The 'residual shear strength' of over-consolidated clay is relevant for:
Choose the correct answer
Fresh compacted fill with no slip history
Existing (reactivated) slip surfaces in OC clay — platelet alignment gives lowest possible φ'r; must use residual for old landslides
Sand liquefaction analysis
Pile design in soft clay
Correct Answer
B. Existing (reactivated) slip surfaces in OC clay — platelet alignment gives lowest possible φ'r; must use residual for old landslides
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Residual strength: after large displacement (existing slip surface) φ'_r = much lower than peak φ'p; c'_r ≈ 0. Clay platelets align parallel to slip plane → very low friction. φ'_r = 8–15° for high-plasticity clays (bentonite); 20–25° for low-plasticity. Used in: stability analysis of first-time and reactivated landslides, old dam failures (sliding along pre-existing slip surface). Must be used if reactivation of old slip is possible.
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