Monitoring the strata behavior in the Destressed Zone of a shallow Indian longwall panel with hard sandstone cover using Mine-Microseismicity and Borehole Televiewer data

Abstract

Massive coal extraction from the longwall panel results in the formation large void space (goaf), which is followed by elevation in the stress level of the overlying strata. The increasing stress leads to rock displacements in form of deformation, fracturing and caving of the roof behind the powered supports for regaining the equilibrium state. These rock displacements divided the longwall roof into three zones known as Caved Zone (CZ), Fractured Zone (FZ) and Continuous Deformation Zone (CD). Previous studies have shown that the overlying strata in the Destressed Zone behaves violently in presence of hard rock (such as compact sandstone, shale etc.), which is a major problem faced in the Indian underground mines. Therefore, the present study intends to monitor the roof behavior of a shallow longwall mine (working depth: 68-74m; mining height: 2–3m) of Central India, which is overlain by a thick sandstone bed (average thickness: 37-48m) using six months (Oct’01-Mar’02) mine-induced Microseismicity containing event coordinates (in 3D plane) and event parameters (such as Magnitude, Energy and Apparent Stress) along with Borehole Televiewer (BHTV) data. The paper discusses the results of some important studies, which are very essential for enhancing the safety in the mining area, such as: [a] The height of Destressed Zone was calculated through multiple approaches (empirical relations, mining height and bulking factor) and, found to be around eighteen times the mining height (hc+hf~18h), [b] Microseismicity study of the longwall roof showed that events corresponding to Fractured Zone had the lowest b-value (b=0.3301) and highest occurrence frequency (a=2.3178), [c] Borehole Televiewer (BHTV) data showed the presence of massive fractures and water seepage level in Fractured Zone at depth of about 40-50m, [d] The integrated approach of b-value, Energy-Magnitude (E-m) and Gutenberg-Richter Frequency-Magnitude relation showed that events corresponding Fractured Zone had the highest magnitude (+1.2) and energy (106.6J), and [e] The behavior of microseismic event properties (such as Event Counts, Energy, Apparent Stress and Magnitude) was also studied with respect to mine geology, and abnormal changes were seen when the roof strata was encountered with hard rock (mainly coarse-grained sandstone with RQD>60%) and fault (FF`). The study successfully helped towards obtaining several useful results related to dynamic roof displacement and failures, which very much enhanced the safety in the longwall mines.