A unique solution had to be developed to permanently seal the Deer Creek mine.

The Deer Creek coal mine in Huntington Canyon, Utah, was closed in 2015 after 41 years of service. It produced coal from two seams, Blind Canyon and Hiawatha, at a depth of between 1,200 feet (ft) and 2,600 ft. PacifiCorp owned the mine and burned the coal at one of its nearby power plants.

The mine closure plan, however, was anything but simple. Due to the location of the mine, the owners faced unique challenges that required resolution prior to sealing the entries to avoid long-term environmental issues.

The Challenges

The volume of groundwater in the mountain, and the predicted additional accumulation over time, was calculated to potentially build up to extreme pressures behind the portal seals, which could have resulted in a catastrophic collapse of the mountain top. Substantial damage from landslides and contamination of the river below were all major concerns.

PacifiCorp turned to Strata Worldwide — which had completed other, more routine projects at Deer Creek during its lifespan — and they discussed and tested possible solutions for a proactive reclamation project. It was decided that instead of sealing the water inside the abandoned mine, Strata would design a way to capture the water and control its flow path down into large pipes to be carried out of the mine. The pipeline would then transport the water into the valley below, where it could be used by the neighboring power plant in their operations.

The primary challenge was the adequate control and management of the discharge water from the mine. All aspects of the post-closure water management program required permits from the Utah Division of Oil, Gas and Mining as well as the U.S. Mine Safety and Health Administration. It was also subject to comment by other stakeholders, including the Sierra Club.

It took some time to get to the starting point at Deer Creek, as the review by regulators and potential contractors to determine a complete scope of work took about two years. Strata, acknowledging the complexity of the project’s elements, took time to ask questions and verify parameters and data prior to submitting what ended up being a successful bid. In the meantime, facets of the work evolved. What was initially set to be a series of interlocking plugs for the groundwater and water discharge from the mine became a series of 26 dams.

Those dams, strategically placed throughout the underground locations at Deer Creek, were intended to direct water toward the portals and prevent discharge outside of the desired flow path. In the end, the flow path to the portals was determined by the slope directions of the entries and the dams constructed by Strata.

Strata’s work involved preparatory site work and constructing the dams, which consisted of placing many 3-ft x 3-ft x 4-ft interlocking concrete blocks. A subfloor foundation of concrete was also added to support the blocks, and once installed, the entire dam was sprayed with a waterproofing membrane. This alleviated and prevented any potential leakage from the direction the groundwater was set to follow.

Miners pump AFS into the rib to seal against water ingress.

At the conclusion of the installation phase, Strata conducted a test of flooding the area to evaluate how the new dams functioned. It was noted that several dams were experiencing seepage, and Strata realized the water was migrating around the dams through the coal ribs in some areas. The crews then brought in the Advanced Foam Solutions (AFS) product

Strata AFS is a rapidly curing hydrophobic polyurethane foam designed to be injected into rock fissures, gravel layers, joints, cracks and voids to fill voids and consolidate strata. It is ideal for controlling and diverting water — including high volumes of gushing water.

The crews drilled into the mine’s ribs and floor, injecting the solution, and the AFS quickly cured to create a solid, impermeable barrier that sealed against the water flow. Afterward, crews mixed a combination of bentonite (an absorbent aluminum phyllosilicate clay) and shotcrete, and sprayed the lower half of the area where water would likely accumulate due to gravity and grade.

A series of 14-in. HDPE drainpipes is placed in each plug to direct water down the mountain.

The dams varied in height from 4 ft to more than 8 ft, according to the surveys and specifications of the mine owner.

Following the completion of the dams, Strata designed and constructed a series of high strength “plugs” in the mine’s portals, each measuring about 25 ft in thickness and containing roughly 200 yards of concrete. The concrete fill for the plugs had to be completed in one continual pour, so Strata engineered and constructed sturdy form walls using EVG 3D panels and No. 9 rebar that would hold the pressure. This project and the plugs were the first of their kind.

Strata crews concreted all ribs, floor, and roof in the impacted areas before building and shotcreting the plug form walls. A series of 14-in. HDPE drainpipes was placed in each plug to catch underground water and allow it to exit the mine and travel down the mountain through a pipeline to the power plant.

This new infrastructure system was proactively placed within the mine to prevent water accumulation from ever becoming an issue for the now-closed coal mine.

Work was completed in December 2017 and took a total of 55 days. It was wrapped up on schedule, under budget and without injuries to the 10 crew members from Strata or any other on-site contractors. It is important to note that price, cost and time schedule were all critical to the client so that the work would be completed during their preset reclamation calendar and fall within compliance with state and federal outlines. No post-work issues arose from regulatory stakeholders.

“We were very satisfied,” reported Mine Manager Devin Leroy. “Strata came in and did a great job in a very time-sensitive situation. Excellent job, and got it done in a safe manner.”

Strata also reported that no sizable obstacles emerged during the course of the project, and all points of completion were met successfully.

This article was written and submitted by Strata Worldwide.