Final answer:
Undue settlement is the factor that can cause a drainage pipe to lose its grade, leading to blockage or ponding. This occurs when the ground settles unevenly, disrupting the pipe's slope and causing water to stagnate, potentially accumulating sediment.
Step-by-step explanation:
The factor that will cause a drainage pipe to lose its grade and increase the possibility of blockage or ponding in the pipe is d. Undue settlement. When the ground surrounding a drainage pipe settles disproportionately, it disrupts the pipe's slope or grade. This can occur due to various reasons such as soil compaction, erosion, or the weight of overlying structures. A proper grade is crucial for the flow of water through the pipe and to avoid stagnation. Undue settlement can create low points or 'bellies' in the pipe where water collects, leading to sediment accumulation and potential blockage.
For example, factors like sediment accumulation may occur as a result of undue settlement, as when the grade of the pipe is lost, the velocity of the water may not be sufficient to carry sediment out of the pipe. This sediment can then build up, contributing further to the blockage. Although excessive water flow due to floods or pH level changes can create other drainage issues, they are not directly responsible for a pipe losing its gradient.