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

Chapter 9 FOUNDATION ENGINEERING
while the last one uses the boundary element method. A brief summary of the features of some of
the computer programs developed for analysis of general pile groups can be found in Poulos (1989b)
and the report by the Institution of Structural Engineers (ISE, 1989). Computer analyses based on
the elastic continuum method generally allow more realistic boundary conditions, variation in pile
stiffness and complex combined loading to be modelled.
Comparisons between results of different computer programs for simple problems have been carried
out, e.g. O'Neill & Ha (1982) and Poulos & Randolph (1983). The comparisons are generally
favourable with discrepancies which are likely to be less than the margin of uncertainty associated
with the input parameters. Comparisons of this kind lend confidence in the use of these programs for
more complex problems.
Pile group analysis programs can be useful to give an insight into the effects of interaction and to
provide a sound basis for rational design decisions. In practice, however, the simplification of the
elastic analyses, together with the assumptions made for the idealisation of the soil profile, soil
properties and construction sequence could potentially lead to misleading results for a complex
problem. Therefore, considerable care must be exercised in the interpretation of the results.
The limitations of the computer programs must be understood and the idealisations and assumptions
made in the analyses must be compatible with the problem being considered. It would be prudent to
carry out parametric studies to investigate the sensitivity of the governing parameters for complex
problems.
b) Choice of Parameters
One of the biggest problems faced by a designer is the choice of appropriate soil parameters for
analysis. Given the differing assumptions and problem formulation between computer programs,
somewhat different soil parameters may be required for different programs for a certain problem.
The appropriate soil parameters should ideally be calibrated against a similar case history or derived
from the back analysis of a site-specific instrumented pile test using the proposed computer program
for a detailed analysis.
9.3.5 Lateral Loading
9.3.5.1 General
The response of piles to lateral loading is sensitive to soil properties near the ground surface. Due to
the proneness to disturbance of these surface layers, reasonably conservative soil parameters should
be adopted in the prediction of pile deflection. An approximate assessment of the effects of soil
layering can be made by reference to the work by Davisson & Gill (1913) or Pise (1982).
Poulos (1972) studied the behaviour of a laterally-loaded pile socketed in rock. He concluded that
socketing of a pile has little influence on the horizontal deflection at working load unless the pile is
sufficiently rigid, with a stiffness factor under lateral loading, K r , greater than 0.01, where
K f = E pI p
E s L 4 (9.20)
and I p and L are the second moment of area and length of the pile respectively.
The effect of sloping ground in front of a laterally-loaded pile was analysed by Poulos (1971) for
clayey soils, and by Nakashima et al (1985) for granular soils. It was concluded that the effect on
pile deformation will not be significant if the pile is beyond a distance of about five (5) to seven (7)
pile diameters from the slope crest.
9-40 March 2009