ABBEY ROAD OBSERVATORY
Telescope Pier Installation Blog
Pouring the rest of the concrete pier was a bit of a push. I thought it was important to do it all in one go so that the bond between layers was as strong as possible (I mixed and poured one 30kg bag at a time). That meant a long night for me as I wasn't able to get started until 7pm.
Before I started I swung by Home Depot again to get the rest of the concrete I would need, 5 more bags for a total of 9 bags (including foundation). That came to another $40 plus tax.
The process began by first making a quick little hat from a piece of the steel fencing that I put temporarily on the top of my cage. The hat was sized just slightly smaller than the sonotube ID, and its purpose was to keep the cage centered in the sonotube while I was pouring the concrete. With the hat in place I put the sonotube into position, and anchored the bottom with another 30kg bag of gravel evenly distributed and packed down. I then added wooden braces to the sonotube, confirming that it was plumb vertical. Now that the sonotube was secured I filled the rest of the hole around it back in using the dirt I dug out initially. I packed the dirt as I went, using a sledgehammer as a tamping tool. I made sure to double check the sonotube was still plumb along the way. With the sonotube now fairly secure I worked out the location of my 1/2" deck support rods, mount anchor bolt height, and concrete fill height, and marked it all on the exterior of the sonotube. I needed to trim 9" off the sonotube so that the mount anchor bolts would be at the correct height. I drilled holes and installed the 1/2" threaded rods, and pre-assembled the piece of plywood that would hold the four mount anchor bolts in the correct position when they were inserted into the concrete. Then began the ardous task of hand mixing up 8 bags of concrete, shoveling it into a bucket, shlepping it up a step ladder and pouring it into the sonotube. Once the sonotube was filled up to my preselected level I inserted the mount anchor bolts. I then covered the whole thing with a tarp and patio umbrella to keep it (relatively) cool while it cures. All this took about 4 hours to complete.
In noted that at the end of all the pouring that the sonotube, despite all the bracing, was still quite wobbly. I think that due to the mass of wet concrete involved, you can not brace too much. This morning when I checked it, the pier was solid as a rock...sweet!
Click on the pictures below to see a larger version.