For a routine consolidation test in the laboratory, the thickness of the specimen is:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2008
Detailed Explanation
The one-dimensional consolidation (oedometer) test is carried out on a thin disc of soil held in a rigid ring, between porous stones, and loaded in stages (each usually kept for 24 hours). IS 2720 (Part 15) specifies the standard specimen size.
Why “20 mm” Is Correct
The routine specimen is 20 mm thick (with a diameter of 60 mm). A thin specimen keeps the drainage path short (10 mm with double drainage), so each load stage reaches the end of primary consolidation within a day. The diameter-to-thickness ratio of about 3 also limits the effect of side friction against the ring.
| Item | Standard value (IS 2720 Part 15) |
|---|---|
| Specimen diameter | 60 mm (larger sizes such as 75 mm also allowed) |
| Specimen thickness | 20 mm |
| Drainage | Top and bottom porous stones (double drainage) |
| Load increment ratio | Each load usually double the previous one (Δσ/σ = 1) |
| Duration of each stage | Usually 24 hours |
Why the Other Options Are Wrong
- A. 10 mm: 10 mm is the drainage path of a 20 mm specimen with double drainage, not the specimen thickness.
- C. 40 mm: 40 mm would make each stage take about four times as long, which is impractical for routine testing.
- D. 60 mm: 60 mm is the diameter of the standard specimen, not its thickness.
Key Point
Oedometer specimen: 60 mm diameter × 20 mm thick, double drainage (d = 10 mm).
Key Concepts for Students
- Results from the test: e – log σ′ curve, Cc, Cr, σ′c, av, mv and cv.
- Because time scales with d², a 2 m thick doubly drained clay layer takes 10,000 times longer than the 20 mm specimen to reach the same U.
- Undisturbed samples are trimmed into the ring carefully to avoid disturbance.
Practice more: this is question 23 in Soil Consolidation MCQs – Part 2. Try the whole set and check every answer instantly.