Protect foundation construction though winter
I’m building (my own) house for the first time in Corvallis, Oregon (Climate zone 4a) with the help of my dad. There is heavy clay soils here. The site is a walk out basement. The tallest part of the excavated walls from final/existing grade down to the gravel pad where the footings will be formed is about 7 feet. The gravel pad is ¾” minus compacted to 90%+, and is anywhere from 6-12 inch thick. The engineered retaining wall footings are 12 inch deep and 5’3” wide, with narrower internal strip footings and a 16 inch wide footing on the norther walkout side of the pad. There’s approximately 11inches between the outside face of the retaining wall footings and the excavated clay pit wall (that will be backfilled with open gravel later).
I expect to have the footings poured in the next month. I’m strongly considering putting a winter halt on construction after that point till next spring due to my dad having other obligations for December and January, and he’s not only my only other labor help but also holds far more construction experience than myself.
Finally my question; If I were to leave these footings open to the elements over the winter should I be concerned about water/freeze damage, and if so in what form?
I’ve been brainstorming methods of keeping them as dry as possible. One idea is draping 6 mil plastic from the top of the surrounding grade, down the face of the excavated pit walls, and over the footings and terminated inside the footings. I’d use weights to hold down the plastic at both ends of the plastic and on top of the footings. I’d also temporarily fill in the 11inch void where the footing drain would eventually go so the plastic couldn’t be weighted down by rain water there. I could also place GPS insulation sheets between the top of the footings and the plastic. The GPS would be used later under the slab. The rebar verts could be capped and covered with more plastic, which could be taped to the other plastic at its base. The rain water that would be pushed to the inside of the footings would have a gravity path out the daylight side, as the top of the footing on that side is flush with the gravel pad the rest of the house footings bear on. I’d love to hook up footing drains too but currently I’m having an extremely hard time sourcing the necessary CB pipe saddles required to attach the drain pipes to the 10 inch private storm drain they need to feed into. It might be over a month until I’m able to get these saddles which is getting into our rainy season.
Is this all unnecessary, and would it be fine to leave the footings exposed from October till I’m able to form the retaining walls and backfill next spring/early summer? We get a decent amount of rain here and it dips into the high 20s often thoughout the winter.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The main concern is that the concrete has finished curing before the freeze sets in. Freezing is a problem when the concrete is still "wet", shortly after the pour, but once it's cured, the concrete itself doesn't really care if it freezes. You may have some other issues though, such as bracing wall sections to deal with frost heave of the earth. Do you have an architect/engineer involved with this project that could advise you as to any structural concerns with the walls here?
Bill
I wouldn't be too concerned about frost heaving. Those low temps are not all that low. Your idea to put GPS down over the footings would certainly help, at not too much cost. Just push it down over the rebar to set flush on top of the footings. Cover it with a layer of stone to hold it down and use the stone later as part of your backfill.
I'd be more worried about the clay walls of the excavation. If you get a lot of rain, those walls might just slump down over your footings and into the slab area. Cleaning that up could be a lot of work. I don't have much of a suggestion for how to stop that. Do everything you can to keep site water flowing around the excavation, not into it.
Your idea of draping plastic down over the face of the excavated walls might work, but would be subject to wind & weather damage. If you already have the space for the footing drains installed, why not let the plastic drape down into that channel and let the water run around to the daylight side? I wouldn't matter much if the plastic leaks - the footings can take it. You'd be most concerned about keeping the excavation walls dry as possible. If you dug a trench uphill of the excavation to help divert surface water around the hole, that dirt could be used to weigh down the upper edge of the plastic, making a dam against surface water at the same time.
Cheap landscape timbers can be used to "secure" a draped poly sheet. Wrap the lower edge of the sheet around the timber, tack it in place with some staples. That will usually hold things in place as long as you don't get any *really* high winds. You'll probably get some holes and tears in that sheet over a season though, so don't want to rely on it to make anything really waterproof.
Bill
Thanks for the input. Bill, it sounds like if freezing temps are in the forecast within 28 days of the pour I should put concrete blankets over the footings.
I understand the concerns about the clay walls surviving. about 70% of the pit excavation was completed this time last year. I put plastic down on top of the ground around the pit and draped down the sides (weighted at the bottom to stop wind from flapping it up). All the walls survived last winter this way, though that was a pretty mild winter. One side of the ground above the pit is a grave driveway pad and the opposite side only has 4 ft to the property boundary so the trench above idea in those areas probably isn't possible.
I do have a geotechnical engineer firm consulting. Their geo report outlined bracing, but the principle engineer told me in person during the pre construction site visit he wouldn't be too concerned about excavated wall collapse. Tomorrow I'm going to ask the concrete contractor I'll be using for the pour, who is also does excavation and general construction, what his opinion is on the pit walls.
Peter, when you say the footings can take the water you mean theres no concern of erosion under them because of the gravel pad and there's also no worry about water making its way into the micro cracks of the footing (especially where the rebar verts enter) and expanding when freezing?
You don't really need to do the pour 28 days before it's supposed to get down to freezing temperatures. About a week is safe, two if you want to be really careful. Remember also that the concrete makes some heat as it cures, and the ground is a bit warmer than the air, so it really needs to get *below* freezing and *stay there* for a while before you are really at risk of problems. It's common practice to just "tent" the concrete immediately after the pour (cover it with some poly with some air space), which will keep things safe unless you dip well under freezing for periods of hours or more.
Bill
Good to know Bill. Thanks for helping me learn. Your idea on landscape timbers weighting down poly is exacatly what I did to winterize the walls last year and it worked well. I used free stickers from the lumber yard.
What I meant was that the concrete won't be damaged by water, and to a certain degree, by freezing water. Like Bill said, after about a week, it is more than strong enough to take wetting and freezing.
Erosion is a different story. If there's enough water flow and velocity to scour out the soil under the footings, that would be bad. Not fatal if they are small areas, but this is still something you would like to avoid. Normal rainfall on and around the excavation shouldn't be a problem in most climates, but if the whole site drains through the excavation it might cause problems.
The majority of footings bear on a compacted 3/4" gravel pad. There is a one foot foundation step down at the daylight side, and those footings also bear on compacted gravel. The tops of those footings are flush with the the main gravel pad that the majority of the footings bear on.