In a plate load test, the minimum size of plate to be used is:
Try it first: pick an option, then read the explanation.
Correct answer: B. 300 mm × 300 mm
Asked in SSC JE 2015
Detailed Explanation
The plate in a plate load test must be big enough to load a representative volume of soil, but small enough to be loaded with a practical jack and reaction.
Why “300 mm × 300 mm” Is Correct
IS 1888 specifies square (or circular) mild steel plates from 300 mm × 300 mm up to 750 mm × 750 mm, at least 25 mm thick. The minimum is therefore 300 mm × 300 mm. The test pit is at least five times the plate width, e.g. 1.5 m × 1.5 m for a 300 mm plate.
| Item (IS 1888) | Value |
|---|---|
| Plate | Square or circular, 300 mm to 750 mm, mild steel, at least 25 mm thick |
| Pit width | At least 5 × plate width |
| Seating load | About 7 kN/m² (0.07 kg/cm²), released before the test |
| Loading | Steps of about one-fifth of the expected safe load |
| Readings | Settlement of the plate on dial gauges, until it nearly stops for each step |
| Result | Load–settlement curve → ultimate bearing capacity and settlement |
Why the Other Options Are Wrong
- A. 250 mm × 250 mm: 250 mm is smaller than the minimum plate size.
- C. 400 mm × 400 mm: 400 mm is allowed but is not the minimum.
- D. 450 mm × 450 mm: 450 mm is allowed but is not the minimum.
Key Point
Plate: 300 mm (min) to 750 mm (max), ≥ 25 mm thick; pit ≥ 5B wide.
Key Concepts for Students
- Larger plates give results closer to real footing behaviour.
- Clay: ultimate bearing capacity is roughly independent of plate size.
- Sand: bearing capacity increases with plate size.
Practice more: this is question 33 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.