A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
zone of 45 o spreading from the pile at higher elevation, further load
checking is required for the lower pile.
This 45 o load spread rule is a common practice of foundation engineers
and may not be incorporated in foundation codes. There are some
situations where this rule may fail. For instance, when two piles at the
same foundation level are situated close to each other, they shall have
load effect on each other. However, based on 45 o load spread rule, it could
not address this concern.
7. Which one is better in driven piles, high hammer/pile weight ratio
or low hammer/pile weight ratio? (DP3)
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.
8. Some piling contractors incline to use drop hammer instead of
hydraulic hammers in setting of piles. Why? (DP3 & DP4)
The use of high grade and heavy steel pipes may not warrant the use of
drop hammer. For pile driving of Grade 55C steel section, it requires
heavier drop hammer with increased drop height so as to comply with
penetration resistance at final set. As such, it is possible that such heavy
hammer may damage the pile and endangers the worker who takes the
final set record.
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