How to dig a post hole and secure the post
For any job where you need to sink a pole or post — such as putting in a letter box or setting up a shade-sail or clothes line — you’ll have to dig a deep hole, and there’s no way round it. So do it in the most economical way.
Short of hiring a machine post-hole borer — a heavy, ungainly wild animal — you’ll have to use your spade, so make sure it is sharp. (Sharpen it with a fine, flat metal file using similar strokes as described in ‘Sharpening a knife’, see 'Using tools correctly' in the Contents List of this website.)
There’s a trick to digging deep and emptying the soil out of the hole as you go. Cut a square hole in the surface of the soil, each side the same length as the width of your spade blade. Continue digging straight down, thrusting your spade to the bottom of your hole on all four sides in turn. Always keep the sides square and one spade-width wide. This way you can draw the loosened soil up out of the hole without it falling off your spade. When the hole is very deep you won’t have to bend down on hands and knees, face in the dirt, with a baked-beans can to empty the bottom!
Generally, the diameter of the hole should be twice the diameter of the post.
The hole should be either as deep as half the length of the post or at least 600
mm. If your hole needs to be wider than the one spade width, dig the hole to the required depth as above. Only then, widen the edges. There's no option now but to up-end yourself to empty the soil from the bottom of the hole.
How to secure the post
To position the post vertically in the hole, brace it with stakes on two sides. For a not-so-high pole, such as the one you’d place your letterbox on, it is often enough to ram gravel into the hole. A post-rammer (a tool most farmers possess) which comprises a heavy steel end on a long steel handle, is the ideal tool. A length of 100x75 timber could also work. Take care to ram the gravel evenly on all sides as you slowly fill the hole. Mix soil in with the upper layers.
To be sure of a tight fit, in the absence of a post-rammer, use a bag or two of ready-mix concrete. Read the instructions on the bag carefully, because some
require pre-mixing with water and others can be set easily by pouring the dry
mix into a set amount of water in the hole. To help you assess the amount you’ll
need and the cost of the concrete, use the following general guide. For posts up
to 1.8 m in total length, use one 25-kg bag. For larger posts use two. Easy!
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