Additionally, a neighboring residence suffered damage in the December 2, 2024 incident that occurred at 6:26 a.m.. At the time of the explosion, the weather was clear with winds of 11 mph; the weather was 50°F with no precipitation.
Atmos Energy Corporation owned and operated the pipeline involved in this incident, according to a report from the National Transportation Safety Board (NTSB). The pipeline consisted of 2-inch coated (wrapped) steel main running under an unpaved area near the western edge of Avondale Garden Road.
The superintendent called 9-1-1 at 5:09 a.m. and provided the same information to Jefferson Parish dispatch.
Damaged Valve Box
The gas valve mentioned by the construction superintendent was about 2 feet underground and about 340 feet from the accident home. The asset at grade-level was a valve box: A lidded, roughly cylindrical steel enclosure extending upward from the valve to the grade to enable access to the valve.
The first emergency responder on the scene was the assistant fire chief of the Herbert Wallace Volunteer Fire Company, who arrived at 5:17 a.m., followed shortly by personnel from the Jefferson Parish Sheriff’s Office (JPSO). When interviewed by the NTSB, the assistant fire chief said the superintendent showed him the leak and confirmed Atmos received notification. The assistant fire chief and local law enforcement consulted the Emergency Response Guidebook published by the US Department of Transportation and blocked off nearby intersections to prevent traffic from approaching within 300–350 feet of the leak.
An Atmos senior service technician arrived on the scene at 5:35 a.m., tested for gas in storm drains near the leak, and performed bar-hole testing between the valve box and 4th Street.5 These tests did not detect natural gas in the ground or drains. When interviewed by the NTSB, the senior service technician said these test results indicated there was “not a threat to life or property at the time.
He still attempted to persuade the construction workers at the site to cease work for their own safety, but they did not leave the scene.
Aerial view of explosion site.
Source: Atmos
During the same period, he obtained a license plate number for the truck that ran over the valve box, viewed a photograph of the driver’s license, and collected contact information for the driver’s supervisor for billing purposes.
Checking Sewer Next
During his interview, the senior service technician said he was planning to check the sewers for gas after recording the billing-related information; he said he had not been able to find a sewer manhole when he first arrived.
Between 6 a.m. and 6:15 a.m. an Atmos crew leader and an Atmos field construction coordinator arrived on the scene. At 6:17 a.m., the crew leader submitted an emergency locate ticket to the 811 call center and retrieved locating equipment from his vehicle to verify the locations of underground assets and support emergency repairs.
About 6:26 a.m., the home exploded and began to burn.
Emergency responders on scene responded to the fire, and Jefferson Parish dispatched personnel from additional agencies to provide aid. Over the next 30 minutes, emergency responders applied water to the fire and searched the accident home for survivors. The Jefferson Parish Sheriff’s Office transported four victims with burn injuries to local hospitals, the last at 6:58 a.m.
At 7:01 a.m., the assistant fire chief informed dispatch the fire was under control. Emergency responders conducted another search of the accident home and found a deceased victim in the kitchen.
After the fire was under control, emergency responders continued to smell natural gas odorant in the air. Another Atmos senior service technician arrived on the scene about 7:25 a.m. and used a combustible gas indicator (CGI) to measure the concentration of natural gas in a house adjacent to the burned out home and in its sewer cleanout.
Gas in Sewer
The readings indicated 100 percent gas in the sewer cleanout and 1 percent in the neighboring house.
Subsequently, Atmos employees removed nearby sewer manhole covers to vent the gas from the sewer.
During the emergency response to the fire and natural gas monitoring, other Atmos personnel continued working to shut off the flow of gas. They isolated the leak at 7:58 a.m. by closing two valves on a connected 4-inch main and squeezing off the damaged 2-inch main north of the leak.
They later restored gas service to much of the area by squeezing off the 2-inch main south of the leak and reopening the valves on the 4-inch main. By 8:10 a.m., the second Atmos senior service technician’s CGI readings in the accident home’s sewer cleanout and nearby structures all indicated 0 percent gas.
