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Pile Group
Number & Spacing of Piles in a Group
Very rarely are structures founded on single piles. Normally,
there will be a minimum of three piles under a column or a
foundation element because of alignment problems and
inadvertent eccentricities.

The spacing of piles in a group depends upon

many factors such as

1. overlapping of stresses of adjacent piles,

2. cost of foundation, &
3. efficiency of the pile group.

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Number & Spacing of Piles in a Group … Cont’d

When piles are placed in a group, there is a possibility the
pressure isobars of adjacent piles will overlap each other.
The soil is highly stressed in the zones of overlapping of
pressures.
With sufficient overlap, either the soil will fail or the pile
group will settle excessively since the combined pressure
bulb extends to a considerable depth below the base of the
piles.
Large spacing's are not recommended since this would result
in a larger pile cap which would increase the cost of the
foundation.
The spacing of piles depends upon the method of installation
and the type of soil.
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Number & Spacing of Piles in a Group … Cont’d Typical Arrangements of Piles in Groups
When piles are cast-in-situ, the soils adjacent to the piles are not
stressed to that extent and as such smaller spicing are permitted.
If the displacement of soil compacts the soil in between the piles as in
the case of loose sandy soils, the piles may be placed at closer
intervals.
But if the piles are driven into saturated clay or silty soils, the
displaced soil will not compact the soil between the piles. As a result
the soil between the piles may move upwards and in this process lift
the pile cap. Greater spacing between piles is required in soils of this
type to avoid lifting of piles.
Generally, the spacing for point bearing piles, such as piles
founded on rock, can be much less than for friction piles since the
high-point-bearing stresses and the superposition effect of overlap of
the point stresses will most likely not overstress the underlying
material nor cause excessive settlements.
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Minimum Pile Spacing According to EBCS-7 Pile Group Efficiency

The ultimate load of the group is generally
# Pile Type S different from the sum of the ultimate loads of
1 End-bearing piles passing through 2.5d individual piles Qu.
relatively compressible strata The factor η is called group efficiency.
2 End-bearing piles passing through 3.5d
compressible strata & resting on stiff The efficiency depends on parameters such as :
clay
oType of soil in which the piles are embedded,
3 Compaction piles 2d oMethod of installation of piles i.e. either
4 Friction piles 3d driven or cast-in-situ piles, &
5 Driven piles 2.5d* oSpacing of piles.
6 Bored piles 3d & > 1.1 m
7 Under-reamed piles 2du

* This holds true for driven piles in loose sand. Otherwise, the spacing has to be > 3. 9 10

Empirical Formula

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This equation is limited to rectangular groups with identifiable values of n1 x n2.

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Driven Group Piles in Sand & Gravel Bored Pile Groups in Sand & Gravel
If piles are driven into loose sand and gravel, the soil around the piles
to a radius of at least three times the pile diameter is compacted. The
efficiency of the pile group becomes greater than unity. There will always be a general loosening of the soil during boring and
It is normally assumed that the efficiency falls to unity when the then too when the boring has to be done below the water table.
spacing is increased to five or six diameters.
Since present knowledge is not sufficient to evaluate the efficiency for Since bored piles do not compact the soil between the piles, the
different spacing of piles, it is conservative to assume an efficiency efficiency factor will never be greater than unity.
factor of unity for all practical purposes. We may, therefore, write
For practical purposes, the efficiency may be taken as 2/3-3/4.

The procedure explained above is not applicable if the pile tips rest on
compressible soil such as silts or clays. The carrying capacity of pile
groups under these conditions is governed by the shear strength and
compressibility of the soil, rather than by the "efficiency” of the
group within the sand or gravel stratum.
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Pile Groups in Cohesive Soils Pile Groups in Cohesive Soils … Cont’d

