In which of the following triaxial tests is drainage from the specimen NOT allowed at any stage?
Try it first: pick an option, then read the explanation.
Asked in SSC JE (previous year paper)
Detailed Explanation
A triaxial test has two stages: (1) applying the cell pressure, during which the specimen may or may not be allowed to consolidate, and (2) shearing by increasing the axial load, during which it may or may not be allowed to drain. The test names describe these two stages.
Why “Unconsolidated undrained” Is Correct
‘Unconsolidated’ means no drainage in stage 1 and ‘undrained’ means no drainage in stage 2. So in the UU test drainage is never allowed. It is fast (often 10–15 minutes) and the water content stays constant throughout.
| 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
- A. Unconfined compression: Unconfined compression is also undrained, but it is a special test with no cell pressure; among the triaxial tests the one defined by no drainage at any stage is the UU test.
- B. Consolidated drained: In a CD test drainage is allowed in both stages.
- C. Consolidated undrained: In a CU test drainage is allowed during consolidation (stage 1).
Key Point
UU: closed–closed. CU: open–closed. CD: open–open.
Key Concepts for Students
- UU on saturated clay gives φu ≈ 0 and strength cu.
- CU with pore pressure measurement gives both total and effective parameters.
- CD tests on clay are slow because shearing must be slow enough for full drainage.
Practice more: this is question 19 in Shear Strength of Soils MCQs – Part 2. Try the whole set and check every answer instantly.