Abstract: According to the applicant's invention a load detector is used, in which a load sensor and a vibration sensor are combined using an AND gate. A load signal from the load detector is used for the feedback control of the cutter drum height and double ranging drum cutter running speed, so that a person who is not skilled can operate the double ranging drum cutter while preventing overload acting on the cutter drum.

Abstract: A double ranging drum cutter for long wall type machines for coal extraction is disclosed, which comprises a drum cutter body, a television camera mounted on the drum cutter body for picking up an image of a coal layer cutting portion of a lower bedrock being cut by a cutter drum, a television camera mounted on the drum cutter body for picking up an image of a coal layer cutting portion of an upper bedrock being cut by a cutter drum, and a video image processing unit mounted on the drum cutter body and connected to the television cameras, whereby the boundary between a coal layer and bedrock can be readily judged to permit extraction of much coal without cutting much bedrock and also the ratio of the area of a bedrock to the area of a coal layer in a portion of the bedrock being cut can be readily detected.

Abstract: The earth pressure in the underground working area is effectively alleviated by injecting high-pressure liquid into a weak ground stratum underlying the ore bed being excavated thereby to enlarge the stress envelope formed around the working face. The liquid injection hole is formed as by drill means to extend from the working face or a neighboring gateway through an intermediate layer into the weak stratum preferably at a point outside of the stress envelope initially formed.