In pile driving practice, the minimum drop (fall) of the drop hammer on piles should be at least how much? Select the most appropriate minimum value used to ensure sufficient driving energy without damaging the pile head.

Introduction / Context:
Pile driving with a drop hammer develops driving energy equal to hammer weight multiplied by the drop height. A practical lower bound on the drop height is adopted to ensure adequate energy transfer for penetration while limiting impact damage to the pile head and minimizing excessive set measurements.

Given Data / Assumptions:

• Conventional drop-hammer pile driving for timber, precast concrete, or steel piles.
• Typical site practice with helmet and cushion blocks in place.
• Question asks for a common minimum drop used in field guidelines.

Concept / Approach:
Driving energy per blow E is approximated by E = W * h, where W is hammer weight and h is drop height. Too small a drop results in low energy, many blows, and poor efficiency; too large a drop risks overstressing or shattering the pile head. Many standard practices adopt a minimum drop around 1 m as a balanced choice, especially for moderate hammer weights, to achieve practical set with reasonable blow counts.

Step-by-Step Solution:
Identify the energy requirement: a minimum drop is needed so that E = W*h is not trivial.Field rules of thumb commonly specify h ≈ 1.0 m as the minimum safe and effective drop.Select the nearest listed value matching the practical minimum.

Verification / Alternative check:
Driving formulae (e.g., engineering practice variants of Hiley/ENR) include hammer efficiency, set per blow, and compressibility; using a drop below about 1 m for standard hammers typically yields inefficient progress and inconsistent sets, supporting the 1 m minimum convention.

Why Other Options Are Wrong:

• 80 cm: Often insufficient energy; leads to excessive blows and unreliable set.
• 120–150 cm or 140 cm: Possible in practice but not the minimum; higher drops are selected based on pile capacity and hammer–pile compatibility.

Common Pitfalls:
Confusing minimum drop with optimum drop; ignoring helmet/cushion condition; using very high drops on fragile pile types without proper cushioning.

Final Answer:
100 cm