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35' & 45' friction angles as appropriate . The safety factors are <br /> shown in the following table using the formula FS = Tan (D =Tan6, <br /> where 6 is the slope angle from the toe of the slope to the <br /> closest structure (permit line) . I have also calculated the <br /> Factor of Safety for the working face and the 3 : 1 slopes . <br /> SLOPE EVALUATED CALCULATED FACTOR OF SAFETY FOR <br /> SLOPE ANGLE NATURAL SLOPE ANGLE <br /> o 0=35° OR 0=45° <br /> DIRT-SLOPE FROM TOE TO 6.5° 6.15 <br /> CLOSEST STRUCTURE <br /> SANDSTONE -SLOPE FROM 23 5'(') 2.30 <br /> TOE TO CLOSEST STRUCTURE <br /> CLAY-SLOPE FROM TOE TO 28 2° 1.31 <br /> CLOSEST STRUCTURE <br /> (1) <br /> 23.5" angle is calculated from 35 feet slope height and 53 feet horizontal <br /> distance from the toe of sandstone layer to the permit line <br /> As shown above, the safety factor for the closest structure is <br /> 1 . 31 when the mining face is mined near vertical and a minimum of <br /> a 50 feet mining setback is maintained. Note that as the mining <br /> face moves away from the structure the safety factor increases . <br /> Instead of risking a slope failure and for safety reasons, Summit <br /> Brick and Tile has chosen to use the method of slope stabiliza- <br /> tion described in the permit . <br />