John & Susan Merrell
41390 Hwy 226
Scio, OR 97374
503-394-3790
503-551-7219 (cell)
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Installing Fence Posts

Installing fence posts is not a difficult task, but there are some basic things to keep in mind. This is true whether one is installing a mile of farm fencing, or a couple of hundred of feet of fence for a backyard. One of the most common mistakes occurs when ground expansion caused by frost and/or moisture is not taken into account.

Fence posts that are not properly installed will, over time, be pushed right out of the ground. How is that possible?

If you have ever squeezed a watermelon seed between your fingers and shot it across the room then you understand the principle behind this phenomenon.

When ground freezes it expands. The expansion of ice is an irresistible force. Over time it will wear down mountains. A lowly fence post stands little chance against such force. But, it is possible to install a fence post so that the expansion of the ground will not gradually remove your posts that you worked so hard to install.

For our farm fencing we use round, treated posts. Round posts almost always have a big end and a small end. The first trick to having stable fence posts is to put the big end in the ground. As you can see in this diagram, frost expansion will try to force the post deeper into the ground (like a watermelon seed shooting back towards you when you squeeze it!). This is even more true if the post extends to below the frost line, since expansion pressure will then be almost entirely lateral.

fence post diagram

Freezing can force a post out of the ground (Taper of posts is exaggerated for clarity)

If the small end of the post is put in the ground, the expansion of freezing ground will tend to push the post upwards. (This will be amplified if the post does not extend below the frost line.) This upwards movement of the post will create space below the post. When the ground thaws, dirt will fall from around the post to the bottom of the post hole. Each cycle of freeze and thaw will lift the post a bit, until the entire post becomes unstable and falls over!

Posts set in concrete are even more susceptible to this phenomenon if they are not properly installed.

Picture the industrious fence builder who carefully mixes concrete to pour around their fence posts. The natural tendency in digging a post hole leaves it smaller at the bottom than at the top. Wanting a nicely finished appearance, the builder puts the post in the ground, fills the hole with concrete, and then fashions a nice, rounded to to the concrete. This usually leaves a lip of concrete above the ground.

In this diagram we can see what happens through a series of freeze and thaw cycles.

frost heave

Frost heave is exaggerated with improper use of concrete!

Not only is lateral pressure applied to the post and its concrete base, but the vertical ground expansion under the lip of concrete amplifies the upward motion of the post. As the concrete moves upwards, its rough edges dislodge surrounding dirt, which falls to the bottom of the hole.

Over time, the fence post is literally lifted right out of the ground!

A better technique is to dig a bell shaped hole - larger at the bottom than the top. Also, the concrete should not extend above ground level. Doing this insures that lateral expansion tends to force the post downwards, and also insures that there is no lip for the expanding ground to get a hold of to lift the post upwards.

proper post hole

A bell shaped hole makes for a stable post.

Following these tips will help to build a fence that is sturdy and lasts for many years with the minimum amount of maintenance.

 
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