Most poured walls in a skatepark will be cantilever walls, where both sides of the wall are vertical.  Walls up to 5 feet high should be a minimum of 8 inches thick.  Walls higher than 5 feet should be a minimum of 12 inches thick.  Any poured walls should be centered on a poured concrete footing that is twice the width of the wall and 4 to 6 inches thicker than the wall with a minimum of 12 inches.  The purpose of a footing is to distribute the weight of the wall over a greater surface area.  This footing is what stabilizes the wall.  You can pour the footing without a form into cut trenches free of debris.  Always reinforce the wall and footing with rebar.  Consult local code for exact building specifications. 

Forms for the wall must be strong in order to withstand the substantial pressure of the poured concrete.  Build the forms from ¾-inch plywood with 2 x 4 stiffeners at least every 2 feet.  Place horizontal stiffeners first, and then nail vertical stiffeners across them.  Brace the vertical stiffeners with 2 x 4 strong-backs secured to the ground with stakes.  Assemble the forms with screws so that they can be removed easily after the concrete has set.  Pour concrete slowly up to the top of the wall.  Work the concrete slightly with a shovel or muck rake and tap the sides of the form to eliminate air bubbles.  If there are to be any embedded handrail anchors add them now.  Follow the manufactures instructions.  Finish the concrete as usual.  Keep the concrete moist as it cures.  After curing, remove the side forms and patch any imperfections.  Wait several days before backfilling with earth or gravel if needed.

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