In reply to pasbury:
Think yourself lucky you've only found one pipe! When I dug the footings for my garage I knew there would be a gas pipe, but not a dead gas pipe, a live water pipe, and a dead water pipe as well! All but the live gas pipe were galvanised and pretty much the same diameter. They were also very shallow (6" in one case) as there's no regulations for minimum depth on private property here. The excavation developed a smell of gas early on before we found the live gas pipe, but there was no leak. the gas board said the gas mains pressure is so low leaks are often undetected and the gas smell will accumulate in the ground and only become apparent when disturbed. Apparently the gas will discolour the earth and 'bleach' it somewhat.
Do your utilities not have a 'dial before you dig' service like ours do over here? Only a plum would dig without consulting them.