Jim,
I will strongly echo the other comments about separating the pier from the walking surface, especially for the light-weight design you show. I would suggest that you take the concrete part in your design and turn it upside-down, putting the wider base below ground and back-filling with drainable soil. You can then walk right up to the base of the pier with minimal vibration.?If you need a place to walk (and drop screws without losing them), and don't want a full slab, you could build a low wooden deck to walk on, protruding the cement pier to the deck surface so as not lose height. Also, make sure the bulk is below the frost level ... not such an issue in central Texas.
Here is my observatory build documentation for reference:?Ignoring the building and the dome, note the shape of the pier and the isolation. Best of luck as a solid pier is a wonderful convenience.
Edward
Edward Plumer