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Mr. Jim Mattern
<br />March l9, 2007
<br />Page 21
<br />Table 3. Water Table Definition
<br />Water Table Origin (X, Y, Z) D
<br />ip Dip Direction Applied Range
<br /> r
<br />O O
<br />1 6860, 1602, 7575 7.9 6.1 X = 6,860 to 9,800
<br /> Y = 1,602 to 6,200
<br />2 6860, 1602, 7575 6.3 187.6 X = 6,860 to 9,800
<br /> Y = 0 to 1,602
<br /> X = 4,900 to 6,860
<br />3 6860, 1602, 7575 10.0 38.2 Y = 1,602 to 6,200
<br /> Above Plane Ori (6860, 1602, 7575)
<br /> Dip = 90, DD = 48.7
<br /> X = 4,900 to 6,800
<br />4 6860, 1602, 7575 11.5 237,7 Y = 0 to 6,200
<br /> Below Plane Ori (6860, 1602, 7575)
<br /> Dip = 90, DD = 48.7
<br />highwall face where the 2005 failure occurred. When a lower strength for the mudstone seam
<br />above the L-Seam coal representing a strength softening due to straining under elevated pore
<br />pressure conditions (referred to as residual strengths conditions) displacement contours, shown in
<br />Figure 13, propagate a significant distance up hill. The residual strength condition assumed
<br />bedding plane shear strength of zero cohesion and tension and friction of 5° for only the weak
<br />mudstone seam.
<br />Comparing model displacements to inclinometer measurements at the time shortly after
<br />the 2005 highwall failure provides an indicator of how well the model matches. Assuming any
<br />absolute displacements are not accounted for in the inclinometer reading (i.e., displacements due
<br />to the entire borehole moving down hill), then Figure 13 results can be compared at a specific
<br />location where GIN19 inclinometer was located. The data from inclinometer GIN19, shown in
<br />Figure 14, indicates a differential movement of about 2.0 inches (0.167 ft) across the mudstone
<br />seam above the L-Seam. At this location the model predicts 1.56 inches (0.13 ft). This 22%
<br />difference is considered a reasonable comparison given the continuum-base analysis.
<br />The predicted failed zones after mining Gs-6 Strike Cut are shown in Figure 15. The
<br />near surface materials are predicted to fail due to their low confinement. At depth, the layer
<br />immediately above the mudstone failure plane is extensively failed updip, Figure 16 shows the
<br />same failure indicators but for only the I2-K Interburden sandstone layer above the failure plane.
<br />The failed region in this figure is similar to the observed region of 2005 failed highwall. The
<br />mode of failure is predicted to be tensile failure (i.e., red and magenta colors). Tensile failure in
<br />sandstone would manifest as opened sub-vertical fractures, which was observed. Although
<br />bedding plane strengths are quite low, the predicted sandstone tensile failure only extends 280 ft
<br />updip from highwall face. This distance corresponds quite well to the limit of observed surface
<br />cracking, shown in Figure 17.
<br />Areas of failed zones do not necessarily indicate major fracturing of the rock mass.
<br />Technically, a failed zone in the model has yielded and it is not clear what that looks like in the
<br />field. Since the model is incapable of predicting rock mass fracturing, we must calibrate some
<br />Agapito Associates, Inc.
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