The appropriate triaxial test to assess the immediate stability of an unloading problem, such as excavation of a clay slope, would be the:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2015
Detailed Explanation
The right test is chosen to copy the drainage conditions in the field. Clay has very low permeability, so for a period after any change in load the pore water has no time to flow; the clay behaves undrained.
Why “Unconsolidated-undrained test” Is Correct
For immediate (short-term) stability of an excavation or cut in clay, the soil is still undrained, at its existing water content. The UU test reproduces exactly that: no consolidation, no drainage. So the UU test is appropriate.
Note: for unloading problems the long-term (drained) case is often more critical, because negative pore pressures dissipate and the clay softens. That is checked with CD or CU-with-pore-pressure tests.
| Test | Drainage during consolidation stage | Drainage during shear | Typical use |
|---|---|---|---|
| UU (unconsolidated undrained, ‘quick’) | Not allowed | Not allowed | End-of-construction / immediate stability of saturated clays |
| CU (consolidated undrained) | Allowed | Not allowed (pore pressure can be measured) | Rapid drawdown, staged loading |
| CD (consolidated drained, ‘slow’) | Allowed | Allowed | Long-term stability; effective-stress parameters c′, φ′ |
Unconfined compression is a special UU test with zero cell pressure (σ₃ = 0).
Why the Other Options Are Wrong
- B. Consolidated-undrained test: CU tests suit cases where the soil has consolidated under one load before a quick change (e.g. rapid drawdown).
- C. Consolidated-drained test: CD tests give long-term (drained) parameters, not immediate stability.
- D. Unconsolidated-drained test: ‘Unconsolidated drained’ is not a standard test.
Key Point
Immediate/end-of-construction stability of saturated clay → UU test (φu = 0 analysis).
Key Concepts for Students
- Embankment on soft clay, end of construction → UU.
- Long-term stability of cuts in clay → CD (effective stress).
- Rapid drawdown of a dam → CU.
Practice more: this is question 35 in Shear Strength of Soils MCQs – Part 3. Try the whole set and check every answer instantly.