Boussinesq’s closed form solution for a rod fixed at its end and hit on its top by a mass shows that compressive stress in the rod increases with the mass of hammer. On the other hand, a larger relative mass of hammer leads to lower tension stress. When an impact is made on friction pile, the compressive stresses are highest at pile top. When an impact is made on end-bearing pile, the compressive stresses may be highest at the top or the bottom of the pile.
To achieve optimal pile driving operation, the piles should be installed quickly with low blow counts. This can be achieved by heavy hammer but it is uneconomical as it requires higher lifting equipment cost and transportation cost. A lighter hammer appears to be more economical but for the same impact energy as heavy hammer, it requires a greater stroke and impact velocity which may cause damage to pile.
In fact, low hammer/pile weight ratio leads to damage at pile top or cracks along the pile. High hammer/pile weight ratio may cause compressive overstressing at the pile bottom.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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