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this is good to refer to.
Step 1: The foundation
for on the grass select a somewhat level aria of ground.
plot out a rectangle that is 8 ft wide 20 ft long.
level it all out make some footers out of bricks the bricks should in the ground so the half pipe will also rest on the ground but the bricks help it a ton.
It's very important that it will be level when it's done. and stay level for a long time.