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<br />WHEXSIopeEvaluation <br />Adrian6rown <br />4. Water. Water has not been observed in drill holes in the west wall area. Based on this finding, <br />water pressure will be less than 10% of hydrostatic. Any water in the mine is expected to drain, <br />and water pressure is not expected to affect slope performance. <br />• <br />Based on the evaluation presented in the Attachments for the holes in this azea, the recommended slope <br />angle for this portion of the wall is 60 degrees, controlled by the ability to mine steep slopes in this <br />district. The slope will be unconditionally stable at a wall scale. <br />6.4.4 North <br />The north wall of the mine is currently constrained by the location of the county road that runs along the <br />south side of Grassy Valley. This wall area has been investigated (to date) with two drill holes: GT03- <br />08, and GR-477. The findings can be summarized as follows: <br />1. Mine Wall Material. Tertiary volcanic phonolite and breccia. Weathering is limited. Intact rock <br />strength is excellent, averaging approximately 13,200 psi. <br />2. Rock Quality. Good. Fracturing is modest, with very few fractures showing evidence of <br />movement, or of clayey coating or infill. <br />3. Structure. Generally neutral or slightly adverse. There is some evidence that in the current <br />location of the wall, there is an orthogonal jointset striking west-northwest, with dips ranging <br />from 25°-50°. This may create some opportunity for local failure into the mine opening, probably <br />on a bench scale. Again, there is no evidence from drilling that these joints are sufficiently <br />extensive to impact overall slope performance. There is very little vertical jointing that would be <br />required to release blocks created by this jointset, reducing their potential for impact on the mine. <br />4. Water. Water has been observed in GR-477, located on the western end of the wall and GR-418, <br />500 feet east. All other holes in the area were either reported as being drilled dry, or no mention <br />was made of water being encountered during drilling. Based on these findings, water pressure <br />may be a factor in the slope, with some locations perhaps experiencing full hydrostatic pressure <br />for some periods of the year. <br />Based on the evaluations presented in Attachment B for the holes in this area, the recommended slope <br />angle for this portion of the wall is 60 degrees, controlled by the ability to mine steep slopes in this <br />district. However, the presence of water in at least some of the wall raises stability and mining issues: <br />1. Water pressure may be more significant than the analysis suggests, particulazly with surface <br />freeze-thaw conditions mobilizing residual rather than peak strengths in the wall material. <br />• <br />2. Ice dams and ice plugging may limit water egress, creating excess pressure in the slope, reducing <br />the stability of the wall below that computed. <br />3. Freeze-thaw conditions may mobilize rock fragments from the wall, requiring greater attention to <br />creating catch capacity during mining to prevent this material becoming a hazard for personnel <br />and equipment below the walls. <br />Report 1385L.200309017 16 <br />