Janbu has extended Bishop's method to free form surfaces. When free form (generic form) sliding surfaces are treated, the arm of the forces changes (in case of circular surfaces it is constant and equal to the radius of the arc) and therefore it is more convenient to evaluate the moment equation at the angle of each slice.
With the method of Janbu are taken into account the forces of interaction between the slices, but they are deemed to act along a predetermined thrust line. The solution is obtained by subsequent iterations.
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Actions on the i-the slice according to Janbu and representation of the whole slice
Assuming ΔXi = 0 is obtained by the ordinary method.
Janbu also proposed a method for the correction of the safety factor obtained by the ordinary method according to the following:
Fcorretto = f0 F
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Calculation of the correction factor f0
where fo , empirical correction factor, depends on the shape of the sliding surface and on the geotechnical parameters.
This correction is very reliable for slightly inclined slopes.
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