The load-settlement (Q-s) response of deep foundations is influenced by the soil stiffness.
One of the most common methods of installing these foundations is the process of driving,
which changes the in situ soil stress and stiffness regime. The stiffness further
reduces in a nonlinear manner as the loads and shearing strains increase within the
soil. The decay in the stiffness of the soil surrounding an axially loaded pile varies
with depth. While a variety of methods is available to predict the nonlinear Q-s response
of piles in relatively simpler soil profiles, only select methods can handle the case
of multi-layered soils, where the stiffness properties vary between layers. As an
alternative, the Randolph analytical pile solution is exploited for (i) developing
a new modulus reduction scheme from the back-analysis of load tests on driven piles
that also accounts for plasticity of the soil, (ii) devising a methodology for generating
modulus reduction curves for individual layers of a multi-layered system, and (iii)
formulating a stacked pile model with integration of modulus reduction curves for
an improved solution. The back-analysis process accounts for the installation effects
on the in situ soil stiffness. A step-wise flowchart and example applications of the
methodology are also presented.