What was your instinct about building on bales? Are they for insulation? Are they compacted before you build? Do you expect any differential settlement, and have you seen this done before? Mold definitely = ewe, but I wonder how it might be done differently... thanks hierony for sharing!
I wanted insulation and portability--this way I'll only leave a brown spot instead of gravel or holes for cement piers when the yurt moves. And much easier to construct. Also fairly cheap. The forums have a few comments about straw platforms--I went and emailed Yves of Groovy Yurts for more details. He is very helpful.
I found a wheat/barley farmer who bales his leftover straw and sells it. The straw bales are moderately compressed: ~40 lb each, 40x18x16", $2.75 a bale (Eastern Washington, USA, Fall 2014). I got 75 bales for my 6m/20 ft dia yurt--an excess of maybe 10. I used 11.5 sheets of 3/4" OSB T&G, each ~$20, plus 1x3" or furring sticks (8 ft long).
My major mistake was making the platform blockish, with bales sticking out from underneath. At least the outermost bales should be in a ring, flush with your yurt/platform (knee/kick them to contour). The yurt then protects both the platform & bales from moisture (and thus mold). Any gaps/unevenness in the bales is stuffed with loose straw (such as the transition from square bale placement to circular). The OSB (plywood or wood siding/decking recommended for lower glue content & less mold susceptibility) and 1x3 connections distribute any weight onto multiple bales.
I'm not sure if the plastic moisture barrier would help or not. Because of the initial mold problem with the squarish setup, I didn't use the plastic when I reshaped the platform to circular. But I could imagine it would help seal up the yurt underside from drafts if placed properly.
As for settling, I haven't seen much. Some numbers I found on the totally-trustworthy internet: ~330 lbs/ft^2 for compressive strength, bale returns to full size after crushing to 1/2 original height. If the ground is level (ie, no 3" high ridges due to roots), the bales should come almost even and make wood platform-making easy.
The extra bales were actually really useful during setting up--just stack a couple up to the right height for crown ring/toono placement.
