I have not actually installed an in-ground trampoline myself, but I have done some research into how to do it. Having the ground cave in around the trampoline is a big issue. To avoid this you will really need some form of reinforcement to hold the earth in place.
Here is a great explanation of one family's in-ground installation using wood to support the walls of the pit, and how that quickly failed due to wood rot: http://simpsonsinboise.blogspot.com/2007/10/in-ground-trampolines-101.ht...
Here is a product from Australia that is designed to eliminate a lot of the problems encountered using wood reinforcement: http://www.trampolinesdownunder.com/pages/aboutthesystem
Here is a creative and cost effective solution that uses non-wood materials similar to the Trampolines Down Under product. There are also some good comments regarding why not to make the trampoline exactly flush with the ground surface. He recommends leaving a 2 in gap to improve airflow, and I absolutely agree with him: http://www.planetchipper.com/2008/08/in-ground-trampoline-installation.h...
Building in Ground Trampolines
I have not actually installed an in-ground trampoline myself, but I have done some research into how to do it. Having the ground cave in around the trampoline is a big issue. To avoid this you will really need some form of reinforcement to hold the earth in place.
Here is a great explanation of one family's in-ground installation using wood to support the walls of the pit, and how that quickly failed due to wood rot:
http://simpsonsinboise.blogspot.com/2007/10/in-ground-trampolines-101.ht...
Here is a product from Australia that is designed to eliminate a lot of the problems encountered using wood reinforcement:
http://www.trampolinesdownunder.com/pages/aboutthesystem
Here is a creative and cost effective solution that uses non-wood materials similar to the Trampolines Down Under product. There are also some good comments regarding why not to make the trampoline exactly flush with the ground surface. He recommends leaving a 2 in gap to improve airflow, and I absolutely agree with him:
http://www.planetchipper.com/2008/08/in-ground-trampoline-installation.h...
Here is a final site that has an excellent article on building in ground trampolines: http://cherkyb.blogspot.com/2007/07/inground-trampoline-design.html
I hope this helps.
Regards,
Justin