What type of pipe, if dewatered rapidly, has the potential to collapse?

• A. PVC
• B. Steel
• C. Copper
• D. Ductile iron

Answer: (B)

When water is removed from a full pipe quickly, the internal pressure inside the pipe can drop very fast, creating a vacuum relative to the surrounding environment. The pipe wall then has to resist external forces (soil, backfill, and atmospheric pressure) while the inside is near zero pressure. If the line isn’t vented or adequately supported, this sudden negative pressure can cause the wall to buckle inward and collapse.

Steel pipe is particularly susceptible in this scenario because its walls are stiff and strong in tension but don’t deform plastically as readily to accommodate a rapid internal pressure drop. That combination makes buckling under external pressure more likely when the internal pressure collapses quickly. More ductile materials, like ductile iron, can relieve some of that stress by deforming, while PVC or copper tend to fail by other modes rather than a clean collapse from negative internal pressure.