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
16. In designing mini-piles, should the strength of grout be neglected
during assessment of loading carrying capacity? (MP3)
In designing min-piles, there are two approaches available:
(i) In the first approach, the axial resistance provided by the grout is
neglected and steel bars take up the design loads only. This approach
is a conservative one which leads to the use of high strength bars e.g.
Dywidag bar. One should note that bending moment is not designed
to be taken up by min-piles because of its slender geometry.
(ii) In the second approach, it involves loads to be taken up by both grout
and steel bars together. In this way, strain compatibility requirement of
grout and steel has to be satisfied.
17. What are the considerations in determining whether casings
should be left in for mini-piles? (MP4)
Contrary to most of pile design, the design of min-piles are controlled by
internal capacity instead of external carrying capacity due to their small
cross-sectional area.
There are mainly two reasons to account for designing mini-piles as friction
piles:
(i) Due to its high slenderness ratio, a pile of 200mm diameter with 5m
long has a shaft area of 100 times greater than cross-sectional area.
Therefore, the shaft friction mobilized should be greater than end
resistance.
(ii) Settlements of 10%-20% of pile diameter are necessary to mobilize
full end bearing capacity, compared with 0.5%-1% of pile diameter to
develop maximum shaft resistance.
Left-in casings for mini-piles have the following advantages:
(i) Improve resistance to corrosion of main bars;
(ii) Provide additional restraint against lateral buckling;
(iii) Improve the grout quality by preventing intrusion of groundwater
during concreting;
(iv) Prevent occurrence of necking during lifting up of casings during
concreting.
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