When an underground gas pipeline is damaged and leaks, it is commonly referred to as soil gas migration. Studies show that gas from a damaged pipeline travels very quickly through the soil and is an immediate danger to nearby structures. Instead of slowly “migrating” through the soil, gas is forced under pressure through the soil to cause explosions. The force, regardless of the initial pipeline pressure, can enter structures within 50-100 feet or more almost immediately.
Gas smells have been recorded as far as 250 feet away within 30 minutes of backhoe damage to a gas pipeline. Explosions have been recorded within 15 minutes of pipeline damage. The word “migration” is not really an accurate term. Migration implies the gas slowly meanders through the soil. It does not. Simulated tests have shown where gas pressure from a leak site blew dust into a structure 40 feet away immediately upon initiation of the leak. This is contrary to common perceptions.
Once a gas pipeline is damaged, there is no time to waste in notifying people within a 50-100 foot radius of the damage, particularly if visible or audible signs of a leak are present such as blowing dust, bubbling puddles of water or hissing. These are indications that a large leak is present and people need to create a safe zone around the leak until it can be repaired.
The time it takes to cause an explosion is not dependent on how long it takes for the gas to enter the structure, but on the ignition source or sources and the gas concentration as it mixes with the air. One should not assume that the gas takes a long time to reach a structure!
David Heldenbrand is president of Bison Engineering, which has been collecting data regarding gas explosions and gas migration for over 30 years and has published several presentations on this subject. Contact David at firstname.lastname@example.org if you have any questions.