| Principal Investigator and Institution
Geoffrey R. Martin, University of Southern California
Objective
The primary objective of Task C2-2 is to determine an appropriate, yet
practical, method for establishing the lateral load-deformation behavior
of large pile groups in soft clays and sands. The focus of the task is on
the use of conventional nonlinear "p-y" curves, coupled with
multipliers appropriate for a given pile spacing. Such characterization is
needed for static pushover analyses. In addition, a secondary objective is
to establish an approach to determine an equivalent linear stiffness
matrix for dynamic response analysis.
Approach
During Research Years 1 and 2, this task served to identify, formulate,
and implement an appropriate numerical analysis methodology necessary to
achieve the primary objective. This required investigation on several
fronts. An extensive literature review provided information on practice
and state-of-the-art approaches used for lateral load design. Suitable
field test data on single piles and small pile groups were also obtained
for calibration purposes. Given the absence of information on large pile
groups, a survey questionnaire form was distributed to selected
consultants and transportation agencies in various States, with a focus on
collecting information on long span bridges and large pile group
foundations typically used for such structures.
The influence of soil type on pile behavior required consideration of
constitutive formulations for sand and clay. The observed influence of
dilatency effects on sand behavior may require a more sophisticated
constitutive formulation compared with the commonly adopted Mohr-Coulomb
constitutive model. A constitutive model based on bounding surface
concepts was therefore identified for implementation.
Numerical modeling of the pile group problem required assessment of
various strategies. An initial investigation used a line element to model
the pile and rigid spokes to account for pile diameter effects, and
invoked symmetry simplifications to reduce the problem to manageable
proportions. This approach provided promising results but, in order to
capture diameter effects in a more rigorous fashion, a move to a continuum
pile model was considered more appropriate. Such a modeling approach was
available in the 3-D nonlinear finite difference program FLAC3D, and this
program was adopted to achieve the primary objective of the research.
The focus of Research Year 3 will be to complete the numerical work and
rationalize the results in the form of p-y curves and multipliers for
given spacing, fixity, and soil conditions; and completely document all
work in the form of a final report with appropriate design guidelines. The
scope of work includes the following three subtasks:
Subtask 1 - The following analyses will be conducted:
| Phase |
Item |
Description |
| Calibration |
Pile Group |
Emulation of field test behavior using small pile
group load tests. |
| Research Analyses |
Internal Pile |
Simulate behavior of internal pile in large pile group
using periodic boundary simplification. |
| External Pile |
Simulate side-by-side behavior using two-pile group
with fixed boundaries. |
| Simulate leading row-trailing row behavior using
two-pile group with fixed boundaries. |
Subtask 2 - Development of appropriate recommendations for
designing large pile groups using p-y curves and multipliers, including
use of the program GROUP for pushover analyses, and development of
recommendations for determination of equivalent linear stiffness matrices.
Subtask 3 - Completion of final report documenting all work on
the task from Research Years 1 through 3, including worked examples on
representative large pile groups.
Products
- Recommendations for designing large pile groups using p-y curves and
multipliers.
- Worked examples on representative large pile groups.
- Final technical report document all aspects of this 3-year task.
Technical Challenges
Most of the technical challenges associated with this task have already
been resolved in connection with the numerical difficulties encountered in
Research Year 2. A remaining challenge is the satisfactory simulation of
the large pile group behavior.
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