The explosion and fire occurred after natural gas leaked from a fractured buried main, migrated through an uncapped underground sewer lateral, entered the accident home, and reached an explosive concentration near an unknown ignition source. The investigation did not identify issues with gas pressures in the area of the leak or the timeliness or efficacy of the local emergency response agencies.
Postaccident bar-hole testing and excavations found only one source of natural gas: A leak in the gas main near a valve box on the eastern edge of the construction site.
On-site examinations of the main found it had a fracture around most of its circumference with a thin strip of metal at the top (12 o’clock position) joining the southern section of pipe to the section still threaded into the valve body.
This indicates tension – a stretching force – at the bottom of the pipe along its long axis.
Bending Stress
Tension along the bottom of the pipe is consistent with a bending stress created by an external downward force. Based on information provided to Atmos by the construction superintendent, a cement truck ran over a valve box at the leak’s location immediately before he heard and smelled natural gas releasing. The investigation did not identify another plausible external force that could have damaged the main.
Valve box after the accident (left) and October 2024 Google Street View image of valve box cover (right).
Source: NTSB
The Materials Laboratory examination of the main found fracture characteristics typical of fast crack growth and no signs of significant corrosion or preexisting structural damage that could have contributed to the failure. The fracture was therefore consistent with a sudden application of force by the valve box to the main.
Based on the fracture’s location, timing, and fast-cracking characteristics, the valve box transmitted part of the cement truck’s weight to the gas main, causing the main to fracture and leak.
Examination of the gas main and valve box involved in this accident found coal tar coating the main and deposited on the bottom edge of the valve box. This indicates that the valve box was in contact with the main.
The presence of coal tar alone does not indicate when the contact occurred, but the evidence is consistent with the valve box being installed in contact with the main and therefore able transmit the cement truck’s weight directly to the main near the fracture’s location.
Federal regulations require valve boxes not be installed in a manner that transmits external loads to mains, and valve boxes normally end up installed with a buffer of soil or other protection between the valve box and the assets immediately below.
New Valve Boxes
For example, Atmos’s newer valve boxes end up installed with support and base pads to prevent load transmission. However, Atmos was not actively replacing cast iron valve boxes with this newer design.
Furthermore, the maintenance crew that removed and reinstalled the valve box about two months before the accident did not have a specific standard to follow for reinstalling cast iron valve boxes other than an expectation the top of the valve box be level with the grade. The lack of a standard likely led to the common reinstallation of valve boxes in contact with mains, as described by an Atmos vice president of operations.
If Atmos had adopted either a standard for safely installing cast iron valve boxes or a policy of replacing cast iron valve boxes with the newer design, the protective buffer of soil or support pad would have reduced the load transmitted to the pipe on the day of the accident.
There is not enough evidence to determine whether a buffer or pad would have prevented the accident, but it would have made the infrastructure more resilient.
Atmos has not reported making changes to its valve box installation practices.
After the main fractured, gas propagated undetected through an uncapped underground sewer lateral into the sewer system and then into the incident home, where it reached an explosive concentration.
The gas in the sewers remained undetected because Atmos personnel did not test the atmosphere in the sewers prior to the explosion.
Checked Storm Drains
For about 35 minutes, most of the time between the initial leak and the explosion, there was one Atmos employee on the scene: An experienced senior service technician. He was equipped with a CGI and checked nearby storm drains for the presence of gas, which was one step in Atmos’s subsurface leak investigation procedure.
During his interview with the NTSB, he described planning to check the sewers, another required step, but said that he had already confirmed there was no threat to life or property. His activities after checking the storm drains are also consistent with a belief he had finished assessing the risk posed by the leak. He then switched to the non-safety-critical work of recording billing information for the damage.
The investigation did not find evidence of a natural gas alarm in the accident home, which did not have gas service but was still impacted by a leak from a gas main. It is likely that a natural gas alarm would have provided the home’s occupants with warning of accumulating natural gas and given them an opportunity to evacuate before the explosion.
Click here to read the full NTSB report.
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