Geotechnical Engineering

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680 GEOTECHNICAL ENGINEERING


tend to provide the group a capacity greater than the sum of the capacities of individual piles.
The capacity of the equivalent large pile is analysed by determining the skin friction resist-
ance around the embedded perimeter of the group and calculating the end-bearing resistance
by assuming a tip area formed by this block, as in Fig. 16.15.
To determine the capacity of a pile group, the sum of the capacities of the individual
piles is compared with the capacity of the single large equivalent (block) pile; the smaller of
the two values is taken. Applying an appropriate factor of safety to this chosen value, the
design load of the pile group is obtained.


The skin friction resistance of the single large equivalent pile (block) is obtained by
multiplying the surface area of the group by the shear strength of the soil around the group.
The end-bearing resistance is computed by using the general bearing capacity equation of
Terzaghi. The bearing capacity factors for deep foundations are used when the length of the
pile is at least ten times the width of the group; otherwise, the factors for shallow foundations
are used.


Obviously, the capacity of the equivalent large pile is affected by soil type and proper-
ties, besides spacing of piles. Generally speaking, there is a greater tendency for the group to
act as a block or large single unit when the piles are close and the soil is firm or compact.


Pile groups in cohesionless soils


For driven piles embedded in cohesionless soils, the capacity of the large equivalent pile (block)
will be almost always greater than the sum of the capacities of individual piles, in view of the
densification that occurs during driving. Consequently, for design, the group capacity is taken
as the sum of the individual pile capacities or the product of the number of piles in the group
and the capacity of the individual pile.


This procedure is not applicable, if the pile tips rest on compressible soils such as clays;
in such cases, the pile group capacity is governed by the shear strength and compressibility of
clay soil, rather than on the characteristics of the cohesionless soil.


Bored piles or cast-in-situ concrete piles are constructed by boring a hole of required
diameter and depth and pouring in of concrete. Boring is accompanied invariably by some
degree of loosening of the soil. In view of this, the group capacity of such piles will be some-
what less than the sum of individual pile capacities typically—about two-thirds of it. It may
also be taken as the sum of individual pile capacities approximately.


Pile groups in cohesive soils


When piles are driven into clay soils, there will be considerable remoulding especially when
the soil is soft and sensitive. The soil between the piles may also heave since compaction
cannot be easily achieved in soils of such low permeability. Bored piles are generally preferred
to driven piles in such soils. However, if driven piles are to be used, spacing of piles must be
relatively large and the driving so adjusted as to minimise the development of pore pressure.


The mode of failure of pile groups in cohesive soils depends primarily upon the spacing
of piles. For smaller spacings, ‘block failure’ may occur, in other words, the group capacity as a
block will be less than the sum of individual pile capacities. For larger spacings, failure of
individual piles may occur; or, it is to say that the group capacity is given by the sum of the
individual pile capacities, which will be smaller than the strength of the group acting as a unit

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