Plastic tubing was pegged onto a loose sand-clay bed, topping rock and gravel. We then poured and trowled 1.5 inches of straw-clay slurry, which embedded the tubing. Once that dries, another 1.5 inches will be put down. The floor itself will be tiles. Finally, modern tubing used for hydronic flooring rarely leaks.
Tubing is flexible, and is laid without joints.
The only leaks I've ever heard of came from freezing.