Abstract
Factors such as the hydrogeological conditions, the lithological characteristics of the columns’ components, and the lithological characteristics and stress conditions of the coal seam roof and floor are interrelated and jointly affect column collapse. In this study, the disaster-causing mechanism of column collapse was studied. Based on the system theory, a collapsed column is divided into the column and the surrounding fissure zone as two subsystems for analysis. And, the permeability coefficient of the broken rock under different conditions was measured by a self-designed equipment. The variations of the permeability coefficient for rock samples with different particle diameters, different axial pressures Pa, and different seepage velocities were further studied. Through phenomena analysis and experimental data processing, it was concluded that, under the same pressure state, smaller particle diameter meant smaller permeability coefficient; with the increase of axial pressure, the permeability coefficient decreased; and the larger the water flow velocity was, the smaller the permeability coefficient became. For particle diameter Φ = 2.5–5 mm or larger, the tiny particles formed by randomly washing and breaking in the water flow blocked some of the channels. For particle diameters smaller than Φ = 2.5–5 mm, the smaller permeability coefficient was attributed to the turbulence resulting from non-Darcy flow. The study on the permeability of the fractured rock mass clarified the mechanism of water inrush from the fissure zone of the collapsed column: the collapsed column itself was impermeable, and the permeability of the fissure zone around the collapsed column was related to the lithological characteristics of the rock within the fissure zone and the sequencing of rock strata. When mining coal in areas with collapsed columns, experiments on collapsed columns and fissure zones are prerequisites. This study has a certain referential value for coal mining in this region.
Highlights
According to incomplete statistics, from January 2001 to December 2017, there were a total of 182 water inrush accidents in China, resulting in 1807 deaths and 73 missing
For strain from 0.3% to 0.9% (AB segment), the axial pressures of rocks from the three sections gradually increased, and their permeability coefficients declined to a certain extent
For strain from 0.9% to 1.23% (BC segment), the axial pressure of rocks from the three sections gradually increased to an extreme value; the ultimate strength of rocks from the middle section was 53 MPa, slightly larger than the 47 MPa of the upper and lower sections; the permeability coefficient of rocks from the three sections increased gradually, and the increasing slope was not a definite value, indicating that it was a stage in which the original fissures were closed and a few new fissures came into being
Summary
From January 2001 to December 2017, there were a total of 182 water inrush accidents in China, resulting in 1807 deaths and 73 missing. Column collapse accounted for more than 90% of these accidents [2, 3]. In developed coal-producing countries [4, 5], it is usually possible to locate coalfields in regions with simple hydrogeological conditions and adopt the method of open-pit mining or room [6] and pillar mining [7] to reduce or totally avoid the occurrence of such water hazards. Karst collapsed columns are distributed in more than 20 coalfields in North China, as well as in the provinces of Shandong, Jiangsu, Shanxi, etc. Column collapse can destroy the coal seam and affect fully mechanized coal mining [13, 14], directly impairing the safety and efficiency of coal production
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