Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
The minimum factors of safety recommended for design are summarised in Table 9.3. For vertical
piles designed to resist lateral load, it is usually governed by the limiting lateral deflection
requirements.
For piles in sloping ground, the ultimate lateral resistance can be affected significantly if the piles are
positioned within a distance of about five to seven pile diameters from the slope crest. Based on fullscale
test results, Bhushan et al (1979) proposed that the lateral resistance for level ground be
factored by 1/(1 + tan θ s ), where θ s is the slope angle. Alternatively, Siu (1992) proposed a
simplifying method for determining the lateral resistance of a pile in sloping ground taking into
account three-dimensional effects.
9.3.3 Inclined Loads
If a vertical pile is subjected to an inclined and eccentric load, the ultimate bearing capacity in the
direction of the applied load is intermediate between that of a lateral load and a vertical load
because the passive earth pressure is increased and the vertical bearing capacity is decreased by the
inclination and eccentricity of the load. Based on model tests, Meyerhof (1981) suggested that the
vertical component Q v , of the ultimate eccentric and inclined load can be expressed in terms of a
reduction factor r f on the ultimate concentric vertical load Q o , as given in Figure 9.13.
The lateral load capacity can be estimated following the methods given in Item 9.3.2 above. Piles,
subjected to inclined loads, should also be checked against possible buckling, pile head deflection
and induced bending moments.
9.3.4 Raking Piles in Soil
Raking piles provide a common method of resisting lateral loads. For the normal range of inclination
of raking piles used in practice, the raking pile may be considered as an equivalent vertical pile
subjected to inclined loading.
Deformations and forces induced in a general pile group comprising vertical and raking piles under
combined loading condition are not amenable to presentation in graphical or equation format. A
detailed analysis will invariably require the use of a computer.
Zhang et al (2002) conducted centrifuge tests to investigate the effect of vertical load on the lateral
response of a pile group with raking piles. The results of the experiments indicated that there was a
slight increase in the lateral resistance of the pile groups with the application of a vertical load.
a) Methodologies for Analysis
i) Stiffness method can be used to analyse pile groups comprising vertical piles and raking piles
installed to any inclination. In this method, the piles and pile cap form a structural frame to carry
axial, lateral and moment loading. The piles are assumed to be pin-jointed and deformed elastically.
The load on each pile is determined based on the analysis of the structural frame. The lateral
restraint of the soil is neglected and this model is not a good representation of the actual behaviour
of the pile group. The design is inherently conservative and other forms of analyses are preferred for
pile groups subjected to large lateral load and moment (Elson, 1984).
ii) A more rational approach is to model the soil as an elastic continuum. A number of
commercial computer programs have been written for general pile group analysis based on idealising
the soil as a linear elastic material, e.g. PIGLET (Randolph, 1980), DEFPIG (Poulos, 1990a), PGROUP
(Bannerjee & Driscoll, 1978). The first two programs are based on the interaction factor method
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