When piles are driven into clay soils, particularly when the soil is Experimental results have indicated that when a pile
soft and sensitive, there will be considerable remolding of the soil. group installed in cohesive soils is loaded, it may fail by
 Besides, there will be heaving of the soil between the piles since any one of the following ways:
compaction during driving cannot be achieved in soils of such low 1. It may fail as a block (called block failure).
permeability. 2. Individual piles in the group may fail.
There is every possibility of lifting of the pile during this
process of heaving of the soil. Bored piles are, therefore, When piles are spaced at closer intervals, the soil
preferred to driven piles in cohesive soils. contained between the piles move downward with the
piles and at failure, piles and soil move together to give
the typical “block failure”. Normally this type of failure
In case driven piles are to be used, the following steps should be
occurs when piles are placed within 2 to 3 pile
favored:
diameters.
1. Piles should be spaced at greater distances apart.
2.Piles should be driven from the center of the group towards  For wider spacings, the piles fail individually. The
the edges, and efficiency ratio is less than unity at closer spacings
3.The rate of driving of each pile should be adjusted as to and may reach unity at a spacing of about 8 diameters.
minimize the development of pore water pressure. 15 16

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Ultimate Capacity of Group Piles in Clay Example 1.6

A group of 3x3 concrete piles was established to support a column.
The soil has a cohesion of 50 kPa with an adhesion factor of α=0.8.
Each pile has a diameter of 0.25 m and a length of 12 m. The spacing
between the piles is 0.75 m. Estimate the allowable load from the
column with a safety factor of 2.5.

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Example 1.7 Allowable Loads on Groups of Piles

The allowable load on a group of piles should be the lesser of the values
For the pile group in Example 1.6, calculate the allowable load in a computed on the basis of the following two criteria:
single pile if Converse-Labarre’s formula works out well. 1. Shear failure , &
2. Settlement.

Procedures have been given in earlier sections as to how to compute the

allowable loads on the basis of a shear failure criterion.
The settlement of pile groups should not exceed the permissible limits
under these loads. This is treated subsequently.

Uniform settlement is usually of little consequence in a building, but

differential settlement can cause severe structural damage

N.B:EBCS-7 recommends a permissible total settlement of 50 mm & 75

mm on sandy and clayey soils respectively. Differential settlements
between adjacent columns up to 20 mm is often acceptable. [for shallow
foundations]
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Settlement of Single Pile in Cohesionless Soils Settlement of Single Pile in Cohesionless … Cont’d

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Settlement of Single Pile in Cohesionless … Cont’d Settlement of Single Pile in Cohesionless … Cont’d

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Settlement of Group Pile in Cohesionless Settlement of Single Pile in Clay

Settlement of a single pile in clay is more difficult to evaluate.
An assumption is required to be made regarding the level to
which the load is transferred.
If the pile penetrates homogeneous clay, the load may be taken to be
transmitted to a depth of 2/3 the embedded length of the pile from the
surface.
o If the pile penetrates a weak stratum and is embedded into a firm
stratum, the load may be taken to be transmitted to a level at a depth
of 2/3 the depth of embedment into the firm stratum from the top of
the firm stratum.
o If the pile penetrates a weak stratum and rests on the top of a firm
stratum, the load may be taken to be transferred from the top of the
firm stratum.

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Settlement of Groups of Piles in Cohesive Soils

The total settlements of pile groups may be calculated by
making use of consolidation settlement equations.
The problem involves evaluating the increase in stress Δp
beneath a pile group when the group is subjected to a vertical
load Q .
The computation of stresses depends on the type of soil
through which the pile passes.

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Settlement Computations (Clay) Example 1.8

Check if the settlement is within a permissible limit for a square
concrete pile of 0.3m width and 20 m length driven into sand if
point bearing takes 400 kN and skin resistance takes 600 kN of the
allowable load. Use Ep=21 x106 kN/m2, Es= 20 x103kN/m2, s=0.32,
=0.67and Iwp=0.85. Take the allowable settlement to be 50 mm.

 vo '  '
For normally consolidated clay, e  Cc log
 vo '
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Example 1.9
Check the adequacy of the 3x3 pile group with regard to settlement criterion.
•Spacing and diameter of piles are 0.6 and 0.2 m respectively
•DL=600 KN & LL= 400 KN
•Allowable settlement= 75 mm

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