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. What is the design approach for the spacing of min-piles? (MP1)
For close spacing of min-piles, it would provide substantial cost savings
with the reduction of pile cap size. However, close spacing of piles implies
the problem of group effect which tends to reduce the load carrying
capacity of each pile member. Notwithstanding this, there is well
established rule which govern the minimum spacing of piles, i.e. for friction
piles like mini-piles, the centre-to-centre spacing should not be less than
the perimeter of the pile.
14. Should a pipe or groups of bars be adopted as load carrying
element of min-piles?
The design of mini-piles somehow differs from other traditional pile types.
For instance, the design of most common pile types is controlled by the
external carrying capacity. However, owing to the small cross sectional
area, the design of mini-piles is limited by internal carrying capacity. Hence,
the choice of suitable load carrying element is of paramount importance in
the design of mini-piles.
For steel pipes used as load carrying element, it is of circular cross section
with a high radius of gyration. Moreover, it possesses a constant section of
modulus in all directions, it serves the properties of excellent column. Bars
are suitable when pure axial loading is required in confined situation.
15. Should engineers consider strain compatibility when designing
mini-piles? (MP2)
In designing the axial capacity of mini-piles, grout may be taken into
account in the contribution of axial load capacity. However, the total load
capacity of min-piles may not be equivalent to the sum of individual
capacity derived from grout and from steel H-section. The reason behind
this is that the vertical loads on mini-piles are shared among grout and
steel sections based on their Young’s modulus and areas. Basically, in
order to comply with strain compatibility criterion, the steel bars and grout
will deform as a whole though they possess different stiffness. A case may
occur in which the sharing of loads for grout may be too high which cracks
the grout section and fails the mini-piles already before the whole pile
section could attain the full design load which is assumed to be the sum of
individual capacities. Hence, strain compatibility has to be checked in
designing the vertical capacity of min-piles [28].
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