Study on the deformation and damage characteristics of the surrounding rock and support countermeasures for the dynamic pressure roadway in deep back mining

Imagine you are digging a tunnel deep underground, far below the surface where the air is cool and the rocks are calm. Now, imagine you are digging that same tunnel, but this time you are 850 meters down, where the Earth is squeezing you with the weight of a mountain, the heat is like a sauna, and the rocks are acting more like soft clay than hard stone. This is the "Three Highs and One Disturbance" world that the researchers at Qujiang Coal Mine are dealing with.

The paper investigates a specific tunnel (the -850m roadway) that is being crushed from above by a mining operation happening right over its head. The team discovered that this tunnel doesn't just break in one way; it breaks in two very different "neighborhoods," and you need different tools to fix each one.

The Two Neighborhoods of Broken Rock

The researchers found that the tunnel's damage splits into two distinct zones based on where the mining face is relative to the tunnel.

1. The "Front-Runner" Zone (Forward Disturbance)
Imagine a giant, invisible hand pushing down on the tunnel just as the mining machine is about to pass over it. This is the Forward Disturbance (Dynamic-Pressure) Deformation Zone.

• What happens: The roof of the tunnel gets pushed down and slides sideways, like a deck of cards being shoved off a table. The sides of the tunnel get squished and folded inward. The floor, made of soft mudstone, gets so squeezed that it puffs up into the tunnel like a rising loaf of bread (a "convex" shape), sometimes swelling up to 60-70 cm in just three months.
• The Fix: You can't just try to hold this rock back with a stiff wall; it will just snap. Instead, the paper suggests a "resistance and yielding" strategy. Think of it like a shock absorber on a car. First, you use flexible support (anchors and nets) that lets the rock move a little bit to release the pressure. Then, once the rock has settled, you reinforce it with a rigid "U-shaped steel shed" and inject grout into the corners to lock everything in place. This combination allows the rock to breathe a little before you lock it down.

2. The "Aftermath" Zone (Rear Sustained)
Now, imagine the mining machine has passed over, but the pressure from the coal pillar left behind is still crushing the tunnel from above. This is the Rear Sustained (High-Pressure) Deformation Zone.

• What happens: The roof doesn't just slide; it starts to peel apart and collapse inward, forming a shape the authors call a "spire-type" or "pylon" damage, looking like a broken tower. The floor keeps swelling up, sometimes reaching 70-120 cm in some sections. The sides of the tunnel fold inward because the rock is so broken it can't hold its shape.
• The Fix: Here, flexibility isn't enough. The rock is already too broken. The paper proposes a "Three-Anchor" super-team. This involves using three different types of anchors working together: standard bolts, long steel cables, and special "grouting anchors" that pump cement into the rock to glue the broken pieces back together. This creates a strong, high-stiffness "bearing ring" that holds the deep, broken rock together.

How They Knew This Was True

The researchers didn't just guess; they watched the tunnel in real life and then tested their ideas on a computer.

• Real Life: They installed sensors on the bolts and cables to measure the force, and they measured how much the tunnel walls and floor moved. They saw that the damage patterns matched their two-zone theory perfectly.
• Computer Simulation: They built a massive 3D model of the mine using software called FLAC3D. The model was 280m long, 240m wide, and 100m high. They simulated the mining process and the rock's behavior.
• The Result: When they applied their new "Three-Anchor" and "Resistance-Yielding" plans in the computer, the results matched the real-world measurements. The simulations showed that the plastic zone (the area where the rock is broken) shrank from a giant, connected "butterfly" shape into smaller, isolated islands. The displacement (movement) of the tunnel walls stopped growing and started to stabilize.

What They Ruled Out

The paper is very clear about what doesn't work. They argue that using a single, standard support method (like just adding more bolts or just using a steel frame) is not enough. In the deep, high-stress environment of this mine, a single type of support fails because the rock is too complex. The old way of just "spraying concrete" or using standard flexible support without the specific zoning strategy leads to bolts snapping and steel frames bending. The paper explicitly states that you cannot simply "resist" all the pressure with a rigid wall in the front zone, nor can you just "let" the rock move without reinforcement in the back zone.

The Bottom Line

The study concludes that to keep a deep mining tunnel safe, you have to treat the "front" and "back" of the mining area as two different problems.

• In the front, use a flexible approach that yields to the pressure before locking it down with steel sheds.
• In the back, use a heavy-duty "Three-Anchor" team to glue the broken rock together.

The authors suggest that this "zonal" approach is the key to stopping the tunnel from collapsing, and their computer models confirm that this strategy works. It's not a magic cure-all for every mine in the world, but for this specific deep, high-pressure environment, it appears to be the right recipe for keeping the rock from turning the tunnel into a pile of rubble.