28 Calculate factor of safety (FS) of the unreinforced slope using equation (1) below to determine the need for rock bolts1. The F.S should be calculated for both the wet and dry conditions2. Decide upon the minimum FS necessary to insure slope stability3. Calculate the total rock bolt force T (equation 2)4. The rock bolt lengths should, in general, be determined such that when using the design bolt spacing, the force cone generated from the bottom of the anchor overlaps
29 6 = angle of rock bolt with horizontal; bolt sloping up above whereW = weight of rock6 = angle of rock bolt with horizontal; bolt sloping up abovehorizontal is of a negative anglec1 = angle of discontinuity to the horizontalE = angle of resultant force due to earthquake or blasting forcesand Wc = cohesion of rockH = vertical distance from point where discontinuity daylights onslope to top of slopeu = groundwater forceT = total force due to rock anchorsEquation (1) was selected for the calculation of the FS since the equation contains most of the parameters required for slope-stability analysis and is straightforward to use.To determine the optimum direction of the rock bolts, equation (1) is differentiated with respect to 6 . This gives a maximum FS when
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