A. Shane Bryant, P.E., an engineer in Tooele, Utah, responds: For a recent lakeside-deck project that involved placing 32 posts in 3 to 4 feet of water, we started by hand-digging out as much of the lake bed in the location of each footing as possible. Based on the 1,000-psf bearing capacity of the clay soil, we used lengths of 8-inch schedule-40 pipe as forms, driving them about 2 feet into the lake bed with a sledgehammer and a portable jackhammer and checking for plumb as we went with a torpedo level.
Once the pipe forms were in place, we used a torch to cut them off a few inches above the water level. Just before filling them with ready-mix delivered with a concrete pump, we suctioned out the water. Once the piers had cured, we used epoxy anchors to fasten galvanized post bases to their tops.
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