A bellied sewer pipe may or may not be broken but can still cause a list of problems. Pipes such as PVC or ABS will bend a lot before cracking. Pipes that do not flex, such as cast iron, terracotta, Transite, or Orangeberg tend to crack when put under pressure.
Bellied pipes create a “trap” of water, this area of standing water slows down incoming sewage and can then lead to a blockage. In worst cases the pipe breaks leaking sewage into the ground, and allowing dirt to enter the pipe creating a blockage.