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Drained vs undrained shear strength

Short answer

Drained shear strength is measured under slow loading allowing drainage, with zero excess pore pressure, using effective stress parameters c' and φ'. Undrained strength is measured under rapid loading with no drainage, causing excess pore pressure; for saturated clay, φ_u = 0 and τ_f = c_u. Drained governs long-term stability, undrained governs short-term.

What are the differences between drained and undrained shear strength?

Drained shear strength is the strength mobilised when the soil is sheared slowly enough that water can drain freely, so no excess pore pressure develops. It is expressed in effective stress. Undrained shear strength is the strength when the soil is sheared quickly with no drainage, so volume cannot change and excess pore pressure develops.

PointDrained strengthUndrained strength
DrainageAllowed during shearingNot allowed during shearing
Excess pore pressureZeroDevelops and changes the effective stress
Stress usedEffective: τf=c′+σ′tan⁡ϕ′\tau_f = c' + \sigma'\tan\phi'Total: τf=cu+σtan⁡ϕu\tau_f = c_u + \sigma\tan\phi_u
Saturated clayc′c' and ϕ′\phi' both presentϕu=0\phi_u = 0, so τf=cu\tau_f = c_u
TestCD test, slow direct shearUU test, UCS test, vane shear
Loading rateSlowRapid
GovernsLong-term stability (after construction)Short-term stability (end of construction)
Soil usuallySand, gravel, clay after long timeSaturated clay under quick loading

✓ Answer checked by Claude Opus 5.5 (high effort)

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