Pile Design and Construction Practice, Fifth edition

272 Pile groups under compressive loading
5.4 Eurocode 7 recommendations for pile groups
Clause 7.6.2.1 of EC7 requires the stability of a pile group to be considered both in
relation to the risk of failure of an individual pile in the group and to the failure of the
group considered as an equivalent block foundation. Sub-clause (4) states that the block
foundation can be considered to act as a single large-diameter pile. However, no guidance
is given as to relationship between the diameter and depth of this pile to the shape, base
area and depth of the group. If it is assumed that the plan area of the large-diameter pile
is equal to the gross area of the group, then in the case of square (or rectangular) groups
the resulting calculations could give an over-conservative value of the design load. This
is because the perimeter and shape factors for rectangular bases are larger than those for
circular sections. Also it is reasonable to assume that the shaft friction of the pile should
be calculated on the basis of a soil-to-soil interface using the undisturbed shear strength
of the surrounding soil. Whereas when calculating the shaft friction on an individual
pile the installation method has an important influence on the resistance of a pile to soil
interface. Where a group of piles are driven into a clay the surrounding soil is strengthened
by expulsion of pore water, and a sand is strengthened by densification.
Conversely, drilling for a group of bored piles could cause weakening of a clay due
to relaxation of a fissured structure or drilling in sand could result in loss of resistance
in friction.
If as an alternative to the large-diameter pile assumption the pile group is treated as an
equivalent block foundation, the partial factors for actions and material properties are the
same as used for piled foundations (Tables 4.1 and 4.2 in Section 4.1.4). The base resistance
factor for spread foundations, � Rv, and the factor for sliding � Rh, are both unity for
set 1 in DA1 (Table A5 in Annex A of EC7). There are no R4 resistance factors for spread
foundations.
Worked Example 5.1 at the end of this chapter shows that the assumption of a single
large-diameter pile under-estimates the resistance of a rectangular group in clay, compared
with calculations assuming a block foundation of the same dimensions as the prototype,
when using the calculation method described in Section 5.2.1.
Clause 7.6.4.2(2)P states that the assessment of settlement of pile groups should take
into account the settlement of the individual piles as well as that of the group, but it does
not make it clear whether the settlement analysis should assume that the group acts as an
equivalent large-diameter pile or as a block foundation. Presumably the latter is the case,
for which Clause 6.6.2, considering the settlement of spread foundations, requires the
depth of the compressible soil layer to be taken normally as the depth at which the effective
vertical stress due to the foundation load is 20% of that of the effective overburden
stress, which may in many cases be roughly estimated as one to two times the foundation
width or less for lightly loaded foundation rafts. In the case of pile groups the authors
assume this to be the depth below the base of the equivalent rafts shown in Figure 5.3.
5.5 Pile groups terminating in rock
The stability of a pile group bearing on a rock formation is governed by that of the individual
pile. For example, one or more of the piles might yield due to the presence of a pocket
of weathered rock beneath the toe. There is no risk of block failure unless the piles are
terminated on a sloping rock formation, when sliding on a weak clay-filled bedding plane