Experimental study on bearing mechanical characteristics of 'hyperbolic'coal samples

After the underground coal gasification is completed, a hyperbolic shaped coal pillar is formed between the two cavities to support the overlying rock and ensure the safety and stability of the gasification area. The work aimed to study the bearing mechanical characteristics of hyperbolic shaped coal pillar. Based on the acoustic emission (AE) monitoring system and the XTDIC 3D full-field strain measure system, 6 sets of “hyperbolic” coal samples with different lateral arch heights (h=0, 3 mm, 7 mm, 10 mm, 13 mm, 17 mm) were tested for uniaxial compression. The influence of h on peak load, deformation damage, and AE characteristics of coal samples were analyzed to reveal the bearing failure mechanism. The results are as follows. ① The “hyperbolic” coal sample can be divided into rectangular structure (main load-bearing body) and lateral arch structure, and its load-bearing failure mechanism is related to the force form and lateral arch structure. As h increases, the bearing capacity of the coal sample decreases. Compared with h=0 mm coal samples, the peak load was reduced by 7.66, 13.56, 26.83, 35.28, and 62.75%. ② The overall force form of coal samples changed from pressure-based to pressure-bending with increased h. Stress concentration occurs in the middle region of the coal sample, forming a weak area. The corresponding horizontal displacement field migrates towards the middle and finally at the edge of the middle. The vertical displacement field changed from a horizontal strip to an inclined strip, and ultimately concentrated at the upper end of the lateral arch structure of the coal sample. ③ Under axial load, the lateral arch structure of the coal sample exerts an equivalent force on the middle area of its rectangular structure, and is affected by the heterogeneity of the coal sample, which exacerbates the damage degree of the weak area in the middle. The effect increases with the increase of h, the coal sample undergoes shear failure when the load does not exceed its tensile strength. The failure mode of the coal samples shifted from tensile-shear mixed failure to shear failure, and all typical coal samples have experienced varying degrees of peeling and local ejection failure. ④ The evolution of the cumulative count-time curve of coal AE can be divided into three types. When h is 0 mm and 3 mm, it can be divided into four stages: “upward convex” growth, relatively rapid growth, rapid growth, and “sudden” growth, and its evolutionary characteristics are consistent with conventional rock samples. When h is 7 mm and 10 mm, it can be divided into three stages: relatively rapid growth, rapid growth, and “sudden” growth. When h is 13 mm and 17 mm, it can be divided into two stages: rapid growth and “sudden” growth. The post-peak stage shows a “sudden” growth pattern, while the inconsistent growth pattern in the pre-peak stage is caused by the stable expansion of coal sample cracks and continuous damage in the central region.