Question
Hello,

I was interested in sinking a trampoline in my backyard.  I have looked on the Internet and found some answers, but nothing talks about after you dig the hole for the trampoline, do you not have to have a retaining wall inside the hole that you are placing the trampoline in??  If you have any suggestions they would be greatly appreciated.
Thanks

Answer
Hi, Tiffany - Here is a company with a patented system.  They use a modified sheet-piling.  Their istruction manual shows how it assembles together:

http://in-groundtrampolines.com/installation.html

You don't need big concrete or timber retaining walls, but if you've already purchased the trampoline, then you'll need some way for the dirt to stay in place.  A substitute might be to use a rigid root barrier product, comprised of plasic panels that lock together in sections to form a circle.  

www.vesproinc.com, www.villarootbarrier.com, to name two; there are others.  

I'd add PVC bracing (irrigation pipe) along the upper and lower rims for added support -- See the installation manual, above, and try and match it as best you can.  Point the ridges of the root barrier OUTWARD - towards the dirt, so there are no sharp edges within the bouncing pit.  Each panel anufacturer is slightly different, but hte ridges are there to deflect the roots downward, but you don't need them in the pit.

A quick solution; Hope it helps.