GATE 2023 CE (CE2) – Question 18
A standard penetration test (SPT) was carried out at a location by using a manually operated hammer dropping system with 50% efficiency. The recorded SPT value at a particular depth is 28. If an automatic hammer dropping system with 70% efficiency is used at the same location, the recorded SPT value will be ______.
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Correct answer: (B) 20
Explanation
The SPT blow count depends on the energy actually delivered to the sampler. The energy delivered is proportional to the hammer efficiency $\eta$ times the blow count needed.
For the same soil (the same resistance to penetration), the product of blow count and efficiency is constant:
$$N_1\eta_1=N_2\eta_2 .$$
$$N_2=N_1\frac{\eta_1}{\eta_2}=28\times\frac{0.50}{0.70}=20 .$$
A more efficient hammer needs fewer blows, so the count drops from 28 to **20** (option B).