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Dickerson Quarry s~ es Page 3 • October 31, 2000 slope angle reduction to 95° (1 vertical to 1 horizontal) modification 1 in the southwest corner of the quarry. Figure 7 presents the recommended final N55°E quarry wall cross section, including the initially planned slope, the quarry floor, the intermediate benches and the potentially adverse joint set in the worst-case location through the toe of recommended slope. Flattening the slope angle to 45° in the southwest corner of the quarry moves the intersection between the steepest measured adverse joint dip, 38°, and the ground surface 31 feet further inside the property line. STABILITY EVALUATION OF ADVERSE NORTHEAST DIPPING JOINT SET The stability of the ultimate south facing, east facing, north facing and northwest facing quarry walls is not at risk because the joint sets present are steeper than the planned slope angles. The stability of the potentially adverse northeast dipping joint set does not represent a hazard for the recommended ultimate southwest corner quarry slope for the following reasons: 1) If the 20° dip of the adverse northeast dipping joint set in the western outcrops, shown on Figure 5, persists below the surface and is less than the residual angle of surface friction of the competent fresh granite the potential failure block cannot slide. The residual angle of surface friction is measured on continuous planar rock surfaces. The typical residual friction angle for granite is between 31° and 33° (Hoek, 1970). 2) In addition, when smooth machined (planar) granite specimen surfaces are tested in direct shear a small, but measurable surface cohesion value of a few psi is typically measured. This probably results from stronger quartz crystals gouging into softer mica and feldspar crystals. This small surface cohesion will result in a relatively large resistance to sliding per foot of face width because of the large surface area along the approximately 300-foot long potential sliding surface (Figure 7). 3) The potentially adverse northeast dipping joint surface exposed in the overhanging granite slope in the existing quarry is not planar, but irregular. The angle of irregularity above the assumed plane of sliding is added to the residual angle of surface friction to calculate the frictional resistance to sliding (Patton, 1966). 4) The potentially adverse 38° dipping joint set is discontinuous, and none of the adverse joints exposed is more longer than four feet in length. The intact rock bridges between individual adverse northeast dipping joints must be