More than 350 miles of brittle, accident-prone natural gas lines run under the Inland Northwest. Shorter sections of the pipe made from the same material join the steel mainline of gas to almost 7,500 homes, a constellation of connections across the region where there can be 55 per mile in Spokane.

On Wednesday, workers contracted with Avista broke through one of these gas lines in Spokane’s North Indian Trail neighborhood. Three homes were evacuated and no one was hurt, but the incident brought attention to the work started last year by the utility company to replace its entire network of lines made from Aldyl A, a polyethylene pipe made by DuPont before 1984. The brittle, easily cracked pipe is responsible for two local gas explosions in the past decade.

“We knew that over time we had to replace them,” said David Howell, Avista’s director of gas delivery. “We’ve always been about trying to identify potential problems.”

Maureen Loucks, a resident of the northwest Spokane neighborhood, said the work has taken longer than she hoped, but overall it was a minor inconvenience.

“Every time they seemed they were done, but then they’d come back,” she said. “It’s been a long project.”

Loucks’ neighborhood is among the first to receive the new lines, and Avista officials said all of Spokane’s neighborhoods will see sections of their roads punched through and dug up. Some might see their yards excavated. Howell said Avista tries to be as “surgical and minimal” as possible, but sometimes an open trench is the only option.

The replacement work is described as “proactive” by Avista representatives, and they note the utility already has replaced 31 miles of the plastic pipe and rebuilt 2,900 of the 7,500 connection points that need to be replaced.

But the work comes after state regulators required four Washington utilities to survey their natural gas lines and develop a 20-year plan to replace pipes at high risk of failure. Avista, Puget Sound Energy, Cascade Natural Gas and Northwest Natural Gas, which in total serve about 1.2 million gas customers, were required by regulators to submit the plans and begin work.

The total cost of Avista’s 20-year project is impossible to estimate, said Laurine Jue, a spokeswoman for the utility. But the next two years of the project, which focus on the riskiest sections of pipe, are expected to cost more than $10 million.

Avista already has removed and replaced 42,000 feet of the brittle pipe in Davenport, Washington, Another 32,000 feet of Aldyl A pipe was replaced in Odessa, Washington, where the plastic line was responsible for a gas explosion in 2008. Two people were injured and Avista was fined $200,000 by state regulators.

Another explosion occurred in 2005 at Cougar Mechanical in Hillyard, where a leaking natural gas line resulted in a man being engulfed in flames when he attempted to light incense. The man, Monte Yockey, eventually was paid $500,000 by Avista.

Joe Subsits, chief pipeline safety engineer with the Washington Utilities and Transportation Commission, said the Seattle area has very few pipelines made from Aldyl A, and the ones that do exist are of a smaller diameter that makes them less prone to cracking. Most of the replacement work is being done on this side of the Cascades.

Subsits said the state and utilities devised ways to prioritize what pipes should be replaced first. In Spokane, areas with more people, older pipes and rocky soil conditions have priority.

“We’ll replace the riskiest pipe first,” Subsits said. “You’re delivering a flammable material, so there’s always risk.”

Avista officials said the risk was no greater than usual, adding that the biggest risk came from people digging before calling Avista to find out if a pipe was beneath them.