Which piping material cannot be used in underground gas piping?

• A. Copper
• B. Steel
• C. Aluminum alloy
• D. PVC

Answer: (C)

Underground gas piping has to survive soil conditions, resist corrosion, and maintain gas-tight joints over time. Aluminum alloy piping is not used for this purpose because it lacks the established code approvals and reliable performance required for underground service. Its joints and material behavior under soil pressure, moisture, and potential chemical exposure aren’t as proven as those of steel or certain plastics, which have long histories of safe underground use in gas piping. Copper, steel, and PVC/plastic options, when permitted by the applicable gas code, are considered suitable for underground installations, whereas aluminum alloy is not.