What type of piping must be installed with designated amounts of gravel?

Flexible thermoplastic sewer piping indeed requires the installation of designated amounts of gravel. This type of piping is often used in various applications due to its flexibility, corrosion resistance, and lightweight properties. When installing flexible thermoplastic sewer piping, it is crucial to surround the pipe with gravel to provide necessary support and facilitate drainage around the pipe. The gravel also helps in preventing soil from entering the piping system, which can lead to blockages and reduce the effectiveness of the drainage system.

The other materials listed typically do not require gravel for installation in the same manner. For instance, cast iron sewer piping is quite durable and has its own structural integrity, so it does not rely on gravel for support. Galvanized steel piping, while robust, is generally used in different plumbing applications and does not have the same requirements as flexible thermoplastic piping. PVC piping is lightweight and often installed in other ways that do not involve designated gravel amounts. Thus, the specific requirements for gravel placement with flexible thermoplastic sewer piping highlight its unique installation needs within plumbing systems.