MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

Task E1-2: Foundation Design for Liquefaction-Induced Displacements

Subject Area: Geotechnical and Foundation Engineering
Research Year 3

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.

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