Shear stress that a saturated soil sustains at failure under conditions in which no pore‑water drainage occurs during the shearing process; expressed as a total‑stress parameter (commonly Su) and used to assess short‑term stability and design where rapid loading precludes excess pore‑pressure dissipation.

Principle

When drainage is prevented, pore pressures generated by shear alter effective stress but do not dissipate during the event; the measurable shear resistance is therefore a total‑stress quantity that controls short‑term failure behaviour.

Demonstration

Illustrative scenario → Situation: A freshly excavated clay slope experiences rapid surcharge from construction traffic. Recognition: The clay is saturated and loading is fast relative to drainage paths. Action: Use Su (from field vane or undrained triaxial test) to compute short‑term factor of safety. Consequence: Predicted stability reflects total‑stress strength; if pore pressures were instead assumed to dissipate, the slope would appear erroneously safer.

Misapplication

Treating Su as equivalent to long‑term (drained) shear strength or using Su obtained on a remoulded sample as the intact soil short‑term strength; the error is conflating a total‑stress, rate‑dependent measurement with effective‑stress parameters that govern drained behaviour.

Consequence

Using Su appropriately yields realistic short‑term stability estimates and design loads for rapid events; using it where drainage occurs leads to conservative or unconservative designs depending on whether long‑term strength is greater or smaller than Su.

Reversal

If drainage is permitted (sufficient time, high permeability, or engineered drains) or the soil is unsaturated so that pore‑pressure generation differs, the undrained shear strength no longer governs behaviour and effective‑stress parameters must be used instead.

Boundary

Clearly within: saturated, fine‑grained soils subjected to rapid loading (vane or UU test conditions). Boundary case: transitional permeability or partial saturation where some pore‑pressure dissipation occurs during shearing. Clearly outside: long‑term drained strength described by effective cohesion and friction (c′, φ′) or coarse, freely draining granular soils under slow loading.

Semantic Tension

Su (total‑stress, short‑term parameter) ↔ effective‑stress strength parameters (c′, φ′) used for long‑term, drained behaviour; designers must choose the parameter consistent with drainage conditions and time scale.

Synthesis

Undrained shear strength is an operational, time‑scale dependent measurement: it represents how a saturated soil resists shear when pore pressures cannot dissipate and must be applied only where the loading and drainage conditions match the measurement assumptions.