Retaining wall stability: If W is the self-weight of the wall and P is the horizontal active earth pressure, what minimum factor of safety against sliding is generally adopted at working loads?
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A1.0
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B1.25
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C1.5
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D2.0
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E2.5
Answer
Correct Answer: 1.5
Explanation
Introduction / Context:Sliding resistance of retaining walls is checked by comparing available resisting forces with the driving horizontal earth pressure. Design guides commonly recommend a minimum factor of safety (FS) at service loads.
Given Data / Assumptions:
- Retaining wall on soil or rock foundation.
- Service load combinations (working state).
- FS_sliding = (frictional resistance + passive resistance, if any) / P.
Concept / Approach:
A typical target is FS ≈ 1.5 for sliding under service loads, unless otherwise required by project codes or seismic provisions.
Step-by-Step Solution:
Compute resisting force = μ (W + any vertical surcharge on base) + passive (if mobilized).Compute FS = Resisting / P and check FS ≥ 1.5.Verification / Alternative check:
Many texts align on 1.5 for sliding and 1.5–2.0 for overturning (varies by standard). Ultimate limit-state methods convert to partial factors instead.
Why Other Options Are Wrong:
1.0 and 1.25 are too low; 2.0–2.5 may be overly conservative for routine static cases unless specified by the client or code.
Common Pitfalls:
Ignoring base key contribution; overestimating passive resistance without proper compaction; neglecting uplift water pressure.
Final Answer:
1.5