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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 25 • 10.0 IMPACTS BENEATH THE MINED B-COAL SEAM Based on mapping and observations by the author in the B-Seam of the Somerset Mine, impacts to the coal and rocks below the mined coal bed are expected to be limited to about one mining thickness. There is no expected mining impact to the Rollins Sandstone because its top (or upper tongue) lies 20 to 50 feet below the base of the B-Seam. Furthermore, impacts to the floors of the mine workings aze expected to be limited to the chain pillazs, because the floors of the longwall panels will be loaded with caved roof rocks and overlying strata before deformation in the floor can occur. Floor heaving, pillar punching (the pillar punches into the floor and roof rocks), and squeezing (also known as plastic flowage, see Dunrud 1976, p. 36) are the only expected deformation in the immediate mine floor, which consists of impure coal, shale, and claystone. Deformation in the floors of the chain pillazs is expected to occur after the longwall panel is mined and the pillazs begin to crush out under the increased overburden load. 11.0 POSSIBLE SUBSIDENCE CONSEQUENCES Predicted subsidence impacts for the Apache Rocks and the Box Canyon mining areas have been described in detail above. The greatest surface impacts are expected to occur along the precipitous slopes and cliffs that flank West Flatiron and in the steep canyon slopes of the Box Canyon mining area (Box Canyon, Sylvester Gulch tributaries to the east, and the unnamed drainage to the west of Box Canyon). Though unlikely, the worst possible consequences foreseen aze that: Cracks as much as 50 to 100 feet deep may develop above the chain pillazs common to the first three northern panels and may divert intermittent flow in Box Canyon and the unnamed canyon to the west of Box Canyon to the mine through local fractures or to permeable rocks in the overburden (see Map 14 for details). Flow, in this unlikely event, would likely be through linear zones of structural weakness (i.e., fractures, joints) which may have caused the canyon to form initially. It is believed, however, that fractures and joints will terminate in the soft, ductile shale and claystone units. 2. Cracks could locally form and be as much as 75 to 200 feet deep above chain pillars and barrier pillazs on the precipitous slopes, ridges, and/or cliffs that flank Wes[ Flatiron and may accelerate the naturally-occumng rock fall and landslide process. The worst possible, but again very unlikely, underground impacts in the Apache Rocks and the Box Canyon mining areas may be that: 1. Interconnected fractures in the fractured zone may form in the Marine Sandstone that • underlies the D-Seam and impact isolated groundwater that may be present. Any water 831-032.181 Wright Water Engineers, Inc.