On piles, the drop of the hammer must be at least:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 29/01/2018 (Evening)
Detailed Explanation
A drop hammer is the simplest pile-driving hammer: a heavy weight is lifted between guides (leads) by a winch and allowed to fall freely onto the pile head. The energy per blow is the hammer weight times the height of fall.
Why “120 cm” Is Correct
Too small a fall gives too little energy and the pile hardly moves; too large a fall can damage the pile head. Building construction texts used for SSC JE give the drop as at least about 1.2 m (120 cm), so option (c) is the expected answer. In practice the fall of a drop hammer is usually in the range of about 1.2 m to 1.5 m, and a heavier hammer with a shorter fall is preferred to a light hammer with a long fall.
Why the Other Options Are Wrong
- A. 80 cm: 80 cm is below the quoted minimum.
- B. 100 cm: 100 cm is below the quoted minimum.
- D. 140 cm: 140 cm is within the usual range but is not the minimum asked for.
Key Point
Drop hammer: free fall ≈ 1.2 m (minimum quoted); prefer heavy hammer, short drop.
Key Concepts for Students
- Energy per blow = W × h.
- Pile driving formulas (Engineering News, Hiley) use the hammer energy and set per blow.
- A helmet and dolly cushion the pile head during driving.
Practice more: this is question 23 in Shallow and Deep Foundation MCQs – Part 2. Try the whole set and check every answer instantly.