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1 <br />' an active slide mass is a composite surface of sliding on existing discontinuities (perhaps in <br />several different orientations), shearing through intact rock, and rotation of discrete blocks. <br />The shear surface could be narrow and well defined or it may be expressed as a wide zone of <br />shear displacement. <br />Stability of the overall pit slope is dependent on the slope height, slope angle, and quality of <br />the rock mass. Rock mass quality is defined by the strength of the parent rock material and <br />the character of discontinuities including frequency, orientation, continuity, roughness, and <br />infilling material. Due to the shape of the surface expression of the active area and the highly <br />fractured character of the rock mass, it was determined that the slide mass is not controlled by <br />a single discontinuity within the rock mass. However, there is a preferred orientation for <br />' joints which could impart an anisotropic character to the rock mass resulting in a preferential <br />directional weakness. The following section presents Haley & Aldrich's evaluation of the <br />stability of the overall pit slope in the subject area. <br />4-02. ENGINEERING ANALYSIS OF OVERALL SLOPE STABILITY <br />A. eneral <br />The following section presents Haley & Aldrich's methods of analysis and results of stability <br />calculations for the subject pit slope. The goal of the analyses is to determine the appropriate <br />slope angle consistent with design safety factors. <br />' B. Design Criteria <br />The safety factor is defined as the resisting force divided by the driving force. Design criteria <br />' for slopes in general including dams, highways, and mining applications vary greatly <br />depending on the situation, the consequences of failure, degree of uncertainty, and the <br />likelihood and duration of the loading condition. In general, 1.4 is considered a high safety <br />factor and is appropriate for high risk structures such as large dams which could cause <br />significant damage and/or endanger people if they fail. Conversely, very low safety factors, <br />as low as I.0 to I.l, are appropriate fur low risk situations, and for short term or infrequent <br />' occurrences. Low risk is defined as little or no loss of either property or life in the event of <br />failure. Many mining operations are designed for very low safety factors since the <br />consequences of failure are minimal and there are usually no third-party risks. Based on this <br />' range of Safety factors, a safety factor of 1.25, representing mid-level risk was chosen as the <br />design criteria for the interim slope configuration. <br />' C'. Stabilit~Analysis and Model <br />Engineering analyses were conducted in a two-step sequence of first determining the strength <br />' of the rock mass, and second, calculating the safety factor against sliding based on rock mass <br />strength, slope height, and slope angle. <br />' Rock mass strength is based on a bi-linear strength envelope which incorporates the strength <br />of the parent rock material and properties of discontinuities at different stress levels. At low <br />stress levels the rock behaves in a combination of friction and sliding over irregularities along <br />2 <br />1 ~`~ ~~ <br />