Funding for this research was provided by:
National Natural Science Foundation of China (11371230)
Shandong Provincial Natural Science Foundation of China (ZR2015AQ001)
Research Funds for Joint Innovative Center for Safe and Effective Mining Technology and Equipment of Coal Resources by Shandong Province and SDUST (2014TDJH102)
SDUST Innovation Fund for Graduate Students (SDKDYC180348)
Article History
Received: 21 March 2018
Accepted: 11 May 2018
First Online: 28 August 2018
Competing interests
: The authors declare that they have no competing interests.