I have used bituthene (sp?), a thick rubber sheet with a super strong mastic glue on the back side. With the larger 12" pipe, I use it to cap off both ends of the pipe. It won't leak, and when you need access, you simply cut it out with your pocket knife. This stuff is very tough and sticky, and it has not given me any trouble yet. I have recovered piped that were buried for over 2 years with no ill effects on the contents. I live in a rain forest like environment. Bituthene is used for many water proofing applications in the building trades.