⚡ electrical engineering

Mechanical Behavior and Dual-Peak Strength Evolution of Coal Pillar–Granular Backfill Composite Body in Highwall Mining

This study investigates the mechanical behavior and dual-peak strength evolution of a coal pillar–granular backfill composite body in highwall mining, revealing how pillar width and granular material height influence strength through established models to support green and safe resource recovery.

Original authors: Ya Tian, Lixiao Tu, Xuyang Shi, Xiang Lu, Yukun Yang, Fuming Liu

Published 2026-07-28

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ya Tian, Lixiao Tu, Xuyang Shi, Xiang Lu, Yukun Yang, Fuming Liu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth's crust as a giant, layered cake. Sometimes, miners want to eat the whole cake, but the top layers are too steep or the bottom layers are trapped in a way that makes them impossible to reach with a normal shovel. This is the world of open-pit mining, where huge machines scoop out coal from the surface. But as they dig deeper, they leave behind a "wall" of rock and coal at the edge of the pit, trapping valuable coal seams right underneath. This trapped coal is called end-slope coal. If left alone, it's lost forever, buried under tons of waste rock. To get it, engineers use a clever trick called highwall mining, which sends a remote-controlled robot miner into the coal seam from the edge of the pit, chewing out long tunnels.

However, this robot can't chew out the entire seam at once, or the roof would collapse. So, it leaves behind thick pillars of coal to hold up the ceiling, like the pillars in an ancient temple. The problem? Over time, these coal pillars can get tired, crack, and fail, causing the ground above to sink or even catch fire. To stop this, miners are trying a new idea: backfilling. Instead of just leaving the tunnels empty, they fill the space around the coal pillars with loose rocks and gravel (granular material). Think of it like stuffing a broken vase with packing peanuts to keep it from shattering. The big question scientists are asking is: How well does this "coal pillar plus packing peanuts" combo actually hold up under pressure? Does the loose rock help, or does it just get in the way?

This paper dives into that exact question by building a miniature version of this underground world in a lab. The researchers, led by Ya Tian and Xuyang Shi from the China University of Mining and Technology, created a special machine to test a Coal Pillar–Granular Backfill Composite Body (CGCB). They didn't just push on a block of coal; they built a sandwich with a coal pillar in the middle and loose, crushed rock on both sides, then squeezed it to see what happened.

What they found is surprisingly dramatic and a bit like a two-act play. When they squeezed their coal-and-rock sandwich, it didn't just break once and give up. Instead, it showed a dual-peak strength, meaning it had two distinct moments of maximum power. The first peak, which they call α-strength, happens when the coal pillar first starts to crack. The second peak, the β-strength, happens later, after the coal has crumbled a bit but the loose rock on the sides has squeezed in tight and taken over the load.

The researchers discovered that the size of the coal pillar matters a lot. If the pillar is too narrow, it breaks easily. If it's just right (around 100 mm wide in their test), the first peak is strong. But if the pillar gets even wider, the first peak actually gets weaker because the coal itself has more hidden cracks inside it (a phenomenon known as the "size effect"). However, the second peak—the β-strength—is a different story. It gets stronger and stronger as the coal pillar gets wider, following a specific mathematical curve.

Even more interesting is the role of the loose rock. The team found that the height of the granular material (how much "packing peanuts" they used) acts like a supercharger. The more rock they piled up, the stronger the second peak became. In fact, when they filled the space up to 95 mm high, the second peak strength was up to 6.48 times stronger than when there was no rock at all! This suggests that the loose rock isn't just filling space; it's actively squeezing the coal, preventing it from falling apart completely, and then taking the weight itself once the coal is damaged.

They also tested different types of rock sizes. They found that rocks with a higher "friction angle" (which is a fancy way of saying the rocks are rougher and grip each other better) made the whole system stronger. The rougher the rocks, the better they held the coal together.

The authors built a mathematical model to explain this, treating the coal pillar like a wall and the loose rock like soil pushing against it. They realized that as the coal cracks, the loose rock pushes back harder (like a spring compressing), creating a "passive earth pressure" that saves the structure from total collapse. This happens in three stages: first, the coal holds the weight; second, the coal cracks but the rock starts squeezing it; and third, the rock and the broken coal work together to hold up the roof.

The paper concludes that this "coal pillar plus backfill" method is a promising way to recover that trapped end-slope coal safely. By using the right amount of loose rock and the right size of coal pillar, miners can create a structure that doesn't just survive the initial break, but actually gets stronger as it settles. While these results come from small lab models and need to be tested in real mines, the findings suggest that filling the gaps with loose rock could be the key to keeping the ground stable and saving valuable coal resources that would otherwise be lost forever.

