From my own experience (water problems aside) use a smaller pipe to
drive a pilot hole. If your ground mount is 1 1/8", use either a 3/4, or
at the most, 7/8" pipe to make a pilot hole. I tried using 1 1/8", and
ended up with quite a bit of "slop". 3/4" seemed to yield the best
results for me. Leaves enough earth to firmly hold the base, yet makes
it very easy to install without beating the heck out of the base Itself!

73
Dave
N0HNJ