We recently converted to public sewers after years of using a septic tank. It appears that the cast iron pipe that connects to the sewer is damaged. Most likely it's cracked, water is coming back into the basement. When we used the septic system the toilet was the only thing connected to it, the gray water went elsewhere. The pipe is probably damaged under the front porch which which can't be accessed. I have a field stone foundation. What is the most economical way to correct this?