Technical Summary: Mechanical Behavior and Dual-Peak Strength Evolution of Coal Pillar–Granular Backfill Composite Body in Highwall Mining

Problem Statement
Highwall mining is a critical technique for recovering stranded "end-slope" coal resources in open-pit mines, which otherwise account for 5–15% of total reserves. However, traditional highwall mining leaves coal pillars that are prone to long-term instability, leading to surface subsidence and potential spontaneous combustion. While backfill mining offers a solution, existing research has largely focused on cohesive paste backfills. The use of in-situ granular spoil (solid backfill) is often viewed with skepticism due to the limitations of traditional analytical methods in capturing post-consolidation strength gains and dynamic lateral confinement mechanisms. There is a critical gap in understanding the strength evolution of a composite body formed by coal pillars and granular backfill under varying mining parameters.

Methodology
To address this gap, the authors developed a novel experimental and theoretical framework centered on a "Coal Pillar–Granular Backfill Composite Body" (CGCB).

Experimental Apparatus: A specialized axial compression testing device was independently designed to simulate the in-situ stress field. The apparatus features a base, front/rear retaining plates, lateral restraint plates, and a lateral pressure transfer mechanism. It incorporates multiple pressure sensors (F5Y and F9Y models) to monitor real-time stress variations in both the granular material (lateral and vertical) and the coal pillar.
Materials: Coal specimens were sourced from the Zhundong Coalfield (uniaxial compressive strength ~11.16 MPa). Granular materials were collected from the Tianchi Energy South Open-Pit Mine, with particle sizes adjusted via a similarity gradation method to a maximum of 5 mm. Three gradation schemes were tested to vary the internal friction angle and cohesion.
Test Matrix: Thirty-five test schemes were conducted using a WAW-600 hydraulic testing machine. Variables included coal pillar widths (50–150 mm), granular material heights (0–95 mm), and particle gradations. Tests were run under displacement control until 10% axial strain.
Theoretical Modeling: A staged mechanical model was established based on earth pressure theory and the Mohr–Coulomb criterion. The model divides the bearing process into three stages: stationary collaborative bearing, dynamic collaborative bearing, and collaborative bearing stabilization. It calculates strength evolution by iteratively adjusting the passive earth pressure exerted by the granular material as the coal pillar fails and consolidates.

Key Results
The study revealed a distinctive "dual-peak" stress–strain response in the CGCB, characterized by an initial peak ( $\alpha$ -strength) and a secondary peak ( $\beta$ -strength).

Dual-Peak Strength Evolution:
- $\alpha$ -strength (Initial Failure): This strength initially increases with coal pillar width (peaking at 100 mm) and then decreases, reflecting a competition between the width effect and the size effect (where larger pillars contain more defects). It shows a linear increase with granular material height.
- $\beta$ -strength (Subsequent Failure): This strength follows a power-law relationship, increasing continuously with coal pillar width. It is significantly more sensitive to granular material height than $\alpha$ -strength; for a 150 mm pillar, $\beta$ -strength at 95 mm height was up to 6.48 times higher than at 0 mm height. In some cases, $\beta$ -strength exceeded $\alpha$ -strength, particularly in wider pillars where the scale of initial failure was small relative to the specimen.
Influence of Granular Gradation:
- While granular cohesion had a negligible effect, the internal friction angle significantly influenced strength. Higher equivalent internal friction angles (achieved through specific gradations) increased both $\alpha$ and $\beta$ strengths.
- The influence of gradation on $\beta$ -strength was most pronounced in narrower pillars (up to 41.67% variation) and diminished as pillar width increased.
Stress and Failure Mechanisms:
- Increasing granular height and pillar width elevated the horizontal stress within the granular layer, providing lateral confinement that constrained crack propagation in the coal pillar.
- Failure modes transitioned from brittle axial splitting (in narrow pillars without backfill) to ductile shear failure (in wider pillars or with high backfill).
- Theoretical modeling confirmed that the transition from $\alpha$ to $\beta$ strength corresponds to the coal pillar's initial failure and the subsequent generation of passive earth pressure by the consolidating granular material.

Significance and Claims
The paper claims to provide fundamental insights into the strength evolution mechanism of coal pillar–granular backfill composites, a subject previously under-researched. By establishing a systematic research framework and a comprehensive strength evolution pathway, the study offers a scientific basis for:

Evaluating the long-term stability of backfilled slopes.
Designing effective subsidence control measures.
Promoting safer and more environmentally sustainable strategies for recovering end-slope coal resources using solid backfill.

The authors maintain a modest tone regarding the scope of their findings, explicitly noting that the conclusions are derived from laboratory-scale specimens. They state that the applicability of these results to full-scale field engineering requires further verification in subsequent studies. The work aims to bridge the gap between theoretical understanding and the practical implementation of green mining practices in open-pit environments.

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