Rocky Plains Observatory
- Enough planning, finally its time to get going -
The first order of business was to accurately layout the locations of the holes for the six footers (for the floor) and the central hole for the telescope pier. Batterboards were staked and mason's lines layed out to mark hole centers. Then the fun started...
The floor footers required 12 inch diameter by 3 foot deep holes, needed to get well below frostline. I rented a 12" tow-able auger that worked out great, but I constantly hit 2-6 inch rocks that required me to remove the auger and break the rocks out by hand (one of the origins of the Rocky Plains in the name of the observatory). I also used the auger to pilot the telescope pier hole by drilling 4 holes which I broke up and dug deeper by hand (the auger only drilled 3 feet deep and I wanted the central pier to be 4 feet deep). The holes were all belled out by shovel, so that they had a wide footing to spread the load and further prevent the piers from heaving out by frost.
- Concrete Forms
The concrete footings and pier were fully reinforced and sonotube used for the above-ground concrete forms. The small forms were driven tightly about 12 inches into the holes (and then backfilled after pouring), but the central pier was a bit more complex. A 2 foot diameter form tube was used for the 18" above ground protrusion. This tube was fastened to a 4'x4' plywood collar, which capped the 3 foot diameter hole. When the central pier hole was filled to ground level, the form cap was added and the remainder poured. A plywood plate had been pre-assembled with the (6) 3/4" J-bolts (for bolting down the steel pier). This plate was also very flat and resulted in a smooth, flat top surface (I carefully leveled the plate as well). The central pier was reinforced with a cage of rebar rods and hoops (formed hoops are available cheap from local concrete supply houses). Batterboards were left in place until after concrete was poured to enable precisely locating post anchors to be pushed into the wet concrete.
- Concrete Pouring
Rather than mix 2 yards of concrete by hand, I chose to pay a little extra to have the concrete delivered (the difference in cost was less than $100 once you figure the cost of renting a small mixer). The tradeoff is that you need to be able to pour quickly, as the concrete company charged extra after 1 hour of site time. I tried to get everything completely ready to pour, lining the path with plywood and making a ramp and wheel stop at the edge of the pier hole for dumping. I assembled a crew of coworkers, bribed with the prospect of cold beer After completing the job... We had 4 wheelbarrows that were used to ferry concrete from the street back to my backyard (don't want the concrete truck driving onto my driveway, as they are not designed to carry that kind of load). I was a little nervous, but as it turned out the whole job went very smooth and fast (it only took about 30 minutes).
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