278 Pile groups under compressive loading
settlement caused by the working load on the piles. The negative skin friction on the piles is
not included in the working load for the latter analysis.
EC 7 gives no specific guidance for the design of pile groups carrying compression load-
ing in filled ground. As in the case of the single pile calculation, the load distribution on
individual piles in the group is best undertaken by an interaction analysis as described
in Section 4.3.8. It is evident that treatment of the group as a single large-diameter pile as
recommended in Clause 7.6.2.1(4) for the determination of group stability is not valid for
application to an interaction analysis.
Clause 7.3.2.2(P) requires account to be taken of the weight density of materials in a
settlement analysis for piles in filled ground. As noted in the case of the single pile the partial
factors for weight in Table A.4 of EC 7 Annex A are unity (see Table 4.2 Section 4.1.4).
5.7 Effects on pile groups of installation methods
When piles are driven in groups into clay the mass of soil within the ground heaves and also
expands laterally, the volume of this expansive movement being approximately equal to the
volume occupied by the piles. High pore pressures are developed in the soil mass, but in the
course of a few days or weeks these pore pressures dissipate and the heaving directly caused
by pore pressure subsides. In soft clays the subsidence of the heaved soil can cause negative
skin friction to develop. It is not usual to add this negative skin friction to the working load
since it is of relatively short duration, but its effect can be allowed for by ignoring any
supportprovided in shaft friction to the portion of the pile shaft within the soft clay.
Methods of calculating the surface heave within a pile group have been discussed by
Hagerty and Peck(5.28). Chow and Teh(5.29)have established a theoretical model relating
the pile head heave/diameter ratio to the pile spacing/diameter ratio for a range of length/
diameter ratios in soft, firm and stiff clays.
It is not good practice to terminate pile groups within a soft clay since the reconsolidation
of the heaved and remoulded soil can result in the substantial settlement of a pile group, and
neighbouring structures can be affected. It may be seen from Figure 5.37 that there is little
difference between the extent of the stressed zone around and beneath a surface raft and
a group of short friction piles. The soil beneath the raft is not disturbed during construction
and hence the settlement of the raft may be much less than that of a pile group carrying the
same overall loading. This was illustrated by Bjerrum(5.30), who compared the settlement of
buildings erected on the two types of foundation construction on the deep soft and sensitive
clays of Drammen near Oslo.
A building where the gross loading of 65 kN/m^2 was reduced by excavation for a base-
ment to a net loading of 25 kN/m^2 was supported on 300 timber friction piles 23 m long. In
10 years the building had settled by 110 mm and the surrounding ground surface had settled
by 80 mm. A nearby building with a gross loading of 55 kN/m^2 had a fully compensated
unpiled foundation, i.e. the weight of the soil removed in excavating for the basement bal-
anced the superstructure and substructure giving a net intensity of loading of zero on the
soil. Nearly 30 mm of heave occurred in the base of excavation and thus the settlement of
the building was limited to the re-consolidation of the heaved soil. The net settlement 9 years
after completing the building was only 5 mm.
Lateral movement of a clay soil and the development of high pore pressures can damage
structures or buried services close to a pile group. Adams and Hanna(5.31)measured the pore