2010-05-07_PERMIT FILE - C2009087 (48)

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-12- the major joint sets are nearly vertical and strike approximately perpen- dicular to the cut face. Minor joint sets also dip at relatively steep angles but strike almost parallel to the cut face. Considering the beneficial bed- rock dip direction, the highwall should be stable from a plane failure mechanism provided joint orientations do not control. Considering that minor joints roughly parallel the cut direction, we believe that there is a risk of mas- sive wedge failure if the highwall is cut at angles steeper than joint dip angles. The likelihood of wedge failure was analyzed using stereographic projec- tion methods based upon Marklin's test. A slope is considered "safe" or stable provided that the line of intersection of any two joint (or bedding) planes do not daylight on the surface of the slope with an angle greater than the • friction angle of the rock surface. Our field observation indicates that major and minor joint sets generally dip at angles greater than 75 degrees. A friction angle of at least 75 degrees would be required for friction between rock faces to contribute significantly to resistance of sliding. Published data suggests that typical friction angles between rock surfaces are generally less than 30 degrees, or less than one -half of the flattest joint dip angle measured. We, therefore, do not believe that friction between rock surfaces will contribute to resistance of wedge or block type failures. With these conditions, Marklin's test can be used to evaluate kinematic stability of the cut. The orientation of major and minor joints varies somewhat depending upon the type of rock and location within the pit. We divided the highwall into three basic formations and analyzed each individually using Marklin's method. The • three formations considered are the brown sandstone exposed in the upper por- tions of the cut; the massive gray sandstone which is present below the brown