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
21. How does false set occur in pile driving? (DP10)
For pile driving in certain soils like dense silt and weathered rock, the
occurrence of false set phenomenon is not uncommon. During the driving
process, negative pore water pressure is developed and the driven piles
appear to have sufficient capacity during pile driving as the built-up of
negative pore water pressure leads to an apparent temporary increase in
driving resistance and strength. However, some time after the pile driving,
the dissipation of this negative pore water pressure would reduce the
bearing strength in resisting the design loads. Sometimes, the presence of
cracks along pile sections may bring about the problem of false set by the
dampening effect of stress waves by these cracks. To avoid the problem of
false set, a certain percentage of constructed piles should be chosen to
perform re-driving to check for the false set phenomenon [26].
22. Why are holes present in steel plates connecting to H-piles?
There are two kinds of holes present in the steel plate connected to H-piles
in the pile cap. The first kind of holes is designed to be filled with welding
for better connection with H-piles. The second kind of holes is present to
facilitate concreting works of the pile caps. In fact, the void space
underneath the steel plate is hardly to be accessed by concrete and these
holes provide alternative paths to gain entry into these hidden void.
23. How can piles be driven through steeply dipping karst surfaces?
The steep dipping and variable nature of karst surfaces poses problems for
installation of driven piles. Very often, the consequences of hard driving
piles over steeply-inclined karst are slipping and buckling of piles. To tackle
these problems, the following two options are mostly adopted:
(i) Pre-boring is carried out in steep dipping karst surface as this
method could penetrate hard layers;
(ii) Reinforcing the end section of driven piles by welding stiffening
plates
24. In the installation of strain gages in driven H-piles to measure
loads, why should they be normally used in pairs?
Strain gages are often installed in driven piles to measure the load
distribution along the piles. They have to be protected from being removed
as the pile is driven into the ground. Protection of strain gages is achieved
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