SWEETWATER COUNTY — Many towns in Sweetwater County were built, literally and figuratively, on the coal mines below them.
Coal mining arrived in the area, along with the railroad, in the 1860s. Towns and coal camps sprung up near and often above the mines. The area prospered as workers extracted the cheap, abundant energy source that was in such high demand. The majority of mining was completed by the Union Pacific Coal Company along with some independent companies.
Those same mines that brought economic prosperity also caused damage to the lands above them. Some of the first evidence of mine subsidence was reported around the turn of the century in Rock Springs. More than 100 years later, Sweetwater County is still confronting problems caused by mine collapse and fires.
There are 78 known historic mine sites in Sweetwater County with 20,000 acres undermined, according to Doug Beahm, owner and principal engineer/geologist for BRS Engineering out of Riverton. Some of the county’s extensive undermining is located under infrastructure, some is in developed areas, and most is out in rural areas.
All of the underground coal mines in the area had closed by about 1964. When the mines were abandoned, open rooms were left behind from which the coal had been removed. These areas were supported by large wooden timber props or unmined coal pillars.
Over time, supports failed, and mine roofs collapsed. The voids and rubble created then began to migrate upwards, eventually causing the ground’s surface to sink. Sinkholes, known as open mine subsidence, started to appear. In some areas, the results of subsidence surfaced as a shallow depression in the ground.
Subsidence has caused damage to structures, including homes and businesses, as well as infrastructure in both developed and rural areas in Sweetwater County. Past problems with mine subsidence in Rock Springs even made it into national news, including the New York Times and Los Angeles Times.
Mine subsidence mitigation by the Wyoming Department of Environmental Quality’s Abandoned Mine Land Division dates to the early 1980s and continues today. AML works with consultants and contractors to reclaim mining lands and mitigate subsidence issues throughout Wyoming. BRS Engineering has managed much of the mine reclamation work done in Sweetwater County along with local companies including Wilbert Engineering and Western Engineers & Geologists.
In rural areas, the AML mitigates mine subsidence features which manifest at the surface and pose a physical hazard. Typically, this is accomplished by the excavation and backfill method. Rural regrading projects also involve drainage stabilization to restore a more natural drainage pattern that promotes vegetation. These types of projects have been completed in the past in areas including Lionkol Road near Rock Springs, Superior and Winton.
In developed areas, structures are often impacted before mine subsidence manifests at the surface. Drilling and grouting is the usual mitigation method in these areas. Grouting involves the injection of a cement grout under low pressure into the shallow voids to reduce the potential for future mine subsidence that could impact structures or infrastructure, according to the 2017 AML magazine. The AML also does investigative drilling of abandoned coal mines to identify potential future subsidence issues before they affect the surface in developed areas.
New areas of mine subsidence are often discovered during site visits. These visits are conducted at least annually at 49 separate mine sites in southwest Wyoming that are known to be prone to ongoing subsidence issues.
Once projects are planned, they are advertised to potential contractors for three weeks prior to a mandatory onsite bid tour to be sure they are familiar with site conditions before preparing bids, Beahm said. Due to COVID-19 restrictions this spring, an online meeting service was used in place of onsite bid tours. During the virtual bid tour, contract requirements were explained and potential contractors could view ground and aerial videos and photos of the work areas.
Approximately 3,800 acres of land are undermined within Rock Springs city limits, according to a BRS Engineering presentation on active and planned projects in Rock Springs. Of those, approximately 440 acres have been mitigated in areas of shallow undermining.
AML mine subsidence work continues in Rock Springs and Sweetwater County with several projects scheduled in 2020.
ROCK SPRINGS 2020
In Rock Springs, investigative drilling for 2020 started April 20. The procedure includes drilling and site mapping. The main focus will be along the No. 1 and 7 coal seams and will involve mainly city rather than private property. Depending on the results, AML may recommend mitigation of areas with a higher risk for future mine subsidence impacts.
A drilling and grouting project at Canyon View Estates is expected to begin in July and will include the area around the First Congregational Church. A project involving the area of Ahsay Park, the Sweetwater County School District No. 1 Head Start building and portions of Miller and Tisdel Streets and Ahsay Ave might also begin the summer of 2020. AML is awaiting archaeological clearance to proceed.
The Wyoming AML program has to comply with environmental protection laws. This requires an assessment of natural resources on the site, including cultural and archaeological resources. Artifacts encountered may be historical or prehistoric and are avoided during construction.
Other areas in Rock Springs that are on the list for future mine subsidence projects include Thompson Street and the Bunning Park area. Emergency projects are always a possibility as well.
BRS Engineering is contracted for two infrastructure protection projects in Sweetwater County in 2020.
