A while back I posted a question about digging a tower base when bedrock
was (probably) just a few feet down. The tower was a U.S. Tower TX 455,
and required a 6.5 foot deep hole. It was likely that much of the 100
acres we had available was on the Niagara escarpment, with limestone at a
depth of about 3-4 feet. We got out a large excavator, and indeed hit rock
at 3.5 feet. Then got out a bigger excavator and tried to smash the rock.
Not only was it impossible to even dent it, but the surface was as smooth
as a highway. Not to be deterred, we took a rotary impact hammer with a
two foot bit and drilled 4 holes at 45 degree angles aiming toward the
center of the pit, and 4 vertical holes, then drove in 7/8 inch
rebar--potted in with hydraulic cement. Over that the prescribed rebar
cage was placed, although that had to be shortened with the help of an
oxy-acetylene torch. Three yards of 3500 psi concrete was placed on all of
that, and allowed to cure for a month. The tower was installed without
incident, and has since survived 60 + mph winds. Thanks to all who offered
advice.......
Chuck, N8CL
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