Design of the Key Bearing Layer and Secondary Mining Technology for Previously Mined Areas of Small Coal Mines

In many underground coal-mine fields, where small scale coal mining had been performed, the available resources were not extracted fully. To extract the remaining coal, it is necessary to perform secondary mining. In this paper, a novel secondary mining technology is suggested incorporating a novel concept of building a key bearing layer (KBL) through backfilling in the previously mined areas to increase the extraction ratio, prolong the mine life, and avoid undesirable impacts on the limited coal resources and ecological environment. To analyze the influence of previously mined areas of small coal mines on the secondary mining, two new methods of determining the thickness of the KBL, choosing of the backfilling material, the slice backfilling method, ABAQUS finite-element analysis, field monitoring of displacement, and secondary mining technology were implemented. The results show that a minimum thickness of 5 m is required for the KBL in B909 panel of Pingshuo No. 2 underground coal mine to ensure stability of both the roof and coal walls during the secondary mining process. Ruimi-backfilling material was used to construct the cutting layer and KBL with different compressive strengths in the previously mined areas to carry out the slice backfilling, which met the mining requirements of the normal coal seam with reduced cost. Numerical simulation results showed that, when the secondary mining face was 20 m away from the previously mined areas, the secondary mining stress overlapped with the stress which was there from the previously mined areas to result in roof collapses. When the secondary mining face approached the previously mined areas, the roof of the goaf had a large area of collapse, which was beneficial to the stability of the KBL. It was possible to use the field-monitored displacements to validate the numerical-modeling results. The proposed novel secondary mining technology with backfilling in the previously mined areas of small coal mines turned out to be a safe, high-yielding and efficient mining method.