Design and field response analysis of double-sided staged hydraulic fracturing in gob-side entry under multi-layer hard roof condition

Deep beneath the earth, where coal is extracted, the rock above the mine faces a constant struggle. When a layer of coal is removed, the heavy rock layers that once rested upon it must find a new way to settle. In many mines, these upper layers are made of thick, hard rock that refuses to break apart easily. Instead of crumbling into the empty space left by the coal, this hard roof hangs suspended, like a heavy ceiling that refuses to fall. Over time, the weight of this hanging rock presses down on the narrow tunnels left behind to transport equipment and people. This pressure causes the tunnel walls to squeeze inward and the floor to heave upward, threatening to crush the passage and halt operations. Engineers have long sought ways to manage this pressure, often by reinforcing the tunnel or by trying to break the rock before it becomes a danger. The challenge lies in knowing exactly which layer of rock is causing the most trouble and where to apply the force to break it safely.

At the Limin Coal Mine in China, researchers faced a complex version of this problem. The mine was extracting a new section of coal next to an area that had already been mined out. This left a narrow strip of coal acting as a pillar between the active tunnel and the empty space, a setup known as a gob-side entry. Above this tunnel, the rock layers were not uniform; they consisted of multiple hard strata, some thick and some thin, all stacked together. The engineers needed to determine which of these layers were bearing the most weight and causing the tunnel to deform. They also needed to figure out exactly where to drill holes to weaken the rock without causing a sudden, dangerous collapse. To solve this, a team from Liaoning Technical University designed a plan to use water pressure to fracture the rock in a controlled, staged manner, testing their theory in the field to see if it could calm the ground pressure.

The researchers began by analyzing the layers of rock above the mine, treating them as a single, thick beam that distributes weight. They calculated how the load was shared among the different layers to identify the three most critical ones. Their analysis pointed to a layer of coarse sandstone just above the coal, a layer of siltstone further up, and another thick sandstone layer even higher. These three layers were the primary culprits holding up the weight of the mountain above. However, the team realized that these layers did not all press down in the same spot. The lower layers concentrated their weight right next to the tunnel, while the highest of the three layers pushed its weight slightly further away, toward the empty space of the old mine. This difference meant that a single approach would not work; they needed a strategy that could target different heights and different horizontal positions simultaneously.

To test their idea, the team created a detailed computer simulation of the mine and the surrounding rock. They modeled the extraction of the coal and the movement of the rock layers to see how stress built up in different areas. The simulation confirmed their suspicion: the stress peaks for the lower layers were indeed right next to the tunnel, while the peak for the highest target layer was shifted toward the old mine. This insight allowed them to design a specific drilling pattern. They decided to drill two types of holes on both sides of the tunnel: short holes aimed at the lower rock layer and long holes reaching up to the higher layers. By staggering these holes and treating them in sections, they could weaken the rock in a precise, step-by-step fashion. The plan involved drilling holes into the solid rock wall and the narrow coal pillar, then using high-pressure water to fracture the rock in isolated segments, starting from the bottom of the hole and working their way up.

The team put this plan into action in a 100-meter section of the tunnel, drilling a total of thirty-six holes. They used a specialized tool to seal off small sections of each hole and injected water at high pressure to create cracks in the rock. This process was repeated for every section of every hole, ensuring that the rock was weakened along its entire length. While they worked, they set up a network of sensors to watch what happened. They placed instruments to measure how much the tunnel walls moved, monitored the pressure on the hydraulic supports that held up the roof, and listened for the tiny sounds of rock fracturing deep underground. They compared this treated section with an untreated section of the tunnel that had been mined earlier, allowing them to see the difference the water fracturing made.

The results were striking. In the untreated section, the tunnel walls and floor moved significantly as the mining face approached, with the roof and floor squeezing together by about 225 millimeters. The side of the tunnel next to the coal pillar moved inward by 180 millimeters, and the solid rock side moved by 125 millimeters. In the section where the rock had been fractured with water, the movement was dramatically reduced. The roof and floor moved only about 22 millimeters, the coal pillar side moved 18 millimeters, and the solid rock side moved just 12 millimeters. The hydraulic supports, which bear the weight of the roof, also showed a clear improvement. In the untreated area, the average force on the supports was 9,725 kilonewtons, but in the treated section, this average dropped to 8,943 kilonewtons, a reduction of about eight percent.

The team also listened to the rock itself. In the untreated section, the sensors recorded several high-energy events, indicating large, sudden breaks in the rock that could be dangerous. In the treated section, the activity was much quieter. The sensors detected only low-energy events, small and frequent cracks that released energy gradually, with no large, high-energy breaks recorded. The researchers also looked inside the holes after the treatment and saw clear evidence that the rock had fractured as intended. While the data showed that the water fracturing successfully reduced deformation and load, the team noted that the comparison was not perfect because the two sections were mined at different times and under slightly different conditions. They could not isolate the effect of the fracturing from every other variable, but the overall trend was clear. The field response matched their design goals: the rock moved less, the supports carried less weight, and the dangerous, high-energy breaks were eliminated.

This study demonstrates that by understanding the specific layers of rock above a mine and targeting them with a tailored drilling strategy, engineers can significantly improve the safety and stability of underground tunnels. The approach of using long and short holes to address different rock layers at different positions proved effective in calming the ground pressure. While the researchers acknowledged that more data would be needed to fully separate the effects of the treatment from other factors, the reduction in movement and load was substantial. The work provides a practical reference for other mines facing similar challenges with hard, hanging roofs, showing that careful planning and precise execution can turn a dangerous geological situation into a manageable one.