I've been asked to design a 12'x15' deck in upstate New York. The home (and fill in the project area) involved is about a year old. The fill is clean material with a large sand component.
The deck would be a conventional design with a beam (3-2x10's) running parallel to the house supporting the 2x8 joists which would have a 19" cantilever. The 15' side would be attached to the house. The 12' side of the deck would be placed along an adjacent sunroom. The deck beam would be supported by three piers placed on 16" footings, 48" deep placed in sonotubes. The distance from the top of the footings to the bottom of the beam would be around 4-1/2'.
The problem I'm looking at is that the end pier of the 16" footing of the deck would need to be placed around 12" from the existing 18" sunroom footing. I'm concerned that digging the hole for the 16" deck footing would threaten the close-by 18" footing of the sunroom. Maybe it could shift toward the hole made for the 16" deck footing, or even buckle at the point where the pier meets the footing at ground level?
The attached photo shows the sunroom and its piers. The deck would be placed to the left of the sunroom in the photo where the stairs are visible.
Does anyone have a sense if placing the new 16" footing (along with the need to hand-excavate for it) would be a problem for the sunroom footing?
If so, any idea how much of a separation between the deck and sunroom footing would be reasonably safe? I've already allowed for a cantilever of the beam to move the footing away from the sunroom as far as possible.
I could conceivably have the contractor stabilize the sunroom pier in question by placing a 2x8 across the bottom of all three sunroom piers while construction is ongoing, and direct him to compact the fill around the sonotube for the 16" deck footing sonotube before and after filling them with concrete. The 2x8 would hold the sunroom footing in place while the potential threat exists.