Hi Maid Marion. Procrastinate no longer. It's time to make a plan and get after it.
You don't need earth bags or any fancy floor if you need to get the yurts up pronto.
Buy two HD cheap poly tarps at WalMart, Home Depot, Harbor Freight or Lowes. Buy the tarps a MINIMUM of two foot larger than the diameter of your yurt.
The yurts will sit atop the tarp. The tarp edge folds up at the wall under the cover, making a tub so water stays out.
Clear and flatten the site. Lay out the tarp. Erect the yurt on top of the tarp. Fold the edge of the tarp up the OUTSIDE of the wall, and tuck it under the lower ropes that snug the yurt. Trim the tarp to the height you want. The yurt cover drapes OVER the tarp.
One better than that is to lay two layers of plywood atop the tarp, each layer at right angles to eachother, and with no seams in alignment. Screw the top layer to the lower layer with plated deck screws. Screw length. If two layers of 1/2 plywood or OSB, use 1" screws. If a layer of 1/2 and 3/4, or two layers of 3/4, use 1.25" screws. The screw are at Depot. Buy them in 5 lb boxes.
To cut to size, find the yurts focal point and pencil the yurt diameter plus 1.5 inches. Cut the circle. Don't cut the tarp underneath. Erect the yurt. To keep the yurt from moving around on the platform, andchor it to the floor with small 'L' brackets about three inches long per side. They come in '2' packs at Depot or wherever. Roll out cheap indoor outdoor carpet for a REALLY nice floor. Your done.
Winters coming.
Good luck.