In a standard penetration test, what are the weight of the hammer and the dropping height as per IS 2131-1981?
Try it first: pick an option, then read the explanation.
Correct answer: D. 63.5 kg, 750 mm
Asked in SSC JE 23/03/2021 (Evening); SSC JE 28/10/2020 (Evening)
Detailed Explanation
The standard penetration test (SPT) measures how hard it is to drive a split-spoon sampler into the soil at the bottom of a borehole. To make results comparable everywhere, the hammer and drop are standardised.
Why “63.5 kg, 750 mm” Is Correct
IS 2131 specifies a hammer of 63.5 kg falling freely through 750 mm. The sampler is driven 450 mm in three 150 mm stages, and the blows for the last 300 mm give the N-value.
Why the Other Options Are Wrong
- A. 2.6 kg, 310 mm: 2.6 kg and 310 mm are the rammer and drop of the light (standard) Proctor compaction test.
- B. 63.5 kg, 450 mm: The hammer weight is right, but 450 mm is the drop of the heavy compaction rammer, not the SPT.
- C. 4.89 kg, 450 mm: 4.89 kg and 450 mm are the rammer and drop of the heavy (modified) Proctor test.
Key Point
SPT: 63.5 kg hammer, 750 mm free fall, N = blows for the last 300 mm.
Key Concepts for Students
- The split-spoon sampler has an outside diameter of about 50 mm.
- SPT is most reliable in sands; less so in clays and gravels.
- N-values are corrected for overburden pressure and dilatancy.
Practice more: this is question 2 in Soil Exploration MCQs – Part 1. Try the whole set and check every answer instantly.