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• <br />-15- <br />higher factor of safety than does Patton. A minimum factor of safety of <br />about 5.0 at an inclination angle of 26 degrees is indicated as shown on <br />Fig. 8. <br />The effect of water infiltrating tension cracks or joints was not con- <br />sidered in any of the above analyses. The ground surface at the top of the <br />highwall slopes downward away from the crest and no evidence of seepage within <br />the exposed formations was evident. We believe that the occurrence of hydro- <br />static forces in tension cracks or joints is remote considering the existing <br />topographic conditions. Adequate consideration must be given, however, to <br />control of surface drainage after preparation of the entry wall to reduce the <br />likelihood of isolated failures. <br />We believe that the potential for rockfall should be considered in design. <br />• Relatively large blocks were noted within the brown and gray sandstones. In <br />some cases, the blocks appeared to be greater than 3 feet in size. It is <br />difficult at this time to determine if the blocks are the result of fracturing <br />during mass blasting for initial mining, or exist due to existing planes of <br />weakness. Marklin's test indicates that there is a potential of small block <br />failure in the brown sandstone for cuts steeper than 55 degrees. This angle <br />corresponds to the existing formation angle observed. <br />There are several methods to reduce the likelihood of rockfall occurring, <br />or to minimize damage. One method of reducing the distance that fallen rock <br />could travel is to drape the face with wire mesh. Provision of wire mesh can <br />be relatively expansive. It may be less expensieve to assume that some rockfall <br />will take place and to provide catch benches in the slope and /or entry extensions <br />• <br />