Fractured zone height of longwall mining and its effects on the overburden aquifers

Abstract

As mining depth becomes deeper and deeper, the possibility of undermining overburden aquifers is increasing. It is very important for coal miners to undertake studies on the height of fractured zone during longwall mining and the effects of longwall mining on the underground water while mining under surface water bodies and underground aquifers. In order to study this problem, piezometers for monitoring underground water levels were installed above the longwall panels in an American coalmine. Large amounts of pre-mining, during mining and post-mining monitoring data were collected. Based on the data, the heights of fractured zones were obtained and the effects of longwall mining on the underground water were studied. The results demonstrate that when the piezometer monitoring wells had an interburden thickness of less than 72.7 m, the groundwater level decreased immediately to immeasurable levels and the wells went dry after undermining the face of longwall. The height of the fractured zone is 72.7–85.3 m in the geological and mining conditions. The results also show that the calculated values of fractured zones by the empirical formulae used in China are smaller than the actual results. Therefore, it is not always safe to use them for analyses while mining under water bodies.

Introduction

As mining depth becomes deeper and deeper, the possibility of undermining overburden aquifers is increasing. Longwall mining was conducted under overburden aquifers in an American coalmine. In order to study the effects of longwall mining on the underground water, piezometers for monitoring underground water levels were installed and large amounts of data for monitoring the groundwater were collected from this mining area before, during and after mining. The major monitoring works included periodic measuring of the water levels in piezometer monitoring wells drilled to various depths. The monitoring data includes 32 monitoring stations. The data collection process lasted nearly 2 years before longwall mining began, and continued through 2004. Mining was completed nearly at the end of 2002.

The variable mining conditions at this longwall district were depending on the location of the active mining sections. This coalmine extracted Pittsburgh #8 coal seam at a mining height of 2.44 m (8 ft) using a longwall retreat mining method. The rib-to-rib widths of the longwall panels were 305 m (1000 ft). The overburden thickness between the coal seam and the surface ranged from 122 to 137 m. The overburden strata are composed of limestone, sandstone, grey clay shale, and so forth. The longwall panels were retreat-mined from east to west. The main entries were mined southward and longwall gateroad entries branched off to the east from the main entries.

Experiments showed that the groundwater level was disturbed by the mining operation to some degrees [1], [2], [3]. However, the severity of disruption was highly variable depending on the geologic conditions and thickness of the overburden between the coal seam and the surface [4], [5], [6]. The goal of this project was to determine the mechanism of groundwater loss (or change) due to undermining. Based on the data analyses, the effects of longwall mining on underground water were studied in this paper.