For the Horses

I'm curious about the footing under the rubber mats, as I've found that gravel under rubber mats in lean-tos at my two volunteer jobs (I feed and muck the stalls and paddocks of a total of 30+ horses per week, thankfully not all in one go, nor alone), gets compacted over time, and creates bubbles. I was wondering, if someone has any experience with a slightly graded concrete under the mats to create natural draining away from the shelter rather than pooling in there.