Potentiometric and Aquifer Property Changes Above Subsiding Longwall Mine Panels, Illinois Basin Coalfield

Abstract

This study examined the response of potentiometric levels and hydraulic properties to subsidence caused by a 725‐ft‐deep active longwall mine in southern Illinois. The overburden is mainly shale but includes a shallow sandstone aquifer, overlain by a shale aquitard, capped by thin drift. Pumping and packer tests indicate that subsidence fracturing increased the hydraulic conductivities of the sandstone aquifer by about an order of magnitude, and of certain horizons of the lower bedrock by several orders. The water table in the drift aquifer was unaffected by mining; however, heads in the bedrock units dropped sharply in response to subsidence, probably because tensional dilation of fractures increased storativities. The sandstone aquifer also displayed gradual potentiometric declines ahead of mining, and a rapid partial recovery afterwards. Depression of water levels and potentially higher well yields represent conflicting negative and positive aspects of the hydrologic impact of longwall mining.

The effects of longwall coal mining on overlying aquifers

“…Subsidence was accompanied by considerable fracturing of the strata, particularly shear fractures in the marginal tension zones and vertical bedding separation in the central trough area. The hydrological effects of the subsidence have been reported by Booth & Spande (1992) and Booth et al (1997Booth et al ( , 1998.…”

“…Previous studies (e.g. Hill & Price 1983;Walker 1988;Matetic & Trevits 1991Booth & Spande 1992) have demonstrated a typical response of rapid (but often temporary) decline in potentiometric heads, caused by the sudden increase in fracture porosity, and alteration of hydraulic gradients, caused by increased permeabilities. These effects are due primarily to in situ hydraulic property changes within the shallow bedrock aquifers, not to drainage into the mine.…”

Formation Mechanism and Reduction Technology of Mining-Induced Fissures in Shallow Thick Coal Seam Mining

“…According to the analysis of key rock M of "step beam" structure in loading key strata, the discriminant of broken rock M in loading key strata when it does not appear downward sliding movement can be analyzed according to = 1.0 in formula (11). The condition which should be satisfied if rock M of "step beam" structure in loading key strata does not appear downward sliding movement is…”

“…Calculated by formula (11), under different values of , if broken rocks of loading key strata do not appear downward sliding movement, the conditions which should be satisfied are shown in Figure 11(a). It is shown that it must meet the following condition: > 2.0 if the downward sliding movement of "step beam" structure does not occur.…”

“…Xing et al and Xu et al classified key strata structures in shallow coal seams and analyzed their structural stability [8,9]. Related scholars used field tests, theoretical analysis, and numerical simulations to study overlying strata movement, mining subsidence, surface mining-induced fissures, and so on, providing principle for supporting design, subsidence trough reclamation, and ecological restoration in the mining area of shallow coal seam [10][11][12][13][14]. Zhang et al, Booth and Bertsch, and Ma et al carried out researches on mininginduced fissures of overlying strata from the perspectives of the structure characteristics of key strata and water-resisting key strata, providing water-preserved mining technologies in shallow buried coal seam [15][16][17].…”