 AML staff works on mine voids under the Hanna football field
DEQ’s AML Staff Work to Improve Safety Throughout Hanna
Hanna – For decades the town of Hanna sat above miles of open underground voids – pockets of space caused by underground coal mining.
Those voids can cave in (an occurrence called subsidence) creating potentially dangerous situations for people and damage to property.
The Wyoming Department of Environmental Quality (DEQ) began working with the town in the 1980s to mitigate the hazards. DEQ’s Abandoned Mine Land Division (AML) has completed a lot of work in the community with more to be completed.
Dave Pendleton, AML program manager, grew up in Elmo, Wyoming, just outside of Hanna (Hanna annexed it in 1979), and he remembers what it was like before AML came and began reclaiming the old mines.
“When I was growing up in the 1970’s, we used to go out east of town where the old mine spoils were. We would ride our bikes out around the mine spoils,” he said.
“There were motorcycle trails, and we would ride them into the bottom of those pits. You would pick up speed even with your brakes locked going down those steep hills.”
Mine spoils are the piles of dirt piled up near the surface strip mine where miners dug it out to get to the coal. The spoils Pendleton played at were from the Nugget Mine east of Elmo. His grandfather excavated some of those pits and placed the spoils with a 1 cubic yard bucket on a dragline.
“There were lots of subsidence features from the Hanna No. 3A underground coal mine out there, but at the time, as a kid, you didn’t know that’s what you were walking around on. It was very hazardous, but we didn’t know any better, nor did the adults as the mine was in production from 1905 to 1920,” Pendleton said.
He remembers finding one vertical subsidence hole that was 15- to 30-feet wide and 15-feet deep. “Evidence of subsidence dotted the ground’s surface between Elmo and the railroad tracks, hidden from sight by sagebrush until you were right on top of them,” he said.
AML filled in those holes around 2006 and 2007, long before David became AML’s Program Manager.
Subsidence holes can open up anywhere there are underground voids. People generally are unaware that the voids exist until subsidence begins to occur on the surface.
Old town Hanna is built above many voids – as were Hanna Elementary School and the football field.
Pendleton explained that Hanna and Elmo have had multiple underground mines, under and adjacent to the towns during the first half of the 20th century and several surface strip mines in the area starting in the 1940’s with the last closing in the early 2000’s.
The older mines, established before 1977, were not reclaimed by the mine companies because no law existed requiring reclamation. Today, mining companies must reclaim the land they disturb while mining.
Pendleton said AML staff injected sand slurry into the mine voids under old town Hanna in the 1980s, and although he was not working with DEQ at the time, he’s seen AML records indicating that 670,000 cubic yards of sand was injected to target certain areas.
Unfortunately, the sand slurry turned out to be an ineffective solution.
“As we later learned, the sand generally goes down deeper into the mine with the movement of groundwater, reopening void spaces in shallower portions of the mine where you really want the voids to be filled,” Pendleton said.
In 2018, AML staff began filling the voids with a cementitious grout that solidifies in place – a much better option than sand slurry – with the help of engineers from Brierley Associates.
Dave Hibbard, project manager for Brierley Associates that worked on the project, said the Hanna project is highly critical, not only to public infrastructure but public’s wellbeing. “The town of Hanna was historically mined to a huge extent, being at one point one of the largest coal mines in the U.S,” he said.
“There has been a lot of undermining in this town, and that naturally will, over time, have a tendency to collapse and propagate upwards, creating sinkholes that lead to damaged property, damaged infrastructure, and in some cases serious injury.”
In 2019, AML staff placed grout underneath a major powerline south of Highway 30 that had been built over voids from the Hanna No. 1 mine. Pendleton said a powerline pole was sinking an inch-and-a-half per year and eventually a hole opened up next to the pole. Pendleton said they later discovered the hole was 59-feet deep under the powerline pole.
The following year, AML and Brierley staff moved on to the elementary school and the football field.
Pendleton said they injected more than 17,000 cubic yards of grout underneath Hanna Elementary School.
Laura Niswender, principal of Hanna Elementary School, said the work under the school was incredibly important to the school and the town.
“Knowing that we’re not going to have potential structural issues because of unstable subterranean spaces; knowing the kids can play on the playground safely; and people can drive in our parking lot safely (is important),” she said.
Pendleton said they injected 30,000 cubic yards under the football field to stabilize it.
“You could see from the aerial photography where there appeared to be troughs of subsidence in the football field that school maintenance staff indicated they would periodically fill with dirt,” Pendleton said.
Hibbard explained that it was nearly possible to line the football field up with maps of the old mine and see the troughs align with the mine.
Niswender added that AML and Brierley staff also redid the field and the track for the school since it was undermined to such an extent.
Since the football field and track were going to be severely damaged by the trucks installing the grout, it was determined that a complete rebuild of the field was warranted.
“They met us halfway, came in, and regraded our football field because, apparently, it was sloped from one end to the other and redid our track,” Niswender said.
To do the grouting work, AML staff needed to set up staging areas to bring the materials to mix the grout. The staff selected three areas to do so.
The first location was east of the elementary school. A hollow steel line was used to push the grout from the plant out to each hole drilled in underground mine voids. The contractor would fill one hole and when finished drag the pipe and place more sections of pipe to the next hole
The second location was the site of the old Hanna High/Middle School, which a portion of the land housed the community garden. In order to use this location, AML staff relocated the garden to a new location in town, which Larry Korkow, Hanna’s public works director, said is actually a preferred location for the garden.
The third location required staff to relocate an unauthorized tire pile. AML staff asked permission from the Town of Hanna to use the lot if they cleaned up the tires and other junk in the area, which the Town agreed to allow.
Korkow said all the grouting work has been frustrating to Hanna residents who essentially have near- constant construction in their town. However, he added, “It’s good to have them here getting some of this addressed.”
Regarding cleaning up the tire pile and moving the community garden, Korkow thought that AML and contractors went well out of their way to make this necessary project have the least amount of inconvenience to the community as possible .
There have been other ancillary measures taken through the course of this project that have enhanced and added to the safety of the community.
AML and Brierley Associates installed grout under the Union Pacific rail line near Hanna. Hibbard said 16 to 35 trains transport goods using that rail line every day, making the voids under the railroad potentially dangerous.
A second access road into Hanna from Highway 30 was created by the AML program. This required constructing an overpass over the railroad and added another level of safety to the community. Previously, there was only one access into the town from Highway 30, limiting ingress/egress for emergency services.
In 2022, the Office of Surface Mining Reclamation and Enforcement honored the Wyoming DEQ’s AML Division during the National Association of Abandoned Mine Land Programs conference for their reclamation work at Hanna’s Elementary School, football field and track.
“It was an honor to receive this recognition,” stated Pendleton. “We still have a few more grouting projects and other work to do in the Town of Hanna before AML will complete remediation of the shallower mine voids, less than 300 feet deep, and start to monitor areas where the deeper portions of the mines are located for evidence of potential subsidence. The grouting work will take a few more years complete and the monitoring will last for many years to come. AML Staff, it’s Consulting Engineers and Contractors are very proud of the work that has been accomplished to date in making Hanna safer for its residents.”
For a complete walk through the Hanna project, please visit the Hanna Football Field and Track StoryMap here.
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