Re: Concrete pier


Arnold C. Ashcraft
 

I have a pier in my observatory that was installed in 2001. It consists of four 6 ft long 4x4’s of pressure treated yellow pine posts fastened together with timber-lock screws and liquid nails to make a composite pier that is 7 inches square. I dug a hole about 18 inches in diameter and 3-1/2 ft deep, put about 4” of gravel in the bottom and placed the wooden pier on top of the gravel. I then filled the hole up to ground level with concrete and built my observatory around it. The observatory is a simple 6’x8’ structure with a fold off roof and the raised floor not touching the pier at any point. There is no appreciable vibration of the pier and it has remained solid without any problem for 20 years, supporting my 10” Meade fork mount and the C11 fork mount reliably over the years.

Note that when I bought the 4x4 posts back in 2001, the chromated copper arsenical pressure impregnant (CCA) was still legal. It was banned in 2003 for uses involving human contact and has been replaced by alkaline copper quaternary (ACQ), copper azole (CA) or micronized copper azole (MCA). The posts you buy in Home Depot are treated with one of these safer impregnants. They are supposed to last a long time in contact with the soil, but I don’t know if they last as long as the CCA treated wood.

Clif Ashcraft, NJ, USA

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