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 Chellis (1961) observed that some of these
formulae were based on the assumptions that the
stress wave due to pile driving travels very fast
down the pile and the associated strains in the
pile are considerably less than those in the soil.

 As a result the action of the blow is to create an
impulse in the pile which then proceeds to travel
into the ground as a rigid body.

 Where these conditions are fulfilled, dynamic
formulae give good predictions.

 As noted by Chellis, if the set becomes small such
that the second condition is not met, then the
formulae may become unreliable.

 Like other driving formulae, Hiley 's formula
does not account for the rate at which the soil
is sheared during pile driving and therefore
cannot differentiate between cohesive and
granular soils.

 The high strain rates in cohesive soils during
pile penetration can cause the viscous
resistance of the soil to be considerably greater
than the static capacity of the pile.

 Poskitt (1991) shows that without considering
soil damping, the driving resistance can be
overestimated by several folds.

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