Funding for this research was provided by:
Shandong Provincial Natural Science Foundation of China (No. ZR2019MA003)
Research Funds for Joint Innovative Center for Safe and Effective Mining Technology and Equipment of Coal Resources by Shandong Province and SDUST Research Fund (2014TDJH102)
Received: 12 October 2019
Accepted: 2 July 2020
First Online: 14 July 2020
: The authors declare that they have no competing interests.