Setting fence posts so that they're plumb and evenly spaced is part of the challenge. But surprisingly, even digging holes and filling them with concrete is fraught with difficulties.
The problem begins with the shape our holes normally take. The top of the average hole that you dig is bound to be wider than the bottom, since you have easier access to the top portion. The result is a V-shaped hole. Novices anticipate no problem in setting fence posts in such holes, then filling the holes up with concrete. But when you fill those V-shaped holes with concrete, you end up with V-shaped hunks of concrete. And in areas subject to frost heaves, this can be an invitation to trouble down the road.
The trouble begins when frost starts to heave your concrete upwards, using the tops of those V-shaped concrete footings to grip them (since the tops form a lip). Dirt then slips in beneath the concrete. Your fencing is raised as a result. The process is then repeated next winter, raising your fencing a bit further. And so on. Eventually, your concrete footings will be significantly elevated beyond where they should be, resulting in instability. Of course, such movement will never occur evenly along the whole course of your fencing, resulting in yet another problem: your fencing will no longer be level.
If you're still intent on setting fence posts yourself, rather than having a professional do it, the key is in filling the hole only halfway up with concrete. Even though it seems this would result in less stability, this is not the case. Instead, you're depriving Jack Frost of a lip he can use to get a tighter grip on the concrete footing. After the concrete sets, fill up the rest of your hole with your excavated soil (or gravel in damp areas), and tamp it down firmly. Another alternative is to buy one of those cylindrical concrete forms, available at home improvement stores. [Source: Lawrence Winterburn (GardenStructure.com).]
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