(b) Rigorous method Fig. 5.25 Example 5.6.

This value was obtained by assuming a value for F of 1.5 in the expression: sec a

To be accurate, columns 5 and 6 should be recalculated using an F value near to the one obtained from the first approximation. In the example F was put equal to 1.3 and the final value for F was

This is near enough to the assumed value of 1.3 to be taken as correct, so the factor of safety for the slope is 1.26.

Only rarely are more than two approximations necessary, and often the procedure need only be carried out once if the assumed value for F was close to the final value obtained.

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