Civil Engineering Reference
In-Depth Information
that the bearing strata are firm throughout the affected depth of this combined bulb, then the bearing
capacity of the group will be equal to the sum of the individual strengths of the piles. However, where a
compressible layer exists within or immediately below the shaded zone, checks must be made to ensure
that the layer will not be overstressed.
ii.
Friction/combination piles
Pile groups in granular soils:
Pile installation in sands and gravels causes compaction of the soil between the piles. This densifica-
tion of the soil leads to an increase in the strength of the soil such that the bearing capacity for the
group exceeds the sum of the bearing capacities of the individual piles that comprise the group.
However, as a conservative approach in design it is usual to take the
group
bearing capacity to be
equal to the sum of the
individual
bearing capacities. The spacing of the piles is usually around two to
three times the diameter, or breadth, of the individual piles.
Pile groups in cohesive soils:
By contrast, in clays, the load-carrying capacity of a group of vertically loaded piles is considerably
less than the sum of the capacities of individual piles comprising the group, and this phenomenon must
be considered in the design else excessive settlement might occur.
An important characteristic of pile groups in cohesive soils is the phenomenon of
block failure
. This is
when the entire block of soil containing the piles fails along the perimeter of the group.
For block failure
Q
=
2
D B L
(
+
)
c
u
+
1 3
.
c N BL
u
b c
where
D
, B, L are the dimensions indicated in Fig.
10.10;
c
u
is the average undrained strength along the sides of the piles;
c
b
is the undrained strength at the base of the piles;
N
c
is the bearing capacity coefficient (usually taken as 9.0)
As mentioned, in clays the capacity of an individual pile within a closely spaced group is lower than that
for an equivalent “isolated” pile. This effect is pretty insignificant and so may be ignored in design. Of
more concern however, is the fact that the block capacity of the group is less than the sum of the individual
pile capacities. The spacing of the piles is thus influential. If the piles are placed close together (i.e. less
than a distance of approximately 1.5d apart) the strength of the group may be governed by the resistance
against block failure and thus block failure becomes a likely failure mode. To prevent block failure, the
piles should be spaced about 2d - 3d apart.
In such cases:
Q E n Q
u
=
up
where
E
=
= efficiency of pile group (0.7 for spacings 2d-3d)
Q
up
=
ultimate bearing capacity of single pile
n
=
number of piles in group.
10.7.2 Settlement effects in pile groups
Quite often it is the allowable settlement, rather than the bearing resistance, that decides the working
load that a pile group may carry.
