VT and a builder has an excavation company who swears by free-draining gravel from just below-grade to footing.
I am checking with an expert on below-grade assemblies and soils--stay tuned.
Peter
Thanks, Peter, I wait with baited breath! One contractor is ready to start backfilling next week, so I'm trying to make my final decisions soon.
Tyler, thanks for your suggestions. We are using filter fabric to keep fine soils out of the gravel, and we will also have roof gutters tied into a yard drainage system. The soil will be properly graded away from the foundation. We have a foundation drain embedded in drainage gravel next to our footing.
Though I didn't plan on using any soil backfill for a "cap", I did plan on installing 15 mil plastic below 2" thick finely crushed stone at the surface, and this would act as a cap to direct water away from the house.
Debra, frost penetrates most quickly due to cold air movement, so it makes sense that a course aggregate could allow air to penetrate deeply, since large, uniformly-sized aggregate has relatively large, interconnected voids. I usually recommend a 4-6" layer of clay or clay-loam mix at the surface to prevent water from penetrating, but it would also prevent air from penetrating. 12", as your contractor recommends, would also work.
When I specify "ground gutters" I include a layer of rubber membrane covered with course aggregate near the surface instead of clay/loam, which would also block air movement. Your 15-mil poly should do the same thing.
Another method of frost penetration is conduction through the soil (or aggregate), and stone does "move" frost more quickly than soil (with all of its tiny, insulating air pockets) but I don't think it would make much difference in this case. I'll be curious what Peter's soils expert has to say about it.
One suggestion, you mention "finely crushed stone at the surface." If you are planning to locate it where rain falling off the roof will land on it, it won't stay in place. 1 1/2" or 3" crushed stone won't wash away as easily.
Thanks for your comments, Michael. I will have a gutter on the roof. The 2 foot width of finely crushed stone around the foundation is mostly to maintain a dry, plant-free area around the foundation.
The shallow layer of crushed stone makes it extremely easy to remove any weeds that try to grow there. The poly barrier not only helps to block the growth of plant roots, but will divert heavy wind-driven rains away from the foundation. I've used this type of crushed stone before, and it holds up well to rainfall without eroding. (Though, as you warned, it wouldn't work well with water falling off the roof.)
Debra,
I too am interested to hear what Peter comes back with,
in the meantime:
It's possible you've already considered this, but what if you simply use your existing design (which seems well thought out) but with the addition of some insulating material under the poly. I don't think it'd need to be very thick or far reaching. Obviously this may change the expense and could introduce termite issues if we're talking foam (if termites are of concern).
That way you won't need to deal with the dirt and you can rest assured the frost penetration issue (no matter how real) is taken care of.
edit: by 'deal with the dirt' I mean to say, if you really don't want dirt there for any number of reasons (weeds being one mentioned). But Nathan is correct that a machine on site wouldn't see placing 12" of native soil on top as much to 'deal with'... and would probably be the cheaper and easier route if that end result is acceptable.
From a practical standpoint, it is easy to use a machine to spread a 12" layer of something.
The best way to control ground water is with grading, pipes and gravel are your backup. The surface around a foundation should be relatively impermeable and sloped away from the structure. 2" of anything, especially machine spread, is impossible to make impermeable.
I would listen to your excavator. He is correct to put down 12" of soil, but his reasoning is probably inaccurate. Ice lensing or frost heaving requires expansive soil, a water source, and freezing temperatures. If you exclude one of those, you don't have a problem. It doesn't matter if the gravel freezes. But, long term it is bad design to direct ground water around your house down to the footing, even if there is a drainage system down there.