Step 4. Install pier foundation

By foundation I am referring to the concrete base of the pier; the cap of an upside down mushroom if you will.  I started by putting one 30kg bag of gravel into the hole, packing it down firmly.  I then cut a section of steel fencing off the roll and formed it into a cylindrical cage of about 1/2 the diameter of the sonotube, so about 5".  On the bottom I made a little mushroom shape so that the metal cage would stand up in the hole by itself plus add some reinforcement to the concrete foundation.  I then mixed up one 30kg bag of concrete and poured it all into the hole.  I had a while to wait while this pour set, so I rested some boards against the cage to give some support, then extended the cage upwards to just below the height of my pier, about 48" above ground level.  I then covered the whole thing with a tarp and went to see Monty Python live via satellite with a buddy.

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