Design News for Mechanical and Design Engineers

A catastrophic gas explosion may have much the same effect as a major air raid. The gas may come from a faulty or improperly used gas appliance, from a leaking container, or perhaps worst, from a fractured gas main. The human and property loss due to the resulting explosion may be horrific.

Nearly thirty years ago there was a major gas explosion in a town Northwest of Boston. A cast iron gas main under a residential street had fractured and led to the explosion. Ironically, the explosion was not at the home nearest the fracture, but at the house next door. I was called in my usual role as a metallurgist, or "trained tin kicker."

I considered the usual causes for such a failure, including surface forces, improper backfilling, poor pipe quality, damage after installation, as by a back hoe, corrosion, and finally frost heaving due to formation of ice lenses under the pipe. The study eliminated surface forces, improper backfilling, poor pipe quality, and damage after installation as contributing to the failure.

The explosion took place in March after a winter cold enough to give rise to frost heaving. The geotechnical engineers calculated that frost heaving could not have fractured a sound pipe, but could fracture a pipe with a strength reduced by about two-thirds.

The pipe had fractured straight across, as opposed to having corroded through. Study of the fracture surface showed "chevrons" which pointed away from the top of the pipe to the bottom. A crack had thus started at the bottom of the pipe and proceeded around both sides till meeting at the top. Optical metallography showed that the pipe wall had corroded most of the way through at the point of fracture initiation.

The estimated reduction in bending strength was 80%, but the remaining 20% was more than enough to contain the pressurized gas. But add the frost heaving to the 80% strength reduction and BINGO!.

I concluded based on metallurgical and geotechnical evidence that the action of frost heaving on the weakened pipe caused the observed failure. I bowed out of the case after submitting my report, so I have no knowledge of any settlements.

The pipe had been in service for about 50 years so that heavy corrosion was to be expected. Was the pipe ever inspected for corrosion or other degradation? I do not know. Who, if anyone was responsible for such inspection? Here again I do not know. What I do know is that fractured gas mains may lead to far worse consequences than those suffered by the gentleman quoted at the start of this article.