Hard Roof's 3D Breaking Characteristics for Longwall Faces Mined via Roof-Cutting Technology with a Chainsaw Arm Machine.

Hard roof's 3D breaking characteristics is vital for mining pressure control. However, the research concerning a longwall face that employs an innovative roof-cutting technology via a chainsaw arm machine (RCTCAM) is scant. To address this gap, this paper thoroughly analyzes RCTCAM's core equipment and roof-cutting procedures. Firstly, the thin elastic plate theory and the superposition principle were applied to establish the first and periodic break mechanical models of hard roofs using the RCTCAM. These mechanical models elucidated the distribution laws of the first and third principal bending moments in a hard roof. Following this, based on tensile failure criteria and crack expansion criteria, the study uncovered that the hard roof's first break transitioned from an 'O–X' shape to a 'U–Y' shape, while its periodic break morphed from a 'C–ン' shape to an 'L–ノ' shape. This transformation led to a unique phenomenon. The rotation of arc-shaped triangular plates formed during the 'O–X' and 'C–ン' -shaped breaks resulted in severe damage to a gob-side roadway. However, a longwall face employing the RCTCAM did not produce such plates, effectively avoiding the associated damage. By implementing the RCTCAM in Workface 8311 of the Yanya Mine in China, the stress peaks of the coal pillars were reduced on average by 22.8%, and the deformation of the roadway decreased by 27.8%. These findings underline the success of the RCTCAM in achieving pressure relief for a gob-side roadway. Highlights: It was given that the theoretical calculation method for deflection, bending moment, and stress of a hard roof with the RCTCAM under the first and periodic breaks. When with the RCTCAM, hard roof's first break exhibits a transition from an 'O-X' shape to an 'U-Y' shape, while its periodic break changes from a 'C-ン' shape to an 'L-ノ' shape. Due to rotation of arc-shaped triangular plates formed in the 'O-X' and 'C-ン'-shaped breaks, a gob-side roadway bear severe damage. However, a longwall face with the RCTCAM has no such plates, thus avoiding damage. With the RCTCAM in Workface 8311 of the Yanya Mine, China, the stress peaks of a gob-side roadway's coal pillars dropped averagely by 22.8%, indicating that the RCTCAM achieve pressure relief for a gob-side roadway. [ABSTRACT FROM AUTHOR]