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

Home Browse Search

• -25- the portion of the pit investigated. Shallow sheet type failure is apparent at some locations within the pit to the west. RECOMMENDED SLOPE CONFIGURATIONS We have been asked to consider short -term and long -term stability of the existing spoil bank. Our analysis indicates that the spoil is stable at this time from a mass failure point of view. No failures were evident in the im- mediate pit area at the time of our investigation even though most of the spoil slopes are at their angles of repose. Some minor failures have occurred, however, on spoil slopes exposed in the open pit to the west. The type of failure which has occurred is relatively shallow and does not appear to have involved massive amounts of material. This is generally the type of failure expected • for most of the existing spoil bank on a short -term basis. We have analyzed the spoil bank for short -term and long -term stability considering that the spoil is about 95 feet thick and resting directly on bedrock at the old pit floor. Strength values used were the same as in the previous section as were assumed ground water conditions. We varied the spoil face slope angle to evaluate the affect slope ratio has on stability. The results of our analysis are presented in Appendix C. Figure 9 presents graphically factor of safety as a function of slope ratio for shallow, infinite slope type failures. Figure 10 presents factors of safety as a function of slope ratio for deep - seated, circular type failures. As shown, the factor of safety is 1.0 for shallow failures and about 1.25 for deep failures at a ratio of 1.5 horizontal to 1 vertical using 0=34 degrees. At the higher bound angle of internal friction (37 the factor of • safety for a 1.5:1 slope is about 1.1 for shallow failures and about 1.4 for deep seated failures. Considering the mixture of materials, the actual factors