Re: ATM Lawn "piles"

[<dk@usmo.com>]

> Frost heave can be a problem, although here down South
> the frost depth is about 1/2".  Ideally, the concrete

It ain't just frost! You need to consider the type of soil you're working
in. Certain clay soils have a high shrink/swell potential due to changing
moisture content (all above freezing). With some clays, there can be
tremendous "hydralic" pressures generated that will produce effects that can
astound those unfamiliar with these particular soil type properties.

Adherence to the conventional wisdom of an isolated deep set pier (lots of
concrete!) destroyed my first observatory. In the present observatory, the
pier is "floating" five feet above the clay and the design also deliberately
traps two feet of standing water at the bottom of the gravel (to prevent the
soil from drying out/ shrinking). The system has been working well since '92
(longer than my first observatory was useful for the intended purpose).

If you find that your site has some of this type of soil (or you just want
to play it safe) use fine dry sand, *not* concrete, to backfill the "lawn
pile" holes.

Doug Kniffen