beamdeckdrainagetrench
I am in the process of building a multi level deck where the second level would be low to grade.
As such, i had to dig some trenches (about 10-12" deep) as part of the beams will be below grade.
My issue is that since the ground is full of clay, this will cause a drainage issue within the trench whenever there is hard rain (see attached picture).
What can I fill this trench with to prevent standing water as well as not impact the beams from any heaving?
Below grade you should not be using EPS foam - that should be XPS (pink or blue, normally) - above grade EPS is fine. EPS is not waterproof, XPS is. You can also increase your subgrade insulation a bit by setting a sheet (or portion of a sheet) going flat or angled slightly downwards away from the house at the top of the footing.
If drainage is a concern (or even if it's not much of one) and you have the trench around the house dug, dig a trench away from the house to a lower point and place drainage tile (not usually actual tile these days - 3-4" perforated plastic pipe - the holes go down) around the footing and out to the lower point, and you'll have a solid drainage system in place. It's one of those things that should be done before the foundation is backfilled in the first place, as there will never be a cheaper time to do it, but rarely is. If filling with 16 grit sand, place filter fabric over the pipe to help keep the sand in place. Filter fabric is a good idea in general, but kind of critical with a sand fill. Pipe is already buried below the insulation, beside the footing, in the picture.
Unless it's going to be noticed by the authorities and cause you hassle for "illegally diverting rain water" (evidently happens in some areas) you might want to drain your foundation drains (and gutters, if any - otherwise it will mostly end up in the foundation drains) to a big buried tank (cistern) so you can use any water collected during the rest of the year, and only drain away the overflow, if any.
No handy references, (I was taught, as grunt labor, by a licensed civil engineer and former SeaBee, but I don't know which books it might have come from to him, or if any of them are on the web) but backfilling with something like road base (crushed rock including fines so it will pack well) and tamping it very well in thin layers (if you tamp 6" of fill, you only properly compact about 2" of it, so tamping 2" layers 3 times gets a better result.) Tamp with something fairly small - a large plate won't really compact as well (unless you rent a vibratory plate compactor.) If you are tamping by hand and it feels frustratingly slow, you are probably doing it right ;-)
Clay is virtually impossible to get properly compacted back into a trench - in non-bearing applications you generally backfill it and mound it a bit, then come back in year to fix the settling. Obviously not such a good idea in a bearing application, thus the use of a readily compactable material like road base.
Best Answer
You will need to dig these trenches deep enough so that the supports for your deck are situated BELOW the frost line for your area. No amount of fill or drainage will be sufficient since it's the freezing of the moisture in the ground that causes frost heave.
Get in touch with your local building codes department and get their specifications for how deep things should be in your area.
I'm also not clear what you're doing here with this trench. Generally decks are supported by piers which would be a lot less excavation than what you're doing.
If you actually intend the beams for your deck to be below grade, I think you're going to have issues. You should probably rethink your plans here if it's not possible to excavate the area and grade it so that water drains away from the area.