One project includes portions of two Kinder Morgan natural gas pipelines south of Rock Springs. Drilling and grouting work will be completed to protect the pipelines. Construction is set to start in June. It has been named the Stable Lane & Sweetwater and Rainbow Mines Pipeline project. Stable Lane comes off the south belt route, and there have been a variety of mine subsidence projects in the area over the years.
In the fall of 2019, drilling in the project area revealed mine rubble that reached within five feet of the ground surface. An open subsidence was also discovered between the two pipelines during a spring of 2020 site inspection. The sinkhole is about 30 feet deep and measured around 11 feet in diameter.
“It underscores the need for this type of a project to protect the infrastructure,” Beahm told Sweetwater County commissioners in April.
Project 17.6B-BRS-3D, also known as the Rock Springs No. 7 & 9 mine pipeline project, includes portions of the Marathon and Phillips 66 pipelines north of Rock Springs and north of Interstate 80 in areas undermined at shallow depths. It is a high priority project since it has been discovered that mine workings in the eastern portion of the project are on fire. Construction is expected to begin in June on the drilling and grouting project. Portions of the mine fire to the south of the project have been mitigated in the past.
Fires can start in abandoned coal mines by spontaneous ignition when oxygen and water are introduced to mine workings through subsidence cracks and pits and unsealed portals or shafts, according to Geological Survey Professional Paper 1164. Beahm said mine fires can also ignite underground due to mine gases, sparks from machinery and other causes. Fires can start in coal slack piles due to human activity or lightning.
Many mine fires have been burning for decades in Sweetwater County and one for nearly a century, Beahm said. The Reliance No. 1 mine fire started underground in 1926 and is still burning.
Most mine fires are monitored rather than mitigated. AML has limited funding and prioritizes projects based on risk to human health and the environment, Beahm said.
Site investigation is one method of monitoring mine fires, which are most active and visible during the winter months. Elevated ground surface temperatures generally keep the ground above mine fires clear of snow cover. Gases and steam are visible at times.
Differences in ground temperature are measured physically and using Forward Looking Infrared (FLIR) cameras mounted on drones. Drilling is also used to determine fire conditions in an underground mine.
When a mine fire is reasonably close to the ground surface, 30 feet or less, Beahm said the burning material is excavated and mixed with local earthen material to cool and smother the fire. In some cases, mine fires have been extinguished by injecting cement grout into the mine workings from drill holes. That method is more expensive and used only when the fire is in immediate proximity to infrastructure, according to Beahm.
Portions of the Rock Springs No. 7 & 9 mine fire have been excavated and backfilled in the past. In 2020, the portion of the mine fire potentially impacting pipelines in the area will be extinguished with drilling and grouting.
Mine fires currently active in Sweetwater County include Rock Springs No. 7 & 9, Reliance, Blairtown, Gunn-Quealy Sweetwater No. 1, Rainbow No. 5, Dines and Winton. The most active fires are the Rock Springs No. 7 & 9 and the mine fire at Dines. Inactive fires include Interstate Camp Mine, Rock Springs North and Superior C Mine.
A mine fire under a county road near Reliance was extinguished during a previous project. Grouting of east Reliance residences was initiated in 2016. Subsidence mitigation work in the small town has continued since then and is expected to be completed this year. The 2020 project started on April 13 with 119 days left on the contract. Mitigation efforts in Reliance have often taken place in residential areas.
Work in Reliance has moved from east to west because mine workings get deeper from east to west, Beam said. One portion of the mine seam is 12.5 feet tall. Upon completion of the 2020 Reliance project, the county road used for primary access to the mitigation site will be improved in cooperation with Sweetwater County.
SAFEGUARDS AND MONITORING
Before and during mine subsidence operations involving drilling and grouting, the AML works with property owners and monitors the effects of work being done.
Conditions are documented at the site of each operation before work begins, and the site is restored to pre-existing conditions after construction. Adjacent property owners are notified of mine subsidence operations. If the work site is on private property, consent is obtained from the owner. The AML offers mine subsidence insurance to property owners at no cost for one year while work is being completed.
During operations, ground vibrations are monitored with a seismograph and recorded continuously between the work area and the nearest structure. Seismographs record the hourly peak vibration and any event greater than 0.25 inches per second. The standard in Wyoming, up to 300 feet from the source, is 1.25 inches per second. No vibrations have ever been observed in excess of standards, the BRS presentation on 2020 Rock Springs projects states. If excessive vibrations are observed, work is halted.
Ground movement is also monitored between the work area and nearest structure using a laser level with an alarm. It will detect movement of one hundredth of a foot. If ground movement occurs, work is halted.
During grouting, down-hole injection pressures and volumes are continuously monitored and recorded. If maximum pressures are reached, work is halted.