Why dynamic pile formulae are inaccurate

Dynamic pile formulae (Engineering News, Hiley, Danish) estimate the capacity from the set per blow, but they are inaccurate for the following reasons.

1. Energy losses are uncertain. The energy lost in the hammer, cap, cushion, pile compression and soil quake cannot be assessed correctly. Efficiency is only assumed.
2. Dynamic resistance differs from static resistance. Pore water pressure, soil damping and strain rate during driving give a resistance that differs from the static capacity, especially in clays and saturated fine sands.
3. Impact is treated as a rigid-body collision. The formulae use a coefficient of restitution, which ignores wave propagation in the pile (the stress wave takes time to travel the length).
4. Time effects. In clays the strength after driving is regained (set-up) with time, whereas in dense saturated sands there may be relaxation, which the set at the end of driving does not show.
5. Soil variation along the pile (layers) and the shaft–tip load distribution are not accounted for.
6. Only suitable for granular soil and not for clay or long piles; the results may differ by as much as 50% or more. For these reasons a factor of safety of 2 to 6 is used, and the pile load test is preferred for important structures.