By Brian New

DALLAS (CBSDFW.COM) – A dangerous combination of old pipes and bad soil is brewing underneath the ground where Atmos Energy crews are shutting off the gas to thousands of customers.

While a section of the gas mainline from the area was collected and shipped to the NTSB laboratory in Washington for examination, Atmos officials point to the dirt in the area as a possible accomplice to the leaks.

The North Dallas neighborhood sit on the Eagle Ford Shale where the soil is an expansive clay.

Jim Fontaine, whose company Tella Firma Foundations studies soil composition in North Texas, said when it rains the ground in the Eagle Ford Shale expands more than any other soil in North Texas.

“These soils can move up and down pretty dramatically,” Fontaine told the CBS 11 I-Team. “They are like a big sponge. When they get wet, they expand. When they dry, they contract. It’s not uncommon in certain areas of DFW for the soil to lift 8 to 12 inches up and down over a period of time.”

That kind of movement puts added pressure on underground pipe lines.

The pipes currently under the North Dallas neighborhood are steel and were installed in the ’40s and ’50s.

Atmos Energy will be replacing them with plastic designed to better withstand shifts in the soil and aging.

According to Atmos Energy Mid-Tex’s latest annual report submitted to the U.S. Department of Transportation, 43 percent of Atmos’ mainlines are still metal along with nearly one in five service lines that run into businesses and homes.

Also, according to the 2016 annual report, more than a third of Atmos Energy Mid-Tex mainlines were installed before 1940.

For years, Atmos has been gradually replacing many of the steel lines with plastic but for the Dallas neighbors, who will be without gas for weeks, the replacement will come all at once.