The 'Swedish slip circle method' (Fellenius method) for slope stability is conservative because:
Choose the correct answer
It over-estimates shear resistance
Neglects inter-slice forces → under-estimates normal stress on slip surface → under-estimates friction resistance → lower FOS
It always gives FOS > 2.0
It assumes no friction (φ=0 always)
Correct Answer
B. Neglects inter-slice forces → under-estimates normal stress on slip surface → under-estimates friction resistance → lower FOS
Why is this the answer?
Fellenius/Ordinary Method of Slices: assumes inter-slice forces are parallel to slice base (zero inter-slice shear and normal). This under-estimates normal force on slip surface → under-estimates shear resistance → FOS lower than actual. Typically FOS(OMS) = 5–20% lower than Bishop's. Very conservative for φ > 0 soils. Still used for preliminary checks. More accurate: Bishop''s simplified or Spencer''s method.
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