Uniaxial compressive strength estimation based on the primary wave velocity in coal: considering scale effect and anisotropy

ABSTRACT

Coal mass strength estimation has attracted broad attentions for decades. Investigating the effect of scale and anisotropy on the Uniaxial Compressive Strength (UCS) estimation via P-wave velocity is crucial in upscaling the lab-obtained correlation to coal mass strength evaluation. Microstructures were considered as the major impact factors for the scale difference and anisotropy of mechanical properties in coal. Thus, the microstructure correlated influence of specimen sizes and anisotropy angles on the UCS estimation based on the primary wave (P-wave) velocity were investigated here. Specimens with four diameters (25 mm to 75 mm) and six anisotropy angles (angles between bedding plane orientation and coring direction, 0° to 90°) were processed. Roles microstructures played for the P-wave propagation were explored by the X-ray computed tomography scanning and P-wave velocity tests. The UCS estimation via P-wave velocity in coal were investigated by regression analysis, significances of scale and anisotropy effect on this correlation were quantitatively evaluated by an introduced one-way analysis of variance method. P-wave velocity for specimens with different diameters exhibits a U-shaped variation against anisotropy angles, it reduces at anisotropy angles 0° ~ 45°, while raises within that of 45° ~ 90°, the minimum, maximum, and second largest value were acquired, respectively, at the anisotropy angle 45°, 90°, and 0°. This was considered caused by the complementary variation of angles between directions of P-wave propagation and orientations of bedding plane and major cleats. P-wave velocity wakens in larger specimens, velocity reductions are significantly at the anisotropy angle of 45°, while less obviously at 0° and 90°. While the anisotropy of P-wave velocity strengthens as diameters gain. Specimen sizes and anisotropy angles were proved to affect the UCS estimation via P-wave velocity by the one-way analysis of variance. Exponential correlation was verified more appropriate in the UCS estimation via P-wave velocity in coal, specimens with diameter 38 mm and anisotropy angle 45° were proved optimal for the UCS estimating by P-wave velocity in coal, since the correlation between them is stronger than other diameters and anisotropy angles. This research facilitates us in quickly and accurately strength estimating in anisotropy coal, meanwhile, it also provides references in extending the lab-obtained mechanical properties of coal/rock specimens to coal/rock mass.

KEYWORDS:

• Scale effect
• anisotropy angle
• strength estimation
• primary wave
• one-way analysis of variance

Additional information

Funding

This research is supported by the National Natural Science Foundation of China [nos. 51874312, 51861145403], Open Project Program of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining [SHJT-16-30.17], Yue Qi Distinguished Scholar Project of China University of Mining & Technology (Beijing), Fundamental Research Funds for the Central Universities.

Notes on contributors

Honghua Song

Honghua Song, a lecturer in the China University of Mining and Technology, Beijing and a member of the G3 group in Pennsylvania State University, University Park, majoring in the scale effect and anisotropy of coal, coal bump, X-ray CT, three-dimensional reconstruction, and acoustic emission analysis.

Yixin Zhao

Yixin Zhao, a professor at the  China University of Mining and Technology, Beijing and a member of the G3 group in Pennsylvania State University, University Park, research interests include coal bump, X-ray CT, multi-field coupling, and fracture mechanics.

Nima Noraei Danesh

Nima Noraei Danesh, a postdoctoral in the China University of Mining and Technology, Beijing and a member of the University of Queensland, majoring in coal gas research.

Yaodong Jiang

Yaodong Jiang, a professor at the  China University of Mining and Technology, Beijing, majoring in coal bump and researches related to rock mechanics and mining engineering.

Yutao Li

Yutao Li, a Ph.D. student in the  China University of Mining and Technology, Beijing, majoring in the anisotropy of coal.