Mechanism and scientific parameters of pressure relief by cutting key blocks above gob-side entry in extra-thick coal seam: A case study

Abstract

The surrounding rock of the gob-side entry driving (GSED) is subjected to multiple high ground pressure effects in extra-thick coal seams, resulting in severe damage and significant control challenges. This study proposes a novel technology of cutting periodically fractured key blocks (CPFKB) to relieve pressure on the surrounding rock. The mechanism of CPFKB in mitigating ground pressure is elucidated, and an analytical model is built. Meanwhile, a discrimination method is given for determining the scientific parameters of CPFKB and when and in which form they fall into a gob. The results indicate that severed blocks exhibit four instability modes: sliding instability, rotational instability, simultaneous rotational-sliding instability, and stable hinge. Cutting angle exerts a significant influence on interfacial stress of severed blocks. Low-inclination cutting angles tend to induce simultaneous rotational-sliding instability, while high-inclination cutting angles typically result in initial rotation followed by sliding instability. The probability of instability markedly increases during mid-to-late stages of rotation compared to early phases. The GSED with narrow coal pillars in extra-thick coal seams using longwall top-coal caving mining is conducive to the implementation of CPFKB. Furthermore, a hydraulic fracturing technique with 75° cutting angle for CPFKB is introduced, and it achieves good practical results.

Keywords

cutting key blocks / gob-side entry driving / rotation / interfacial stress / extra-thick coal seam

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