The longwall mining at Kushiro Colliery is characterized by highly mechanized face equipments and the majority of output comes from this coal winning method. The rate of face advance has, however, declined with increase of mining depth by a main reason why driving and successive maintenance of gate roads has been confronted with diffiulties. To secure a solution of this problem, a new type of gate road system investigated and adopted to retreat mining panels.
In the new system only one of gate roads is driven before winning coal, and another is extended with advance of coal face as if in advance longwall mining system (Fig. 1). At the situation the main gate road is driven in solidcoal by the same way as existing method, but the auxiliary road is constructed in goaf with ring or arch-invert support elements and tubular shield elements so that this gate road can be named as a liner road (Fig.10).
The structure and size of a liner road was determined so as to satisfy both requirements from ventilation and strucrural analysis. The preliminary design lead us to make use of the standard steel beam for the support element and a thin steel plate for shield element.
Up to the present this system has been adopted to five mining panels so that total coal production amounted to 550, 000 tons. In all cases liner roads were perfectly maintained during working of mining panels without any sort of trouble. The new system also revealed the economic feasibility through the result that the expenditure for gate roads in a mining panel could be reduced by about 27% in average comparing with existing longwall system.
