Study on the properties and reasonable width of a phosphogypsum backfill body for gob-side entry retaining in a thick coal seam

Deep beneath the earth, where coal seams lie waiting to be mined, engineers face a persistent and costly dilemma. When a layer of coal is extracted, the roof above it naturally wants to collapse. To keep the tunnels safe for future work, miners traditionally leave behind thick walls of unmined coal to hold up the ceiling. While this keeps the ground stable, it also leaves a significant portion of the valuable resource locked away forever, trapped underground as a safety measure. A more efficient alternative, known as gob-side entry retaining, involves driving a new tunnel right along the edge of the empty space left by the previous mining operation. Instead of leaving a wall of coal, engineers build an artificial wall using a special filling material. This man-made barrier supports the roof just as well as a coal pillar would, but it allows miners to extract nearly all the coal from the seam, turning what was once waste into profit. The challenge lies in finding a material that is strong enough to hold up the earth, flows easily enough to be pumped into tight spaces, and costs less than traditional options.

In a coal mine in Guizhou, China, researchers have been testing a solution that turns an industrial problem into a mining asset. The region produces vast amounts of phosphogypsum, a byproduct of fertilizer manufacturing that usually sits in massive, land-consuming piles. This material poses environmental risks, including the potential to contaminate groundwater with fluoride and radioactive elements. The team, led by engineers from the Guizhou Institute of Technology and local mine operators, asked a simple question: could this abundant waste product be transformed into the perfect material for building these artificial tunnel walls? Their investigation combined laboratory experiments with complex computer modeling to determine not only if the material worked, but exactly how thick the wall needed to be to ensure safety.

The researchers began by treating the raw phosphogypsum, which arrives as a dry powder of flaky crystals, into a usable construction material. They mixed it with water and small amounts of chemical additives designed to make the mixture flow smoothly and set at the right speed. In the lab, they tested dozens of variations, adjusting the amounts of a substance called a superplasticizer to improve flow and an alkali to control how quickly the mixture hardened. They found that a specific combination worked best: a low-alkali environment paired with a precise dose of the flow-enhancing chemical. This recipe allowed the mixture to remain fluid enough to be pumped into the narrow space between the tunnel wall and the empty mine, yet it hardened into a solid structure with a compressive strength of 12.5 megapascals after 28 days. This strength was sufficient to meet the rigorous safety standards required for underground mining, proving that the local waste could indeed serve as a viable replacement for conventional filling materials.

With the material's properties established, the team turned to the question of size. In mining, every meter of width counts. A wider wall is safer but requires more material and leaves less room for coal extraction. A narrower wall saves resources but risks collapse. To find the sweet spot, the researchers used two different mathematical approaches to estimate the load the wall would need to bear. One method, which they described as more conservative and safer, calculated the weight of the rock layers above the tunnel and how that weight would press down on the new wall. They compared this with a second method that looked at how individual blocks of rock might shift and settle. The calculations showed that the wall's required strength depended heavily on the thickness of the coal seam being mined and the weight of the rock above it. The researchers discovered that the thickness of the coal seam was the single most influential factor; the thicker the seam, the stronger the wall needed to be. Other factors, such as the width of the tunnel itself and the friction between rock layers, played a role but were less critical.

To visualize how the wall would behave under real-world conditions, the team built a detailed computer simulation of the mine's geology. They modeled the tunnel, the surrounding rock, and the artificial wall, then watched how the ground shifted as the wall width changed. They tested widths ranging from less than a meter to over three meters. The simulation revealed a clear pattern: when the wall was narrow, the rock on the side of the tunnel moved significantly, squeezing inward. As the wall grew wider, this movement dropped sharply. The most dramatic improvement happened as the wall grew from a narrow 0.8 meters to a more substantial 2.0 meters. Beyond 2.0 meters, the reduction in movement became much smaller, suggesting that making the wall any wider would offer diminishing returns. The computer models showed that at a width of 2.0 meters, the wall could support the overlying rock effectively, keeping the tunnel stable without wasting material.

The implications of this finding extend beyond just keeping a tunnel open. By using a material that is cheap and locally available, and by determining that a 2.0-meter wall is sufficient, the mine can recover significantly more coal. The researchers calculated that by optimizing the wall width and using this new material, the mine could recover an additional 9,681.1 tons of coal from a single mining face compared to using a wider, less efficient wall. This represents a direct economic benefit, turning a waste disposal problem into a source of extra revenue. While the study confirms that the material is strong and the width is safe, the authors note that further investigation is needed to ensure the long-term durability of the wall and to fully understand how the material interacts with groundwater over many years. For now, the work provides a solid technical foundation for using this local industrial byproduct to make coal mining more efficient and less wasteful.