Speaking as somebody with 45 years' experience mucking out . . . there's a new invention that has really caught on here. An enterprising manufacturer turns old tires into thick rubber mats about a foot square, that have tabs on the edges like a jigsaw puzzle. You assemble them and place them on the clay floor of the stall. Any liquid drains down between the mats, and a much smaller amount of straw or wood shavings is needed on top for absorption. Plus it cushions the horses' feet, and their elbows & hocks when they lie down. And you can haul them out and scrub them when they get impossible.
If it were MY stable, I would alter the clay floor under the mats and add drain channels with gravel . . . but it's not my stable. And I do like the mats, it takes next to no time to muck out.