I just had a dry lot put in over the last few weeks. I put some before and after pictures up on my blog, as well as a bunch of the construction process. You can see them here if you’re interested: http://thesmallhorsefarm.blogspot.com/2016/10/dry-lot-construction-with-lots-of.html
The person who did it also did my arena. He is a horseman as well as an engineer so he is very knowledgeable on multiple dimensions and has been doing this a long time. He stripped the topsoil then added stonedust and compacted it. I’ve asked him about geotextile and he has never recommended it as worth the cost. He did use flexible grids across the middle of the dry lot and around the lower edge, since it was built on a slight slope. The hope is that it will slow down water running across the surface and reduce erosion.
This (minus the grids) is the same way he made the small runs behind my barn, which have been in for almost two years and are holding up great. The stonedust is very easy to keep clean and there hasn’t been significant erosion or footing loss. My only concern is that it compacts quite hard, so I do want to put in a sandbox or something so the horses have somewhere soft to nap when confined to the dry lot.
This is all in contrast to when he made a feeder pad for me on a spot that was already muddy in the middle of winter. Since it was already muddy, he used a large quantity of CR-6 (basically gravel with fines). Then in the summer I added stonedust on top myself, because it was a pain in the butt to pick manure out of the CR-6 and not take all the rocks with it.