The effectiveness of screw anchors depends on soil conditions,
which I know little about. But there are people who do.
I've seen maps of soil conditions of my hometown;
ask at your town hall, county seat, etc.
Ask contractors, septic system contractors, building inspectors,
electric/telephone co. workers, or whoever in your area should
know about soils. I suspect in some parts of the country soil
conditions are very uniform. In most of New England the biggest
problem is getting the things further than 3 inches into the ground.
P.S. My screw anchors are strictly temporary and for only 30 or 40' of
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