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
13. Which one is a better choice, a large diameter piles or a system of
several smaller piles with the same load capacity?
The choice of a large diameter pile suffers from the disadvantage that
serious consequences would occur in case there is setting out error of the
pile. Moreover, in terms of cost consideration, for the same load capacity
the cost of a group of small diameter piles is generally lower than that of a
large diameter pile. On the other hand, for small diameter piles i.e.
mini-piles, they are advantageous in site locations with limited headroom
and space. In addition, in some structures with only a few piles, it is
uneconomic because of its high mobilization cost. Reference is made to Dr.
Edmund C Hambly (1979).
14. In designing the lateral resistance of piles, should engineers only
use the earth pressure against pile caps only?
In some design lateral loads are assumed to be resisted by earth pressure
exerted against the side of pile caps only. However, it is demonstrated that
the soil resistance of pile lengths do contribute a substantial part of lateral
resistance. Therefore, in designing lateral resistance of piles, earth
pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs
are adopted with modulus of reaction kept constant or varying with depth.
The normal practice of using a constant modulus of reaction for soils is
incorrect because it overestimates the maximum reaction force and
underestimates the maximum bending moment. To obtain the profile of
modulus of subgrade reaction, pressuremeter tests shall be conducted in
boreholes in site investigation. Reference is made to Bryan Leach (1980).
15. How does the pile installation method affect the load carrying
capacity of piles? (FD10)
The construction of piles by driving method causes an increase in density
of the surrounding soils. Hence, for loose soils this results in improved
compaction of soils between the piles. The sum of the capacities of all piles
as a whole is generally greater than the sum of individual pile capacities
provided that the effect of pile spacing is not taken into account. However,
for bored piles the boring operation induces considerable stress relief and
this causes a substantial reduction in shear strength of soils.
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