When I ordered a spa from Infinity, I got an e-mail with a link to the Owner's Manual. In it, there is a section on "Site Selection" on page 10.
Infinity Spa Owner's Manual
In the manual, the manufacturer recommends a poured concrete slab, however, wooden decking is also permitted provided it meets certain requirements as outlined in the owner's manual.
Regarding your specific question, the manual lays out various options for a foundation and bare ground is not one of them. They do allow the following:
Concrete Stepping Stones or Bricks: Another foundation choice is pre-cast concrete
stepping stones and/or bricks. Available inexpensively at any home center, they are simple to
set up. Depending on the size of your spa and the stones, two rows of four (8) or three rows of
three (9) is usually enough, if evenly spaced. Make sure they are well seated and level.
Good luck with your new spa. Please continue to post questions here - there are many experts here that are very helpful. Infinity is also pretty responsive - if you ask them a question, please let the board know your question and Infinity's answer so the knowledge can be learned by many.
Regards,
Ken
Hey Ken !
Thanks a lot - it is really help to make decision for me...
regards
A.Midak