Failure mechanism of a water jet impacting coal with different bedding plane angle under true triaxial stresses

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Journal of Vibration and Shock ›› 2024, Vol. 43 ›› Issue (22) : 10-19.

GE Zhaolong1,2，MA Shitan1,2，ZHOU Zhe*1,2，ZHANG Xu1,2，CHANG Wenxu1,2

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Abstract

The angle of coal’s bedding planes significantly affects the performance of jetting in rock fracturing. To investigate the mechanism of failure and damage characteristics of coal with different bedding plane angles (BDA) under true triaxial stress conditions, erosion experiments using pure water jets were conducted at various BDA and triaxial stress levels. The experimental results indicate that the angle of coal’s bedding planes has a pronounced impact on the fracturing capacity of the water jet. For coal samples with lower and higher BDA, the water jet impact tends to form cone-shaped erosion pits and crack-like pits, respectively. Additionally, the opening of the erosion pits transitions from circular to elliptical as the BDA increases, and the failure mode shifts from shear-dominated to water-wedge effect-dominated. With the increase in coal’s BDA, the fracturing performance improves by 154.35%, reaching its peak when the BDA is 60°. As the BDA of coal increases, the orifice of the broken pit transitions from circular to elliptical. Under the application of triaxial stress, the destruction of coal samples with various BDA only presents circular erosion pits, and the shear damage caused by water hammer pressure becomes the primary reason for the failure of coal samples under triaxial stress. The difference in jetting damage among samples with different BDA is significantly reduced, and the influence of coal’s bedding planes on the fracturing capacity of the water jet is significantly weakened. The reduction in the fracturing volume of 60 BDA samples reaches 95.6%, the maximum decrease observed. CT scan results reveal that the tension damage caused by the water wedge effect is the primary factor affecting the failure characteristics, with the most severe impact at 60 BDA, and the area of jet damage decreases with the increase in triaxial stress. Scanning electron microscope(SEM） results indicate that triaxial stress causes the closure of pre-existing cracks and micro-damage in coal samples with different BDA, promoting a transformation from brittle shear failure to ductile shear failure in the pit wall of 0 BDA coal samples, and significantly reducing the water wedge effect in 60 BDA coal samples, shifting towards shear failure.

Key words

Radial jet drilling technology / coal bedding plane / triaxial stress / damage failure

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GE Zhaolong1, 2, MA Shitan1, 2, ZHOU Zhe1, 2, ZHANG Xu1, 2, CHANG Wenxu1, 2. Failure mechanism of a water jet impacting coal with different bedding plane angle under true triaxial stresses[J]. Journal of Vibration and Shock, 2024, 43(22): 10-19

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