drywell

On a rainwater drywell system where the water runs off from the roof to the ground where stone surrounds 4" top-perforated PVC pipe wrapped in fabric which connects to solid 4" PVC pipe for about 20 feet to a drywell. My question is: How far down do I have to dig to bury the 4" PVC pipe ( maintaining 1/4 inch slope per every foot ) below grade? Also should I surround the entire pipe length with stone and fabric or just at the perforated sections only and sand at the solid pipe sections or just dirt around the solid sections of the pipe? Thanks
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Gotta frost line?
The problem solves itself. You know the depth of the well do the math back to the beginning..
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