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The basis of all dynamic formulae is the conservation of energy in which energy applied to the piling system by driving hammer is equal to the energy required to penetrate the soils with energy losses during the process. The Hiley’s formula is the most popular dynamic formula used in Hong Kong.
The Hiley’s formula is developed for conditions of soils consisting of sands and gravels. Under this soil condition, it is assumed that there is no time dependency between dynamic and static resistance of piles. However, for clayey soils the static strength development varies greatly with time and the load capacity of driven piles in clayey soils may be lower than that of the instantaneous ones.