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Nuddltahw -Berry <br />In addition, HBET understands that the underdrain is functional. As a result, the <br />piezometric surface will be below the floor of the excavation prior to placement of coal <br />mine waste. However, although groundwater is not anticipated to exist within the coal <br />mine waste, for conservatism, HBET assumed an elevated piezometric surface consistent <br />with the groundwater elevation reported from piezometer BG06 -02 prior to installation of <br />the underdrain. <br />At the south edge of gob pile #3, piezometer BG06 -01 encountered groundwater <br />at an elevation of 5813.9 feet. The three other piezometers installed below the southern <br />portion of the gob pile footprint did not encounter groundwater. However, the bottoms of <br />these piezometers were much higher, at elevations between 5867 feet and 5872.6 feet. <br />Although BG06 -01 encountered groundwater fairly deep, for conservatism, HBET <br />assumed a groundwater elevation of 5870 feet, at the location of BG06 -06 along the cross <br />section, in our analysis. <br />4.4 Results <br />Based upon DRMS Rule 4.10.4(2), a minimum Factor -of- Safety (FS) of 1.5 is <br />required for the analyses to indicate a stable condition. <br />The results of the analyses are included in Appendix B and are summarized <br />below. <br />Gob Pile #3 Circular — minimum FS = 1.525 <br />Gob Pile #3 Block — minimum FS = 1.517 <br />4.5 Discussion <br />As indicated above, the results of the analyses yielded factors -of- safety for the <br />proposed slopes slightly above 1.5. However, the critical failure surfaces were very <br />shallow. The critical failure surfaces were not indicative of a major failure of the gob <br />pile slopes. In addition, as discussed previously, HBET believes that the shear strength <br />of the gob is higher than indicated in the laboratory testing. As a result, the actual <br />factors -of- safety for gob pile #3 are likely higher than indicated in the analyses. <br />It is important to note that the factors -of- safety for the proposed gob pile #3 will <br />be impacted by changes in the piezometric surface elevation. Based upon data from gob <br />pile #2, pore pressures will likely increase in the gob and/or foundation soils during <br />placement of gob at pile #3. It is also important to note that HBET does not anticipate <br />`free' groundwater within the coal waste. Changes in the piezometric surface within the <br />model is intended to reflect the changes to the stability of the gob associated with <br />increases in pore pressures within the gob. <br />X: 2008 ALL PROJECTS100489 - lE Stm-et Associates W00489-0003 Bo%ie .%1ine�200 - Ged.Gob 3'Revision 2'00489.0003 R032114 R2 doe 4 <br />