which refers to the cushion material
that is used between the hammer and the pile so that the top of the pile will not be damaged from the
impact of the hammer. The cushion material often consists of wood blocks placed between the pile
top and hammer, such as shown in Fig. 6.9. Initially, the wood blocks absorb a considerable amount
of the impact energy, but as driving progresses, the wood blocks become compressed and transfer
more of the energy to the pile (see Figs. 6.10 and 6.11). Hence the amount of energy transmitted to
the piles can be highly variable both due to equipment operator performance and due to compression
of the cushion material. These and other parameters that are constantly changing or hard to predict
have led to the conclusion from Bowles (1982) that for the wave equation “any comparison between
the computer output and predicted pile capacity within a 30 percent deviation is likely to be a happy
coincidence of input data.”