[TowerTalk] deep base for MA-550

[David Giuliani]

I'm installing a concrete base for a Universal Towers MA-550.  The mfg's dwg
calls for a base 2.5' x 2.5' x 5' deep in undisturbed soil, using 2.5 cubic
yards.  I passed this by my soils engineer, who's recommending a different
design -- drill a 2' hole with an augur, 25' deep.  Takes similar amount of
concrete, easier to make hole and not worry about backfilling, compacting,
etc., and much more resistance to being pushed over.  This would be a
deviation from the adage: "do what the mfg recommends", replacing it with
"do what the professional soil engineer recommends."

Any experience with this form (pun!) of base?  Advice?  

David Giuliani, WA6PXX
Mercer Island, WA