2008-04-15_REVISION - M1980244 (193)

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tl~; • MainCressonSlo eEvaluation AdrianBrown 7. SLOPE DESIGN Design of a mine slope requires prediction of future performance of the slope, and adjustment of the physical conditions of the slope (such as slope angle, geometry, location and water conditions) to create an adequately stable slope. This process requires that the information that has bee collected be analyzed to quantify the parameters which control the performance of the slope, and then the analysis of the performance of a range of slope designs using those parameters: This process is performed in this section of the report. 7.1 Overall slope parameters The parameters which govern rockmass behavior are the overall effective stress friction angle, and the effective stress cohesion. These can be derived by a variety of methods, of which the Hoek and Bray method is selected for use here (Hoek and Bray, 1981). This approach is empirically based, and uses relationships which have been developed. from the evaluation of the performance of a large number of rock slopes. The Hoek and Bray method requires as input rockmass characteristics which are found using the CSIR and the NGI classification methods. 7.1.1 CSIR Classification • The CSIR method of rockmass classification is described in Hoek and B Town, 1980. pp. 22-27. This method develops a classification for the slope materials, based on six factors (Table 8): Strength of intact material. The average unconfined compressive strength for intact mine rocks is 17,400 psi (120 MPa), sufficient for the slope to stand vertically for over 5,000 feet. Intact material strength is therefore not a limiting factor for these slopes. It does, however, enter into the overall rockmass strength computation, resulting in a value of 12 on the CSIR scale. 2. Rock Quality Designation (RQD). The RQD ofthe rock is generally high; in the order of 90% for most core runs in drilling. Based on observations of joint persistence in the mine wall exposures, joints aze generally not persistent, with a small percentage of joints intersecting more than a single bench in the mine. 3. Joint spacing. Joint spacing is close; in core drilling it was approximately 0.3 m, or occasionally less. The joint system exhibited by the mine materials is favorable for mine stability; most of the fabric is vertical, and major structural features strike approximately parallel to the faces of the main slopes. To date, few fractures or faults of more than 20 feet persistence are adversely oriented with respect to slopes. Based on the lack of persistence of most joints, any overall catastrophic failure of the slope will require at least some fracturing through the intact rock mass. • 12 These factors aze described in Table 5 of Hoek and Brown (1980) ,entitled "CSIR classification for a Class II Rockmass" .The table is reproduced here as Table 8. 1385D.98o612 37