PERMFILE67051 - Laserfiche WebLink

Home Browse Search

West Elk Mine Subsidence Effects The most direct mining impact to groundwater is from overburden subsidence resulting from longwall coal extraction. Subsidence is simplistically defined as the downward displacement of strata overlying the mine workings under the influence of gravity. For purposes of characterizing subsidence effects, the four subsidence zones, as discussed eazlier in Section 2.05.6, aze again described as follows (see Figure I, Exhibit 60}: (1) Caved zone, (2) Fractured zone, (3} Continuous deformation zone, and (4) Near-surface zone. 1. Caved Zone - As coal is extracted and a void is produced, the immediate roof rocks break along bedding planes, joints, and fractures and fall to the mine floor. This is defined as the "Caved Zone." Within this zone, existing fractures tend to open ("dilate") and many new fractures are created. Therefore, this zone will be associated with a relatively high secondary permeability and is directly tied to the highly permeable rubble zones in the workings. Any groundwater present in the caved zone will drain into the mine immediately after caving occurs. To date, no such observation of significant groundwater inflow has occurred in the caved zone created by longwall mining indicating that there is little to no groundwater in the materials overlying the mined B Seam coal. 2. Fractured Zone - A zone of fracturing and local separation along rock bedding planes and joints occurs above the zone of caving and is termed the "Fractured Zone." Here, lateral and vertical constraints within the adjacent overburden strata and the caved rocks below minimize further displacement or rotation of the fractured rock. As such, displacement and severity of fracturing tends to decrease upward as lateral and vertical confining stresses increase. As a transitional zone, groundwater within permeable units closer to the caved zone will tend to be more affected (lost to the mine workings) than those at greater distances above the caned zone. In many cases, induced groundwater drainage (particularly in the upper part of the fractured zone, in the shale sequences between sandstone layers) will cease as the water-beazing zone is restored following bed recompression and closure in response to overburden load and subsidence completion. Evidence of restored water levels has been reported in some wells in the West Elk Mine area after completion of F Seam retreat mining and subsidence. 3. Continuous Deformation Zone -The continuous deformation zone is located between the fractured zone and the surface. In this zone the strata deform without causing any major continuous fracturing through the thickness of the strata, as is the case in the fracture zone. Thus, the strata behave essentially as a continuous medium causing little, if any, disruption of groundwater movement. Surface or tension cracking may disrupt water-bearing formations within this zone, potentially causing downwazd migration of groundwater between permeable units. 4. Near-Surface Zone (or Zone of Surface Crackinp~ -The "near-surface zone" typically consists of weathered bedrock, colluvium, and soil ranging in depth from a few feet to a few tens of feet, which may deform differently than the underlying bedrock, especially on steep slopes. Within the near surface zone, movement can best be described as "elasto-plastic" in • nature. The near-surface strata are typically draped over the underlying strata, and they 2.05-208 Revised November 1004 PRIO