Coupled Effects of Moisture Content and Inherent Clay Minerals on the Cohesive Strength of Remodelled Coal

Abstract

Injecting water into a coal seam to enhance the cohesive strength of coal and thus minimize and reduce the coal wall spalling risk must be considered in underground coal mining systems. In general, coal with low cohesive strength contains clay minerals which may affect the stability of coal by interacting with water. Therefore, the coupled effects of moisture content and inherent clay minerals on the physical properties (i.e., cohesive strength and internal friction angle) of coal samples should be addressed. In this paper, direct shear tests were conducted by remodelling the Yiluo coal with various moisture contents ranging from 6.6% to 20.7%. According to Mohr-Coulomb failure criterion, cohesive strength and internal friction angle of coal were obtained. Afterwards, effects of moisture content and clay minerals (i.e., Kaolinite, Smectite and Illite) on the cohesive strength of coal were analysed using X-ray diffraction (XRD) method. The results show that cohesive strength increases when the moisture content rises from 6.6% to 17.6%, after which it decreases with increasing moisture content. This trend can be well illustrated by the relationship between typical water retention curve (WRC) and suction stress of soil. Therefore, a moisture content of 17.6% would be an optimal value to enhance the stability of the Yiluo coal seam.

Experimental Study on the Efficacy of Water Infusion for Underground Mining of a Coal Seam

“…However, the cohesion of coal being reduced after the threshold moisture content was explained by the reduction of friction between adjacent coal particles. In addition to the similar trend, the ranges of cohesive strength and moisture content in this study are very close to those in the study conducted by Zhang et al (2017) [20], interestingly. Meanwhile, the effect of moisture content on the friction angle of the coal specimen was also studied (Figure 7b).…”

“…The effect of moisture content on the friction angle was much less pronounced than that on cohesion, interestingly. This finding agreed well with that of Zhang et al (2017) [20], as they concluded that the internal friction angle was almost independent of the moisture content. In conclusion, the moisture content of 22.58% was recommended as the optimum moisture content for further tasks (e.g., water injection time and field water injection).…”

“…This value decreased with a gradual increase in moisture content (22.58-29.5%), due to a reduction of the bonding effect (i.e., a coal particle is easy to detach with a high moisture content). A similar effect of moisture content on the value of cohesion has been observed in other materials, such as, soil examined by Huang et al (2012) [17], mudstone investigated by Liu and Zhu (2012) [18], and coal explored by Wang et al (2016) [19], remodeled coal studied by Zhang et al (2017) [20]. In the study by Huang et al (2012) [17], the value of cohesion increased as the moisture content varied in the range of 3.0-12.0% and then dropped rapidly after the peak due to an increase in moisture content (12.0-27.5%).…”

Influence of cyclic wetting‐drying on the mechanical strength characteristics of coal samples: A laboratory‐scale study

“…However, within the other areas, the coal surface morphology shows high integrity and compactness. These indicate that the clay minerals promote the growth of internal fissures and the formation of flaky structures via interactions with water …”

“…As wetting‐drying cycles could induce mechanical property deterioration via the original defects in a rock, the effect of wetting‐drying on coal would be more complex because coal is a porous and low wettability material . In addition, there are usually various clay minerals inside coal mass, and so the water‐coal interaction does not merely involve soaking and water adsorption.…”