| Principal Investigator and Institution
Geoffrey R. Martin, University of Southern California
Objective
The primary objective of this task is to develop practical analysis
approaches that can be used to evaluate the ability of pile foundations in
short- to medium-span bridges to accommodate displacement demands
associated with liquefaction-induced lateral spreads. Emphasis is on piles
or pile groups where the pile spacing does not lead to pile interaction
under lateral loading (i.e., spacing greater than about five pile
diameters).
Approach
During Research Years 1 and 2, this task developed: a case history
database including available centrifuge model test results; simplified
analysis methods for computing free field lateral spread deformations
based on the Newmark approach; and a practical analysis approach to
evaluate deformation demands on pile foundations. The deformation demand
approach was incorporated into the recently completed seismic design
specifications developed under NCHRP Project 12-49. A key component of
this design methodology is the concept of allowing plastic hinge
development in piles up to the ductility capacity, and incorporating the
pile resistance to deformation in estimates of displacement demands
related to ground deformation potential.
Results from Research Years 1 and 2 include the following:
- Earthquake-induced lateral spread displacement computations using
nonlinear site response analyses, which were conducted for sloping
ground containing weak soil layers of varying thickness, showed
displacement time histories and magnitudes which could be reasonably
approximated by Newmark sliding block analyses, assuming a failure
plane at the base of the weak or liquefied layer.
- Case history data and centrifuge model tests demonstrated that: (1)
the maximum pile moment and plastic hinge development occur at
interfaces between non liquefied and liquefied layers; (2) limit
equilibrium can be established between maximum passive pressures on
piles and pile caps, and pile shear forces and moments; and (3) piles
can successfully resist large lateral spread displacements in a state
of equilibrium, provided hinge ductility demands are less than hinge
capacity.
- A simplified design guideline based on a limiting equilibrium
approach for lateral spread demands was developed, which was
incorporated into the recommended seismic design specifications
resulting from NCHRP Project 12-49.
During Research Year 3, the focus of the research will be on the
completion of a final report, which includes detailed design guidelines.
The scope of the work for Research Year 3 includes:
- Conducting comparisons between the NCHRP Project 12-49 simplified
approach and results of analytical simulations (using the computer
programs LPILE or BSTRUCT), with results from centrifuge model tests.
- Development of recommendations for plastic hinge simulations in
model analysis.
- Completion of a final report documenting the three-year research
effort, and providing a recommended design guideline and worked
examples.
Products
- Recommendations for plastic hinge simulations in model analysis.
- Recommended design guidelines and worked examples.
Technical Challenges
Although the numerical and modeling methods that are being used here
are not new, they must be carefully employed in order to not induce errors
into the simulations and results. |