Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 268

rigid. The material behavior of these elements may vary from linearly elastic to
nonlinear, hysteretic, and rate dependent. According to Bea and Audibert
(1979) and Karlsrud et al. (1986) , the soil elements are commonly referred to
as shaft resistance-displacement (t-z) and tip resistance-displacement (Q-z)
elements.
The pile can be modeled as a series of discrete elements; for example, rigid
masses interconnected by springs, or it can be modeled as a continuous element
(rod), either linear or nonlinear. In these models, material properties for soil and
pile can vary along the pile.
This type of model is the most famous in fixed offshore structure analysis.
The primary steps in performing an analysis of cyclic axial loading effects
on a pile using discrete element models are summarized in the following:

1. The pile-head loadings should be characterized in terms of the magnitudes,
durations and numbers of cycles. This includes both long-term loadings and
short-term cyclic loadings. Typically, the design static and cyclic loadings
expected during a design event are chosen.
2. The properties of the pile, including its diameter, wall thickness, stiffness
properties, weight and length, must be defined. Therefore, this will need
an initial estimate of the pile penetration that might be appropriate for the
design loadings. Note that an empirical, pseudostatic method based on
pile load tests or soil tests might be used to make such estimates.
3. Soil properties should be defined, as any analytical approaches will require
different soil parameters.

For practical reasons, discrete element models solved numerically have seen
the most use in evaluation of piles subjected to high-intensity cyclic loadings.
Based on Poulos (1983) and Karlsrud et al. (1986) , results from these models
are used to develop information on pile accumulated displacements and on
pile capacity following high-intensity cyclic loadings.

Continuum Models
The soil around the pile is idealized as a continuum attached continuously to
the pile.
The pile is typically modeled as a continuous rod, either linear or nonlinear.
According to Novak and Sharnouby (1983) and Desai and Holloway (1972) ,
the model material properties can vary in any direction. A wide range of
assumptions can be used regarding boundary conditions, solution characteristics
and others that lead to an unlimited number of variations for either of the two
approaches.
The main key to establishing the model is to define the elastic properties of
the soil (E, G,

,D). However, in the discrete element model, soil resistance-
displacement relationships along the pile shaft (t-z)andatitstip(Q-z) should
be determined. In-situ and laboratory soil tests, and model and prototype pile

ν