BACKGROUND
NCHRP Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions revealed that one-dimensional (1-D) equivalent-linear analysis is the de facto standard for state DOT highway facilities at those locations where site-specific ground response analyses are conducted in accordance with provisions in the AASHTO LRFD Bridge Design Specifications (2014) and the AASHTO Design Guidelines for Seismic Bridge Design (2011). However, users have concerns about the applicability of equivalent-linear analyses for the cases for which site-specific response analyses are most useful (i.e., soft soil sites, liquefiable sites, and sites subjected to very strong shaking). While nonlinear 1-D site response analyses are beginning to be used in practice to address these concerns, considerable uncertainty exists with respect to the appropriate manner in which to employ and interpret such analyses. To assist bridge and foundation designers, guidance on the use and selection of 1-D nonlinear software with pore water pressure generation is needed for the effective and economical seismic design of all types of highway structures.
OBJECTIVE
The objective of this research was to provide guidance on the use and selection of analytical methods for 1-D nonlinear seismic site response analysis with pore water pressures generation to quantify the effects of site-specific conditions on earthquake ground response.
The guidance considered the following: (1) input parameters required for the analyses (e.g., site characterizations, seismic loading); (2) limitation, selection, and validation of analytical methods; (3) the process of model setup; and (4) how to use the results of the analytical methods.
Accomplishment of the project objective included the following tasks.
PHASE I—Planning
Task 1. Conduct a literature review of relevant domestic and international studies for analytical methods for one-dimensional nonlinear seismic site response analysis with pore water pressures generation. The review should include research conducted through the NCHRP; Strategic Highway Research Program 2 (SHRP 2); FHWA; and other relevant national, international, and state departments. The review shall include the identification and collection of available field and laboratory data to achieve the research objective.
Task 2. Synthesize results of Task 1 to identify the knowledge gaps and uncertainties related to the analytical methods for I-D nonlinear seismic site response analysis with pore water pressures generation. These gaps should be addressed in the final product and in the recommended future research as budget permits.
Task 3. Propose analytical and testing programs, to be executed in Phases II and III, to achieve the project objective. The analytical program shall consider at a minimum: different site characterizations, seismic loading, and soil constitutive models (e.g., the effect of soil dilation and pore pressure dissipation); and comparison of equivalent-linear and total stress nonlinear analyses to effective stress nonlinear analyses. At least four commercial software packages shall be used in the analytical modeling and validation.
Task 4. Prepare Interim Report No. 1 that documents the results of Tasks 1 through 3 and provides an updated work plan for the remainder of the project. This report is due no later than 4 months after the contract award. The updated work plan must describe the process and rationale for the work proposed for Phases II through IV.
PHASE II—Analytical Program
Task 5. Execute the analytical program according to the approved Interim Report No.1.
Task 6. Finalize the testing program work plan and provide a matrix of the testing specimens and parameters. The testing specimens shall consider the scale effect and testing configurations on the applicability of the results. Describe how the testing results will be utilized to achieve the research objective. The testing program may include field exploration [e.g., cone penetrometer test (CPT), and sounding, cyclic lab testing].
Task 7. Prepare an annotated table of contents for the proposed guidance, to be developed in Phase IV. Propose case studies to demonstrate the application of proposed guidance.
Task 8. Prepare Interim Report No. 2 that documents the results of Tasks 5 through 7 and provides an updated work plan for the remainder of the project. This report is due no later than 9 months after approval of Phase I. The updated plan must describe the work proposed for Phases III and IV.
PHASE III—Testing Program and Validation
Task 9. Execute the testing program according to the approved Interim Report No.2.
Task 10. Finalize the analytical program based on the testing program and observed data from the field array. Conduct sensitivity analyses as needed to achieve the research objective.
Task 11. Based on the analytical and testing program, summarize the pros and cons of every analytical method including when and how to use each method and update the table of contents for the proposed guidance.
Task 12. Prepare Interim Report No. 3 that documents Tasks 9 through 11 and provides an updated work plan for the remainder of the project. This report must be submitted no later than 15 months after Phase II approval.
PHASE IV—Final Products
Task 13. Develop the draft guidance and case studies according to the approved Interim Report No. 3. The draft guidance is due no later than 3 months after approval of Phase III.
Task 14. Prepare final deliverables including: (1) the revised guidance that reflects consideration of the panel’s review comments; (2) a final report that documents the entire research effort; and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products.” See Special Note D for additional information.
STATUS: Completed. The final deliverables have been received. A publication decision is pending.
