A footing is resting on fully saturated clayey strata. For checking the initial stability, shear parameters are used from:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2008
Detailed Explanation
When a footing is built on saturated clay, the load is applied much faster than the clay can drain. Immediately after construction the clay carries the load undrained, and its strength is the undrained shear strength cu.
Why “Unconsolidated undrained test” Is Correct
Because the field condition is ‘no time to consolidate, no drainage during loading’, the matching laboratory test is the unconsolidated undrained (UU) test. For saturated clay this gives φu ≈ 0 and cu, which are used for the initial bearing capacity check (qu ≈ 5.7cu or 5.14cu).
| 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. Consolidated undrained test: A CU test assumes the clay has consolidated under the load before shearing, which is not true immediately after construction.
- B. Unconsolidated drained test: ‘Unconsolidated drained’ is not a standard test.
- D. Unconsolidated undrained test with pore pressure measurement: Pore pressure measurement is not needed for a total-stress (φu = 0) analysis; the plain UU test is the standard answer.
Key Point
Short-term on saturated clay → UU (total stress, φu = 0). Long-term → CD (effective stress).
Key Concepts for Students
- For a footing on clay the short-term case is usually the most critical.
- Clay gains strength with time as it consolidates under the load.
- Unconfined compression can replace UU for intact clays.
Practice more: this is question 36 in Shear Strength of Soils MCQs – Part 3. Try the whole set and check every answer instantly.