What type of conduit must underground piping be encased in to withstand superimposed loads?

The selection of a conduit made of wrought iron, plastic pipe, or steel pipe for encasing underground piping is essential due to its ability to withstand superimposed loads. Underground installations are often subject to various external pressures, such as the weight of soil, vehicles, and other heavy objects above the conduit.

Wrought iron and steel provide structural strength and rigidity, making them suitable for high-load environments. Plastic pipe, particularly if it's designed for pressure applications, can also deliver adequate protection against physical forces while being resistant to corrosion.

In contrast, options such as flexible PVC or standard PVC may lack the necessary structural integrity to endure these loads in an underground setting. Open installation would not provide any protection against these pressures and is generally not compliant with safety codes. Hence, the most robust and compliant choice involves using a more durable material as outlined in the correct answer.