Posted 01 February 2013 - 08:54 PM
We rebuilt a falling-down deck a few years ago, that was built over a fairly steeply slanted backfill against a foundation, where the piers had been destroyed by frost action. There is a phenomenon called 'ground creep' that happens on hillsides, where when the ground freezes, it lifts perpendicular to the surface - it goes up at an angle - but when it thaws, it drops straight down. The piers we replaced, which were not re-enforced, had been pulled apart about 12-15" down, the bottom sections staying in place, the top parts having moved downhill in 20 or so years about 6", which of course made the 4x4 posts connected to them very crooked, and started the deck collapsing. There was enough dirt between the upper and lower part that at first we thought the initial piers were indeed only 18" tall, only after digging some more did we find the bottom halves.
Properly re-enforced ones would not have pulled apart, but in this hillside case, would likely still have begun to tip because of the creep action. We replaced the old 6" ones with 12" versions, with much larger footings, planning that the larger footing diameter would better resist tipping forces, having more area on the uphill side to press against the soil above. Because of the higher-than-normal side thrust on them, we bent 3/4" rebar into a 'u' shape, with 3 shorter pieces laid in the bottom of the U half way down the footing, spread out into a 6-pointed cross. So far, they're still straight up.