Cast iron underground pipes have been used for sewage and drainage systems for over a century, and their expected lifespan can vary widely depending on factors such as the quality of the installation, the soil conditions, and the level of maintenance and repair.

Generally, cast iron pipes can last anywhere from 50 to 100 years or more. However, if the pipes were installed in areas with acidic soils, corrosive groundwater, or high traffic loads, their lifespan may be reduced. Additionally, factors such as improper installation or lack of maintenance can accelerate the deterioration of the pipes.

One of the most common causes of deterioration and repeated backups in cast iron pipes is corrosion. Over time, the cast iron can rust and weaken, leading to cracks and leaks in the pipe. This can allow roots to grow into the pipe, causing clogs and backups. Corrosion can also cause the pipe to collapse or develop holes, which can cause sewage to leak into the surrounding soil.

Another common cause of backups in cast iron pipes is the buildup of debris and sediment. As water flows through the pipe, solid materials such as grease, hair, and food particles can accumulate on the walls of the pipe, eventually causing blockages that lead to backups.

Actually, the most important cause of sewer pipe backups, is the result of pipe corrosion which is accelerated on the floor of the pipe where it becomes wet most often. Over time, the pipe becomes so thin that a crack develops, and as the pipe corrodes further, the crack widens and eventually the thin serrated edges of the pipe catches toilet paper and other debris, which in turn catches on more debris causing repeated backups. The old solution to the problem would be to dig up the floor, dig down about 3 feet for the entire length of the pipe, then take out the old pipe and replace it with new pipe, then repair the concrete floor and restore the floor surface with new tile or wood, and replace all kitchen and bathroom cabinets that had to be torn out for this process. 

This normally takes about two months for 2000 square foot house, and it costs about $70,000. The alternative to this process is a more modern process, which is to restore the inside of the pipe using trenchless pipe lining technology. For the same size house, this would take about 3 days, and would cost about $20,000.

Finally, shifts or settling of the soil can cause the pipes to become misaligned or damaged. If the soil around the pipes is unstable, the pipes may shift or crack, leading to leaks or backups.

In conclusion, the lifespan of cast iron underground pipes can vary depending on several factors, and they can last anywhere from 50 to 100 years or more. However, corrosion, buildup of debris and sediment, and soil shifts are some of the common causes of deterioration and repeated backups in cast iron pipes. Regular maintenance and inspection of the pipes can help identify and address issues before they become major problems.