Have you used 'floating footings'?
Question:Am planning to do a deck, cannot drill the required depth for the posts due to a sewer line in the vicinity. Hear the 'floating' ones go down 12"...sand, then concrete, & the 4x4 post sits on this. How many inches of sand is good? How are the posts attached to the concrete block? How stable is it? Thanks :)
Answers:
Freezing soils not withstanding, if your sewer line is a private laterial to your home you can install deck over, but if it is within an easement that a jurisdictional body which performs maintenance on it, you cannot construct a permanent structure over it.
That said, you can call USA (underground service alert) at 1-800-227-2600, tell them you plan on digging footings within a couple days (no, you haven't done it yet, yes you will not be diggining within the next 48 hours, yes, you have marked the area with white paint (lath, stakes, flagging, etc.) ((white marking paint is standard)). Tell them your project, they will send out the appropriate responsible party to locate your sewer line within 48 hours. They will mark the location with green paint (for sewer), yellow for gas, red for electric, blue for domestic water, etc. Now you know where not to dig your footings.
NOTE: If your sewer line is private (septic) they will not mark your line.
If your proposed deck foundation element(s) are in the general vicinity (USA only accurate to within 3' horizontally). you still may need to hand dig your footings.
Regarding floating foundations (isolated pier) sand generally falls at 90% relative compaction, so consolidation shouldn't be a factor. The hole the sand is poured into keeps the sand supporting the pier block during sesimic events. Although gophers may have something to say about it. If you still want to try floating, dig hole 12", add sand poured from sack at 12"-36" free fall, tamp, wet and pour 4" concrete cap. Place (wet-set) pier block into concrete, let cure 2-3 weeks, construct deck. The footing spacing should be determined by the span of the deck and girder.
It depends on where you live. If the frost goes deaper than 12" then it will heave and move your footings causing problems in the yrs. to come. Where ever you are try to get the footings deeper than the possible frost line. Call dig safe and they will mark any lines that you might run into. You only need an 8" hole so if you can expand or lessen the size to avoid any lines you will be better off. Make sure that the holes are where you want them and when you pour the cement place an anchore bolt where you want the post to go. Make sure that the bolts only stick up an inch or so because you will bolt down a 4X4 post anchor that has a shallow cover for the bolt. Have a post anchor handy to see how it works. you can also fasten the post anchors with tapcon type screws in most cases but you will want a hammer drill for this aplication. Hope this helps and merry decking
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