An engineer friend did that in his greenhouse, PVC irrigation pipes in a concrete floor. It works very well unless the slab cracks and moves. In gravel this is less an issue.
A friend in St Louis dug down to have a 3 foot/0.9 meter deep layer of pea gravel as her greenhouse floor. Aside from excellent drainage, it absorbs a lot of heat on sunny winter days.
A cycad grower I know arranged his benches in long rows, with the surface closer to the ground than most orchid growers have them. He draped heavy plastic landscape sheeting over the benches, so it hung down to the ground on both sides, and was held down by bricks here and there. He put a high-output heater with powerful fan at one end. This provided bottom heat the entire length of a long series of benches.
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