Cast iron pipe is still used in many areas for sewer lines, though it has been mostly replaced with either PVC or ABS plastic pipes. If iron pipe breaks, the broken section can be replaced with PVC and joined to the existing pipe with a flexible rubber coupling. Metal hose clamps are tightened around the coupling with either a screwdriver or ratchet wrench, to create a water tight seal between the two different pipes.
Dig a trench in the required area at the same depth as the existing cast iron pipe.
Measure and mark the PVC pipe to the required length. Saw through the PVC pipe at the mark using a circular saw --- make sure the cut is straight and perpendicular to the pipe. Scrape off all plastic burrs from the pipe.
Wet around the outside end of the cast iron pipe. Push a flexible rubber coupling half way onto the pipe end. Wet one outside end of the PVC pipe. Push the PVC pipe into the other end of the rubber coupling --- the coupling is now half way on each pipe.
Tighten the metal hose clamps surrounding each end of the rubber coupling. Loop a piece of metal flashing around the rubber coupling; secure the flashing in place with wire.
A handsaw or mitre saw can also be used to cut through the PVC pipe. The metal flashing helps protect the rubber coupling after the trench has been filled in. Make sure the PVC pipe and rubber coupling are the same diameter as the existing cast iron pipe.