Internal deformation characteristics and full section monitoring for extremely thick loose layers under mining conditions

Mining subsidence represents the primary environmental geological challenge in coal mining, particularly at high water level mining areas in the eastern regions of China. The control of movement deformation and the assessment of ecological losses in the context of extremely thick loose layers have garnered significant attention. To investigate the internal movement mechanisms of extremely thick loose layers and ascertain their impact on surface movement and deformation, this study focuses on the Xinji mining area in Huainan. A comprehensive 600 m depth full-section drilling monitoring system is established using the combined testing technology of distributed optical fiber and parallel electrical methods. This system aims to explore the deformation characteristics and internal movement patterns of the extremely thick loose layer. The full-section monitoring system captures internal strain and displacement information within the coal seam mining area, monitors changes in resistivity around the borehole, quantifies the spatio-temporal relationship between deformation in the stratum monitoring section, and analyzes the deformation characteristics and development forms of the inner stratum of the loose layer. Results indicate that the application of multi-parameter joint testing technology significantly enhances the monitoring efficiency and the accuracy of deformation location in the extremely thick loose layer. A relationship between the mining position of the working face and the internal deformation of the loose layer is established, dividing the mining influence process into four periods: the pre-influence period, weak mining influence period, strong mining influence period, and post-mining settlement period. The observed “reverse 3-shaped” shape movement model during advance influence deformation is verified, and the constitutive conditions and influencing factors of this model are analyzed. This model reveals the law of the accumulation and release of stratified stress during the process of coal mining in the extremely thick loose layer. The research outcome provides an essential technical support for the fine monitoring and analysis of the internal movement and deformation of the extremely thick loose layer. The acquired technical data serves as a crucial reference for monitoring and evaluating the progression of mining-induced damage, mitigating losses, reducing subsidence in ecological mining areas, devising land planning strategies for subsidence regions, and assessing the effectiveness of grouting transformations in loose geological layers.