The plan was to bring the carpenter back at this point. We made arrangments, but the morning we were to start work, he woke up sick. With 4 adults ready to work, we didn't want to wait until he got better. What it if took weeks? We decided to get started and see how it went.
The yurt manufacturer has a document on building the platform, which includes this idealized diagram:
Way back before digging the holes, we set up batter boards to map the positions of the pier blocks. The idea was to use these to place the pier blocks correctly. This turned out to be rather difficult.
The batter boards were close enough to the ground that the strings bumped right in to the pier blocks and their brackets. The strings couldn't spring to their correct position that way. You could approximate the pier blocks positions, but that's it.
(Here's an example where the pier block was low enough to let the string pass without touching. Confusing, I know.)
One option would be to build a new set of batter boards substantially higher. Use a plumb bob to line up the new strings with the old, then use it again to place the pier blocks. I don't know how to do that without buying a whole lot of lumber and doing a lot of additional building for just this purpose.
This is all 50% overkill. That's because the placement of pier blocks in the North/South orientation is not critical. Most of them can be off by a couple inches with no effect on the strength of the deck; only the ones under a joint of two joists need to be aligned well, and even those could vary by an inch without trouble. There were 13 strings running East/West, to give North/South positions, which was lot of work for very little gain. I decided to keep the centerline and discard the rest.
However, the East/West orientation matters because we want the joists 4' on center, so the 8' plywood decking will align with it properly. The joists are 4" x 6" nominal (3.5" wide, actual). The edge of a sheet of plywood rests on only half of that, so ideally it gets 1.75" bearing surface. Being off by an inch would mean only having 0.75" bearing - too little! I figure that 0.25" was close enough, and decided to aim for 0.125" (1/8th).
We used the strings to give us the best East/West location we could get. We used the centerline and tape measure to approximate the North/South position. While doing that, we also added a little small gravel (3/4" minus) and/or sand to even out the surface that the pier block would rest on. I figured we'd adjust the pier block locations as the joists went in, to make the joists be 48" on center (within 0.125"), and to compensate for joists that aren't perfectly straight. I knew I would start installing joists, and then measure joist-to-joist and adjust for the actual outcome.
In my idealized model of construction, the ground surface would be perfectly level. This is really hard to create, especially in a hole. Dragging a level across a surface while keeping it level is just about impossible. At best, you can measure level at several points, shift the material around, and level again. It's iterative.
Anyway, this approach was pointless. The pier blocks were imperfect. If you put one on level ground, its top won't be level. The metal brackets aren't perfect either, so any post you put on the bracket is not going to be plumg, or anywhere near it. I figured we'd need to add/remove material under each pier block to get the post to come out plumb. We couldn't do that until the post was installed. Installing the posts means cutting them out of the stock 4x4, which means getting their lengths right. Cutting the posts to length means knowing they're in just the right spot. We couldn't know that until joists are up, right?
I'm just talking about leveling each hole relative to itself. What if you wanted all the pier blocks level with each other? That would be very hard indeed!
Now I know a trick that makes this slightly easier: the metal brackets can be bent. So, level the surface in each hole as best you can. Place the pier blocks accoring to strings and measuring tape. As you install posts, bend the bracket to make the post plumb. Now you don't have to add/remove material to plumb the post. I didn't figure this out until I was almost done.
Next: installing the posts.
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