Observation pipes shall be provided. Such pipes shall be not less than 4 inches (102 mm) in diameter, not less than 12 inches (305 mm) above final grade and shall terminate with an approved vent cap.
The bottom 12 inches (305 mm) of the observation pipe shall be perforated and extend to the bottom of the aggregate. Observation pipes shall be located at least 25 feet (7620 mm) from any window, door or air intake of any building used for human occupancy. A maximum of four distribution pipelines shall be served by one common 4-inch (102 mm) observation pipe when interconnected by a common header pipe (see Appendix-A, Figure A-4).
Exception: Where approved and where the location of the observation pipe is permanently recorded, the observation pipe shall be not more than 2 inches (51 mm) below the finished grade.