BB4TB,

Luckily the system racks sit right on a poured concrete floor / foundation.

However my neck of the woods (the Ottawa Valley) is famous for its Leda clay. To quote from a magazine article:

"This clay holds a lot of trapped water. And when you bang it or shake it hard enough, its particles lose their grip on each other. The whole piece of ground you stand on can turn almost instantly to something like wet jelly."

That's why I'm stopping at four subs

Jeff Mackwood